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Exercised Book Summary by Daniel Lieberman

What you will learn from reading Exercised:

– 12 common myths of exercise and how they mislead us.

– Why we never evolved to exercise and what this means for the motivation and keeping fit. 

– How the modern environment is mismatched with our biology and what chronic diseases this has caused.

Exercised Book Summary:

We never evolved to exercise:

Consider the treadmill, it serves as an example of the peculiar nature of exercise in today’s industrialised world. It would be a challenge to explain to a hunter-gatherer or even your great-great-great-grandparents how you spend the majority of your time sitting in chairs and then use your hard-earned money to go to a gym, where you exert yourself on a machine that causes you to sweat, become tired, and uncomfortable just to stay in one place. It’s a peculiar concept to explain without appearing insane or foolish.

The weirdness of treadmills as a concept encapsulate the main theme of this book: we never evolved to exercise. But, what does Daniel Lieberman mean by this statement?

Essentially, exercise is now widely understood as a voluntary physical activity carried out to promote health and fitness. However, this understanding is relatively new. Our ancestors, who were mainly hunter-gatherers and farmers, had to engage in physical activity for hours each day to obtain sufficient food. Although they occasionally engaged in leisure activities like dancing and playing, they never engaged in running or walking several miles solely for the purpose of maintaining their health.


The Myths of Exercise:

Based on Liebermans experiences and research, he concluded that industrialised societies, such as the United States, have propagated many myths about exercise (by “myth,” Lieberman means a claim that is widely believed but inaccurate and exaggerated). It’s important to note that he is not suggesting that exercise isn’t beneficial or that everything you have read about exercise is incorrect – that would be wrong.

However, it can be argued that the contemporary, industrial approach to exercise is flawed due to the ignorance or misinterpretation of evolutionary and anthropological perspectives on physical activity. As a result, this approach is marred by misconceptions, overstatements, faulty logic, occasional mistruths, and inexcusable finger-pointing.

One of the most prevalent myths regarding exercise is the idea that we should naturally want to do it. There is a group of individuals that the Lieberman refers to as “exercists” who take pleasure in boasting about their physical activity and consistently emphasise the benefits of exercise as a form of medicine – a magical cure that can delay aging and extend life. You’ve likely come across these types of people.

According to exercists, humans were destined to exercise, given that our hunter-gatherer ancestors relied on activities such as walking, running, and climbing for survival over millions of years. Even those exercists who reject the theory of evolution still believe that we are predestined to engage in physical activity.


How this book approaches Exercise:

To begin, it’s worth noting how Lieberman approaches this topic. If you’ve perused any exercise-related websites, articles, or books, you’ll quickly notice that much of what we know stems from observing individuals in modern, industrialised countries like Japan, Sweden, England, and the United States.

Many of these studies are epidemiological, meaning they investigate correlations between health and physical activity in large groups of individuals. For example, hundreds of studies have explored connections between heart disease, exercise habits, and factors such as age, gender, and income. It’s crucial to note that these analyses only reveal correlations, not causation.

Lieberman however, utilises evolutionary and anthropological viewpoints to challenge and reconsider numerous myths regarding physical inactivity, activity, and exercise. As well as answer the following questions:

Are humans naturally inclined to exercise? Is sitting as harmful as smoking? Does poor posture lead to health problems? Is eight hours of sleep a necessity? Are humans relatively slow and weak? Does walking fail to aid in weight loss? Does running damage your knees? Is it normal to exercise less as we age? What is the best method to persuade people to exercise? Is there an ideal form and amount of exercise?

As Lieberman says in the Introduction:“The mantra of this book is that nothing about the biology of exercise makes sense except in the light of evolution, and nothing about exercise as a behaviour makes sense except in the light of anthropology.”


CHAPTER ONE – Are We Born to Rest or Run?

MYTH #1 We Evolved to Exercise


The Natural Human:

One of the most prevalent and appealing perspectives on the influence of nurture on physical activity stems from a concept called the theory of the natural human. This notion, advanced by the 18th-century philosopher Jean-Jacques Rousseau, suggests that humans living in a “savage” state of nature embody our true and innate selves, unspoiled by civilisation. Although this belief has been discredited, it has taken on different forms, such as the notion of the noble savage, which proposes that individuals in non-Western societies whose minds have not been tainted by the moral and social evils of civilised societies are inherently good and decent.

This myth has persisted, and it has been rejuvenated in the context of exercise, dubbed the myth of the athletic savage by Lieberman. The core concept of this myth is that people like the Tarahumara (see below), whose bodies are untainted by modern, decadent lifestyles, are natural super athletes who can accomplish remarkable physical feats and are free from laziness.

By asserting that individuals like those I observed running 70 miles without training do so without effort, this myth suggests that people like you and me, who are neither capable nor willing to achieve such feats, are abnormal from an evolutionary standpoint because civilisation has transformed us into weaklings.


The Tarahumara Tribe:

The Rarámuri or Tarahumara is a group of indigenous people of the Americas living in the state of Chihuahua in Mexico. They are renowned for their long-distance running ability.

How do some Tarahumara manage to run several back-to-back marathons without training, while Ironmen practice and prepare obsessively for years to accomplish similar feats of endurance?

While it’s true that the Tarahumara and other nonindustrial people don’t follow a structured exercise routine, they engage in hours of physical labor every day as part of their daily existence. They walk many miles on rugged terrain and perform manual tasks like plowing, digging, and carrying since they lack modern labor-saving devices. When researchers attached accelerometers to over twenty Tarahumara men, they found that they walked on average ten miles per day. Therefore, the training that enables them to run back-to-back marathons is the physical work that is an integral part of their everyday life.


Analysing the Hadza:

The Hadza, or Hadzabe are a Tanzanian indigenous ethnic group mostly based in southwest Karatu. There are, as of 2015, between 1,200 and 1,300 Hadza people living in Tanzania, however only around 400 Hadza still survive exclusively based on the traditional means of foraging.

Daniel Liebermen spent time observing the Hadza men and women in camp, where he noticed, and various published studies have confirmed, that they spend most of their time engaging in light activities, sitting on the ground, gossiping, caring for children, and engaging in leisurely activities. However, they typically go out to the bush to hunt or gather food almost every day.

In one study, forty-six Hadza adults wore heart rate monitors for several days, and the results showed that they spent an average of three hours and forty minutes a day engaged in light activities and two hours and fourteen minutes a day in moderate or vigorous activities. Although they are about twelve times more active than the average American or European, their workload cannot be described as strenuous. On average, women walk five miles a day and dig for a few hours, while men walk between seven and ten miles a day. When not very active, they usually rest or engage in light work.

But debunking the myth of the athletic savage doesn’t address the fundamental question: What kind of physical activity and how much of it is normal for a “normal” human being?


Physical Activity Levels across humanity:

Assuming that what hunter-gatherers do is evolutionarily “normal,” comprehensive studies of contemporary foraging populations from Africa, Asia, and the Americas suggest that a typical human workday used to last about seven hours. This workday consisted mostly of light activities with at most an hour of vigorous activity. Although there is variation from group to group and season to season, most hunter-gatherers engage in moderate levels of physical effort, some of which is done while sitting. How different is this “normal” level of physical activity from that of postindustrial people, such as farmers like the Tarahumara, factory workers, and others whose lives have been transformed by civilisation, including me (and possibly you)?

The PAL, or physical activity level, is determined by dividing the energy you expend during a twenty-four-hour period by the amount of energy needed to maintain your body if you were in bed all day. This measurement is not influenced by differences in body size, making it impartial.

Hunter-gatherers typically have PALS of 1.9 for men and 1.8 for women, slightly lower than subsistence farmers, whose PALS average 2.1 for men and 1.9 for women. To provide perspective, the PALS of hunter-gatherers is similar to those of factory workers and farmers in the developed world (1.8), and about 15% higher than PALS of individuals with sedentary desk jobs in developed countries (1.6). Thus, the physical activity levels of typical hunter-gatherers are comparable to those of Americans or Europeans who exercise for an hour a day.

It is worth noting that most wild mammals have PALS of 3.3 or higher, which is almost twice as high as that of hunter-gatherers. Therefore, humans who must forage and gather their food and create all their possessions by hand are significantly less active than the average free-ranging mammal.


Conclusion: Lieberman believes two generalisations can be made about human exercise. Firstly, while sports have been popular among young people throughout history, non-sport-related exercise was not common until recent times. Secondly, with modern advancements in technology and changes in society, physical activity is no longer as necessary for people in industrialised societies. As a result, many experts have become increasingly concerned about the lack of physical activity among the general population.


CHAPTER TWO – Inactivity: The Importance of Being Lazy

MYTH #2 It Is Unnatural to Be Indolent


Chimpanzees – Our lazy ancestors:

When chimpanzees travel on the forest floor, it can be a challenge to keep up with them, but they spend most of their day either feeding or digesting. Chimpanzees typically devote about half their waking hours to filling their stomachs with highly fibrous food, and for much of the rest of the day they rest, digest, groom each other, and take long naps. On an average day, they climb only about a hundred meters and walk just two to three miles. To be sure, chimpanzees are highly social animals, and they occasionally fight, copulate, and do other exciting things, but for most of the time our closest ape relatives are sluggards that live a sort of perpetual Sabbath.

Compared to most mammals, including humans, a mile of walking requires more than twice the energy for a typical chimpanzee. Since walking is so expensive in terms of calories, apes are naturally driven to use as little energy as possible while moving around the forest, allowing them to allocate more energy to reproduction. Therefore, apes have evolved to be adapted to a sedentary lifestyle.

Hunter-gatherers like the Hadza, while not engaging in intensive physical labor and often being inactive for several hours during the day, are still more active than apes who make them seem like workaholics. This is particularly interesting as humans evolved from apelike ancestors similar to chimpanzees and gorillas, which implies that it is the modern sedentary lifestyle of humans that is an anomaly. This discovery poses several questions about why non-industrial humans are less active than wild apes, despite being relatively more active. However, before answering this question, it is necessary to examine what happens in our bodies and how much energy we expend while at rest, which requires laboratory investigations.


Energy Expenditure:

Assuming you are an average 180-pound (82-kilogram) adult American male, your body burns about seventy calories per hour when you are resting and inactive, which is known as your resting metabolic rate (RMR). This RMR contributes to all the chemical reactions that occur in your body when you are not physically active. If you continue to sit in a chair for twenty-four hours straight without any physical activity, your body will burn approximately 1,700 calories based on your RMR.

Your basal metabolic rate (BMR) is approximately 10% lower than your RMR, which is about 1,530 calories in the given example. Your BMR is the amount of energy required to maintain the basic functions of your body to keep you alive in a comatose-like state.

In summary, even if you engage in high levels of physical activity, your body still expends more energy to maintain itself than to perform activities, which may seem counterintuitive.


Effects of starvation:

The Minnesota Starvation Experiment was a clinical study performed at the University of Minnesota between November 19, 1944 and December 20, 1945. The investigation was designed to determine the physiological effects of severe and prolonged dietary restriction and the effectiveness of dietary rehabilitation strategies.

The individuals who volunteered for this scientific study in which they starved themselves were able to survive because of their body’s capacity to adapt by utilising less energy even when at rest. Over the course of twenty-four weeks, the volunteers’ resting and basal metabolic rates dropped by 40 percent, a transformation that was not easily observable.

The essential takeaway from this experiment is that human resting metabolisms are adaptable. Resting metabolism represents the amount the body spends on maintenance, rather than what it requires to spend. One of the primary ways in which the volunteers reduced their energy expenditure was by cutting back on maintenance processes that help maintain the body’s balance. Their heart rates dropped by one-third, and their body temperatures decreased from the normal 98.6°F to 95.8°F, making them feel constantly cold, even in rooms that were adequately heated.

It took five years for the Minnesota Starvation Experiment to be analysed and published. One of the critical insights gained from the brave volunteers was that resting is not merely a state of physical inactivity. Despite appearing to do nothing, our bodies continue to actively expend a significant amount of energy on many dynamic and costly processes. Resting is also a crucial way our bodies make trade-offs since we cannot spend a calorie more than once.

As you read this text, you are using approximately sixty calories per hour (equivalent to the energy in a typical orange) to tend to various bodily functions, such as your brain, liver, muscles, kidneys, intestines, and more. If you decide to put down this book and climb a mountain, some of those basic functions will necessarily lose calories as you ascend and descend the mountain. Later, when you return home, you will likely need to eat and rest to replenish those extra calories spent.


The Calorie Trade-off:

It is important to understand the trade-offs between being inactive and active, and a fundamental concept to keep in mind is that a calorie can only be spent once. There are only five ways in which a calorie can be utilised: to grow your body, maintain your body during resting metabolism, store energy as fat, engage in physical activity, or reproduce.

The way in which your body divides and prioritises these functions is determined by various factors, such as age and energetic circumstances. If you are still in the growing stage, it is likely that you won’t have enough energy to reproduce, which is why animals generally start having offspring only after they have completed their growth phase.

From the perspective of natural selection, when there is a shortage of calories, it is always advantageous to allocate energy away from nonessential physical activity towards reproduction or other functions that maximise reproductive success, even if these trade-offs result in poor health and shorter lifespans.

In essence, our bodies have evolved to be as inactive as necessary, but not more. Of course, it is essential to move to survive and thrive, but it is equally crucial not to expend more energy than is necessary to achieve necessary activities.

As a young hunter-gatherer, playing to develop athletic skills, strength, and stamina would have been critical. As an adult, finding food, doing chores, finding mates, and avoiding danger would have been non-negotiable. Participating in important social rituals, such as dancing, would likely be necessary and desirable. However, when energy is scarce, as it typically is, any extraneous physical activity would reduce the amount of energy that can be allocated to survival and reproduction. Sensible adult hunter-gatherers would not waste five hundred calories running five miles just for fun.


Is being lazy a good thing?

As you sit in a chair or lie in bed reading this, feeling guilty about being inactive, take comfort in the fact that your current state of physical stillness is a time-tested, basic approach to sensibly conserve limited energy. Aside from youthful inclinations to play and other social factors, the inclination to avoid unnecessary physical activity has been a sensible adaptation for countless generations. In reality, in contrast to other animals, humans may have evolved to be particularly disinclined to exercise.

The word “sloth” originates from the Latin term “acedia,” meaning “without care,” and was once viewed by early Christian thinkers such as Thomas Aquinas as a form of mental apathy rather than physical laziness. Sloth was deemed sinful because it led to a disregard for the pursuit of God’s work. It wasn’t until later that sloth was associated with physical inactivity, as almost everyone, except for a few elites, engaged in regular physical labor.

Today, “laziness” is commonly equated with sloth, but it lacks the same spiritual connotations. Being disinclined to perform an activity due to the effort involved is not the same as failing to fulfill one’s duties to others. Opting to park in a more convenient spot to conserve energy is merely an instinct and should not be regarded as a spiritual offense.


CHAPTER THREE Sitting: Is It the New Smoking?

MYTH #3 Sitting Is Intrinsically Unhealthy


All humans sit:

To fully grasp how our bodies have evolved to adapt to physical inactivity and activity, it is essential to comprehend the significance of sitting. If our bodies have developed to avoid unnecessary physical activity, including exercise, then how can sitting have such detrimental effects? Additionally, is it accurate that the average American spends thirteen hours a day sitting, and how does this compare to the sitting habits of our ancestors?

While standing may require more energy than sitting, the practice of sitting for extended periods is a widespread habit among humans and other animals due to the long-term benefits of energy conservation. Historically, people sat on the ground without the use of chairs, as furniture was not commonly used by hunter-gatherers. Today, many individuals in non-Western cultures continue to sit on the ground regularly


The activity of Chimpanzees:

On average, adult male and female chimpanzees spend nearly 87% of their day engaged in sedentary activities like grooming, resting, feeding, and nesting. During a 12-hour day, chimpanzees are physically inactive for almost 10.5 hours, with the most active days still consisting of about 8 hours of rest and the least active days consisting of over 11 hours of rest. After building a nest, they sleep for approximately 12 hours until sunrise.

Compared to wild chimpanzees, even the most sedentary American couch potatoes are significantly more active. If being inactive is a natural and adaptive part of both the human and ape conditions, then why and how could several hours of sitting each day be considered unhealthy?


The health concerns of sitting:

Prolonged sitting can raise three significant health concerns that are interrelated. The first is the lack of physical activity. When sitting for long hours, we are not engaging in any form of exercise or activity. The second concern is that extended periods of inactivity can increase the levels of sugar and fat in the bloodstream, which can be harmful to our health. Finally, the most alarming risk of prolonged sitting is the potential for our immune systems to attack our bodies, which can lead to inflammation.

However, a 15-year study that involved over 10,000 Danish individuals did not establish any link between the time spent sitting at work and heart disease. Similarly, a larger study of 66,000 middle-aged Japanese office workers also produced similar findings. These studies indicate that mortality is more accurately predicted by the duration of sitting during leisure time, implying that factors such as socioeconomic status and exercise routines during mornings, evenings, and weekends have significant health implications that surpass the amount of time spent sitting at work during weekdays.

Also, the notion that slouching in chairs causes back pain is not supported by scientific evidence. People without chairs commonly adopt postures that are comfortable and don’t slouch. Furthermore, numerous meta-analyses and systematic reviews have been conducted on the relationship between sitting posture and back pain, and high-quality studies have consistently failed to find a link between habitual sitting in flexed or slouched postures and back pain. In fact, there is no good evidence to support the idea that sitting for long periods or using special chairs increases the risk of back pain. The best predictor of avoiding back pain is having a strong lower back with muscles that are resistant to fatigue. People with strong, fatigue-resistant backs are more likely to have better posture, which can help prevent back pain.


CHAPTER FOUR – Sleep: Why Stress Thwarts Rest

MYTH #4 You Need Eight Hours of Sleep Every Night


The Myth of Eight Hours

In the modern world, we tend to medicalise certain behaviours by prescribing specific doses, such as a minimum of 150 minutes of physical activity per week, twenty-five grams of fibre per day, and eight hours of sleep per night. However, it is unclear when and where the recommendation of eight hours of sleep per night originated.


Patterns of Sleep:

A quick observation would show that there is no uniform sleeping pattern among humans or mammals. Donkeys only sleep for two hours a day, while armadillos sleep for as long as twenty hours. While some animals like giraffes take frequent naps, other species have uninterrupted sleep. A few large animals, such as elephants, can nap while standing up. Moreover, some marine mammals, including whales and dolphins, have evolved the ability to put half of their brain to sleep while swimming.

The reality is that most biological processes are extremely diverse, and sleep is not an exception to this. Due to variations in circadian rhythms and the mechanisms that control wakefulness and sleepiness, sleeping patterns in humans are just as diverse as they are in other animals.

The absence of a consistent sleep pattern applies to populations living in areas with electric lights such as New York and Tokyo, as well as to those living without electricity in the African savanna or Amazonian rainforest. Anthropologist David Samson’s twenty-day study of twenty-two Hadza hunter-gatherers revealed a wide range of sleep activity, with someone in the camp awake at different times. Based on this, Samson estimated that someone was awake for all but eighteen minutes per night. Such variation in sleep patterns is probably adaptive from an evolutionary standpoint, given that we are most vulnerable when asleep at night. Having at least one watchful sentinel, usually an older individual, would have reduced the risks of sleeping in a world full of leopards, lions, and other humans who could harm us.

The UCLA sleep researcher Jerome Siegel and his colleagues conducted a study that was particularly intriguing. They fitted wearable sensors on ten Hadza hunter-gatherers from Tanzania, thirty San forager-farmers from the Kalahari Desert, and fifty-four hunter-farmers from the Amazon rainforest in Bolivia, none of whom had access to electricity, clocks, or the internet. Despite this, Siegel was surprised to discover that they slept less than industrialised people. During warmer months, these foragers slept for an average of 5.7 to 6.5 hours per day, and during colder months they slept for an average of 6.6 to 7.1 hours per night. Furthermore, they seldom took naps.


The uncomfortable sleeping of the past:

The disorderly sleeping conditions of the Stone Age may seem counterproductive to a good night’s sleep from a modern perspective, but anthropologist Carol Worthman suggests that the opposite could be true. As we begin to enter the initial stages of NREM sleep, our awareness of the environment gradually decreases. This process of gradually tuning out might be beneficial because our brain is monitoring our surroundings as we fall asleep, potentially to evaluate if it is safe to sleep. The slow disappearance of sounds like friends and family talking, a crackling fire, crying infants, and the fact that hyenas are far away signal to the brain that it is secure to enter a deeper stage of unconscious sleep. Ironically, by insulating ourselves so effectively from these comforting stimuli, we may be making ourselves more susceptible to experiencing sleep-related stress


The Sleeping System:

The regulation of wakefulness and sleep involves two main biological processes that interact within the brain. Normally, these processes ensure that we wake up feeling refreshed in the morning, stay alert throughout the day, and fall asleep peacefully at night. But when they are disrupted, we may nap inappropriately, struggle to fall asleep at night, wake up too early, or experience sleepless nights.

The first process is our circadian rhythm, a cycle that lasts about 24 hours and is regulated by a group of cells in the hypothalamus called the suprachiasmatic nucleus. These cells stimulate the production of cortisol, a hormone that promotes wakefulness, in the morning. As night falls, the hypothalamus directs the pineal gland to produce melatonin, a hormone that helps induce sleep. The circadian rhythm is synchronised daily by environmental cues such as light, which can reset it gradually (by about an hour per day).

However, relying solely on the circadian rhythm to regulate sleep would be problematic.For this reason, the regulation of our sleep-wake states is influenced by a second system that is closely tied to our activity levels.

This system, called the homeostatic system, functions like an hourglass that measures how long we’ve been awake, gradually building up pressure for us to sleep. As we remain awake, the accumulation of molecules such as adenosine left behind when the brain uses energy increases our sleep pressure. When we sleep, particularly during NREM sleep, we reset the hourglass. The homeostatic system helps balance the time we spend awake versus asleep, and if we are awake for too long, it will eventually take over our circadian systems and help us make up for lost sleep time.

Numerous studies have shown that while physical activity may not be a panacea for all sleep issues, engaging in a single session of exercise (excluding right before bed) generally helps individuals sleep better, and consistent exercise produces even greater benefits.


The sleep-industrial complex:

The sleep industry, also known as the sleep-industrial complex, has taken advantage of people’s concerns about their sleep, persuading them to spend a considerable amount of money on various high-tech devices like noise-canceling headphones, light-blocking curtains, and specialised mattresses. Although these gadgets may seem amusing to our ancestors who slept on the ground, the excessive use of sleeping pills is a cause for alarm. These highly addictive pills are a multi-billion dollar industry, and the number of prescriptions in the United States has increased more than threefold since 1998, not counting over-the-counter medications.

While it’s important to acknowledge that many people suffer from sleep deprivation and its negative consequences, we must also consider the evolutionary and anthropological context of sleep. Some people tend to exaggerate the extent to which modern sleep patterns are abnormal, similarly to how sitting has been demonised. This fear mongering can be profitable, and our society has a tendency to judge physical activity and inactivity behaviours. However, both sitting and sleeping are natural and common behaviours that vary greatly based on environmental and cultural factors, and have complex advantages and drawbacks.


Diagnosing your own sleep health:

If you are unsure about your own sleep health, sleep researchers suggest you ask yourself five simple questions:

Are you satisfied with your sleep?

Do you stay awake all day without dozing?

Are you asleep between 2:00 and 4:00 a.m.?

Do you spend less than thirty minutes awake at night?

Do you get between six and eight hours of sleep?

If your answers to these questions are “usually or always,” then you should sleep contentedly knowing that you generally get enough sleep.


PART II Speed, Strength, and Power


CHAPTER FIVE –  Speed: Neither Tortoise nor Hare

MYTH #5 Normal Humans Trade Off Speed for Endurance


Why are humans so slow?

The fastest human Usain Bolt would have no chance against the vast majority of quadrupeds like zebras, giraffes, wildebeests, white-tailed deer, or even wild goats. As for carnivores, even slower predators like grizzly bears and hyenas could eat Bolt’s lunch-not to mention Bolt-on the track.

Sprinting has a limitation that we quickly run out of energy. Even the fastest animals cannot sustain top speed for a long duration, and humans are no exception. For instance, the fastest thousand-meter run was completed at 27.3 kilometers per hour, which is less than three-quarters the speed of the fastest hundred-meter dash. Elite sprinters like Bolt can sustain maximum velocity for about twenty seconds only, after which they must significantly slow down. Similarly, cheetahs in the wild can only maintain peak speed for a maximum of about thirty seconds before slowing down. Conversely, many other mammals can run much faster than humans and can chase or flee from us at submaximal speeds that they can sustain for more extended periods.

Unless you are a world-class sprinter, you have little chance of outrunning a squirrel. Why are humans so comparatively slow?

Bipedalism has left us with slow running speed due to the force required to walk or run being generated by our legs against the ground. Unlike animals like dogs and chimpanzees, who have four legs to push against the ground and generate power, humans only have two legs, which means less power and less speed. In fact, only one leg is on the ground at any given moment to push us forward. This disadvantage limits our running speed to half that of similar-sized quadrupeds, such as greyhounds that are twice as fast as elite sprinters. To put it simply, having two legs is the reason why humans are relatively slow runners.


How do our muscles recharge?

Muscles recharge ATP through different processes that occur over time. Initially, the energy comes almost instantly from the stored ATP and creatine phosphate (CrP). Then, as the stored ATP and CrP get used up, energy is provided relatively rapidly from glycolysis. However, as the body continues to exercise, glycolysis also becomes insufficient to meet the energy demands, and the body begins to rely on slow and steady aerobic metabolism, which occurs in the mitochondria. Aerobic metabolism liberates energy either from pyruvate, which is an end product of glycolysis, or from fatty acids.


The Phospaghen system:

The phosphagen system, which is the first process, is the fastest but short-lived method of providing energy. When Usain Bolt and I begin to run, our muscles have only a small amount of ATP, barely enough to power a few steps. Although having such a small amount of ATP appears imprudent, it is impossible for cells to produce and store large amounts of these large and heavy organic batteries, which store only one charge each. During a one-hour walk, you use over thirty pounds of ATP, and during an average day, you consume more ATP than your entire body weight. Therefore, the human body stores only about one hundred grams of ATP at any given time. Fortunately, before the muscles’ meager ATP supply is depleted, they quickly access creatine phosphate, a molecule similar to ATP that stores energy. Unfortunately, these creatine phosphate reserves are also limited and are depleted by 60% after ten seconds of sprinting and exhausted after thirty seconds. Nevertheless, this vital burst of fuel gives the muscles time to initiate the second energy-recharging process: breaking down sugar.



The second process of muscle energy regeneration involves the breakdown of sugar, which is a rapid process that does not require oxygen. Glycolysis, which is a part of this process, splits sugar molecules into two halves, generating two ATPs that can provide almost half the energy required for a thirty-second sprint. A fit individual can store sufficient sugar to run nearly fifteen miles. However, the cells can’t handle the accumulation of pyruvates, the leftover halves of each sugar molecule generated during glycolysis, leading to the production of lactate.

Although lactate is harmless and can be used to recharge ATP, the accumulation of hydrogen ions during this process makes the muscles more acidic, leading to fatigue, pain, and decreased function. After around thirty seconds of sprinting, the legs begin to feel as though they are burning due to the increased acidity. It takes a while to neutralize the acid and transport the excess lactate into the third and final process, which is the long-term aerobic energy process that occurs in mitochondria. During this process, energy is liberated from pyruvate or fatty acids.


Aerobic system:

In order to run long distances, oxygen is essential for the body. Burning a molecule of sugar with oxygen produces a staggering eighteen times more ATP compared to glycolysis. However, the process of aerobic metabolism takes longer to provide energy as it involves a long sequence of steps and requires multiple enzymes. This process occurs in mitochondria, which can burn not only pyruvates from sugar but also fats and, in urgent situations, proteins. Nevertheless, the burning of sugars and fats occurs at different rates.

While the body has enough stored fat to run over thirteen hundred miles, breaking down and burning fat takes much more time and effort than sugar. At rest, about 70 percent of the body’s energy comes from slowly burning fat. However, the faster an individual runs, the more sugar they must burn. At maximum aerobic capacity, the body exclusively burns sugar for energy.


VO2 Max: When exercising, the body activates the aerobic system, but the amount of energy generated through this process can vary greatly from person to person. At the upper limit of this process, known as VO₂ max, the muscles require glycolysis to provide additional fuel. If the speed exceeds this limit, the muscles become too acidic and cannot be sustained for long. However, during short bursts of high-intensity exercise, such as a thirty-second sprint, VO₂ max has a minimal effect on speed. As the distance increases, VO₂ max becomes increasingly important, with only 10 percent of energy coming from aerobic respiration during a hundred-meter dash, increasing to 30 percent for four hundred meters, 60 percent for eight hundred meters, and 80 percent for a mile.23 Therefore, the longer the distance, the greater the benefit of a high VO₂ max, which can be improved through training.


Muscle Fibres:

While all muscle cells work similarly, there are several types of fibres in the skeletal muscles that move our bones. Slow-twitch fibres , also known as type I fibres , use energy aerobically and do not fatigue easily, although they do not contract rapidly or powerfully. These fibres  have a darker tinge and are ideal for sustained low-intensity activities such as marathon running. Fast-twitch fibres , on the other hand, come in two types: white and pink. White muscle fibres  (type IIX) burn sugar to generate powerful and rapid forces but fatigue rapidly, while pink muscle fibres  (type IIA) produce moderately powerful forces aerobically and fatigue at an intermediate rate. Fast-twitch fibres  are essential for bursts of extreme power but short duration, such as sprinting a hundred meters.

The percentages of these fibres  types vary from person to person, with most people having slightly more slow-twitch fibres than fast-twitch fibres. Elite athletes who excel at speed and power sports like Usain Bolt tend to be dominated by fast-twitch fibres, while those who specialise in endurance sports like the legendary marathoner Frank Shorter tend to have a preponderance of slow-twitch fibres. A study from 1976 that biopsied the outer calf muscles of forty people found that ordinary nonathletes tend to have equal percentages of fast- and slow-twitch fibres, elite sprinters have about 73 percent fast-twitch fibres, and professional distance runners average 70 percent slow-twitch fibres.


Can you be both fast and have endurance?

The prevailing belief that exceptional athletic abilities result from a combination of inherent talent and practice, and that skills such as dunking that necessitate generating high, swift forces are incompatible with abilities that require endurance. Nevertheless, the consensus does not necessarily represent the truth. While it may be true that the majority of individuals are limited to being either average tortoises or hares, our perception of speed versus endurance may have been distorted by over emphasising the performance of elite, professional athletes.

While it’s true that top marathoners and world-class sprinters have completely different skill sets and cannot compete with each other, their extreme abilities have little relevance to the majority of us who are just regular pedestrians. For instance, the fastest marathon runners can maintain a 4:40 pace for 26.2 miles, completing the race in about two hours. Can you run even a single mile at that speed? It’s unlikely, as only a small fraction of people can maintain that pace for a mile. Unlike the majority of us, these marathoners do not show any trade-off between speed and endurance. They prove that it’s possible to run both fast and far.


The power of HIIT:

We are capable of training our bodies to perform an impressive variety of activities, and there’s no reason why we can’t train for both speed and endurance. In fact, this type of training can be highly effective. For those of us who feel more inclined towards endurance exercises rather than extreme speed, research shows that regular, occasional sessions of high-intensity interval training (HIIT) can not only increase our strength and speed but also our fitness and overall health. HIIT consists of alternating short intervals of intense anaerobic exercise, such as sprinting, with less intense periods of recovery. It’s worth noting that HIIT is not weight training but rather an intense form of cardio. So, to further explore the topic of speed, let’s take a look at how HIIT can help us run faster sprints without sacrificing our endurance.

Let’s start with plyometric exercises, also known as jumping training drills. A typical plyometric routine may involve a sequence of around ten exaggerated skips, in which you jump as high and fast as possible on one leg at a time, raising the opposite knee along with both arms.

As you land, your hip, knee, and ankle joints flex, stretching your leg muscles and making it highly challenging for them to contract explosively. This type of jumping quickly tires out your fast-twitch fibres. Following that, do an equal number of butt kicks. Finally, try sprinting a hundred or two hundred meters repeatedly as fast as you can, requiring your muscles to contract rapidly and forcefully while depleting their ATP and phosphate stores. These HIIT workouts are intense and may cause muscle soreness for a few days.

While high-intensity interval training (HIIT) can’t increase the number of fast-twitch muscle fibres, it can make the ones you have thicker, resulting in increased strength and speed. In fact, sprinters tend to have muscles that are over 20 percent thicker than those of distance runners. HIIT can also transform slower, fatigue-resistant pink fibres into more fatigable white fibres, as well as lengthen fibres, enhancing their shortening speed, and increase the percentage of muscle fibres that contract, boosting force.

However, these adaptations do not occur automatically and require continuous effort to maintain. If you want to improve your speed, you must make a concerted effort to run faster.


CHAPTER SIX  – Strength: From Brawny to Scrawny

MYTH #6 We Evolved to Be Extremely Strong


Primal Strength:

CrossFit enthusiasts believe that their training is rooted in an ancient tradition of full-body athleticism that required significant strength for human survival. According to a devoted CrossFit friend of mine, “Being strong is primal.”

However, as we have previously discussed, our ancestors rarely engaged in exercise specifically for health and fitness purposes, so it is worth asking whether the strenuous workouts of CrossFit even remotely resemble the physical activities of the past. Were our hunter-gatherer ancestors truly as strong as we imagine, or would the intense and exhausting WODs of CrossFit be just as foreign to them as modern practices like paying taxes or reading books?

Measurements of strength in hunter-gatherer men and women are limited but suggest that, in general, they are lean and moderately strong but not bulky. Tropical hunter-gatherers, in particular, tend to be more slender than muscular. For example, the average height and weight of Hadza men are five feet four inches and 117 pounds, respectively, while Hadza women average four feet eleven inches and 103 pounds. Their body fat percentages are about 10 percent in men and 20 percent in women, which is just on the borderline of being classified as underweight.

Studies on the Hadza tribe’s grip strength, upper-body strength, and muscle size indicate that they fall within Western norms for their age and below those of highly trained athletes. The Hadza are not known for their muscular build, but they are in good physical shape and possess the overall strength required for the various physical activities they engage in daily, from digging and running to climbing trees. It’s worth noting that while the Hadza are a single population, they share similar physical characteristics to other hunter-gatherer groups like the San of the Kalahari, the Mbuti of central Africa, the Batek of Malaysia, and the Aché of Paraguay.

While hunter-gatherers do engage in occasional heavy lifting, most of the resistance their muscles face comes from carrying, digging, and lifting their own body weight. They rely on equipment-free, body-weight-based exercises such as push-ups, pull-ups, squats, and lunges to develop strength. However, there is a drawback to these exercises: as you gain strength, the weight you lift remains constant. Hunter-gatherers do not have access to gyms or other tools to help them achieve a super-strong physique like that of Charles Atlas or Milo.


CHAPTER SEVEN Fighting and Sports: From Fangs to Football

MYTH #7 Sports = Exercise


Are humans stronger than chimps:

U.S. Air Force scientists created a peculiar contraption that resembled a mixture of a metal cage and an electric chair to measure the amount of force generated by humans and chimpanzees when flexing their elbows. Out of the adult chimpanzees trained to use this equipment, only one was able to do so, and was found to be approximately 30 percent stronger than the strongest human that was also tested.

More recently, Belgian researchers demonstrated that bonobos weighing seventy five pounds can jump twice as high as humans who weigh twice as much, suggesting that both species can jump to the same height per pound.

Lastly, a laboratory examination of muscle fibres demonstrated that a chimpanzee’s muscles can generate a maximum of 30 percent more force and power than the muscles of an average human. While these studies vary in their approaches, they collectively indicate that adult chimpanzees are not more than a third stronger than humans. Therefore, contrary to the popular belief, a chimp would not be able to dislocate your arm in an arm-wrestling match. However, it is still likely that you would lose.


Ageing and Muscles:

As we grow older, our muscle fibres tend to decrease in size and quantity, and our nerves deteriorate. This results in a decline in strength and power. In industrialised countries like the United States and the U.K., grip strength typically decreases by about 25 percent from the age of twenty-five to seventy-five.

In Framingham, Massachusetts, which is a few miles from the author’s house, the percentage of women who are unable to lift ten pounds has been observed to increase from 40 percent among fifty five- to sixty-four-year-olds to 65 percent among seventy-five- to eighty-four-year-olds. This trend is concerning because as people lose strength, they become less capable of performing basic tasks such as getting up from a chair, climbing stairs, and walking normally. The resulting feebleness also causes people to be less active, leading to a vicious cycle of deterioration.

However, elderly hunter-gatherers and others who remain physically active throughout their lives demonstrate the encouraging news that using our muscles can slow down muscle loss as we age. In fact, ageing does not stop our muscles from responding to resistance exercise. Even modest levels of resistance exercise can slow down and sometimes reverse sarcopenia, regardless of age, thanks to the mechanisms that have been previously discussed.


Violent and Peaceful:

We have been brought up to recognise humanity’s inclinations and abilities for violence, but to trust that humans evolved to be predominantly moral, peaceful, and cooperative. We should be content with being a mostly nonviolent human rather than an ape since, if we were chimpanzees, we would spend a considerable portion of our day attempting to evade being attacked or killed. Only humans are willing to risk their lives by rushing into a burning building to save the life of an unrelated individual or a pet. Even aggressive sports like cage fighting are governed by regulations and referees to limit the amount of harm that participants can inflict on one another. In this regard, we are collectively attracted to the beliefs of Jean-Jacques Rousseau and his supporters, who claim that behaving morally is our natural inclination and that many instances of human violence are the result of corrupt cultural attitudes and circumstances.

However, violence is ingrained in every culture, including hunter-gatherer societies, which challenges the presumption that we are innately gentle and non-aggressive. Therefore, I also acknowledge the contributions of Thomas Hobbes and his adherents, who regard human tendencies toward aggression as archaic, inherent, and sometimes beneficial.

The question remains: how can we reconcile our remarkable abilities for cooperation and conflict prevention (Rousseau) with our potential for aggression (Hobbes)?

Richard Wrangham offers a compelling solution to this long-standing debate by highlighting the critical difference between two fundamentally distinct types of aggression: proactive and reactive. According to Wrangham, humans have substantially lower levels of reactive aggression than other animals, particularly our primate relatives, but much higher levels of proactive aggression. In terms of reactive aggression, we align with Rousseau, while in terms of proactive aggression, we align with Hobbes.

Summary: In summary, humans are not as physically strong as our ancestors because we did not evolve to fight in the same way. Our fighting methods became more proactive, with the use of weapons and sports. Similarly, sports were not originally developed for exercise purposes but rather as organised play to teach survival skills and cooperation. It was only when physical labor decreased that sports became a means of exercise. In modern times, we promote sports as a way to stay healthy, although the effectiveness of some activities like darts may be debatable.


Part 3: Endurance


Chapter 8 – Walking: All in a Day’s Walk

MYTH #8 You Can’t Lose Weight by Walking


Built to walk:

If there is a single physical activity that perfectly encapsulates the key message of this book – that our evolutionary history didn’t involve exercising for the sake of exercise, but instead engaging in physical activity only when necessary – it is walking. The average hunter-gatherer man and woman, including the Hadza people, walk approximately nine and six miles per day, respectively, not for the purpose of improving their health or physical fitness, but to stay alive. Each year, the average hunter-gatherer walks the distance equivalent to that between New York and Los Angeles. Humans are natural endurance walkers.

Although many weight-loss programs recommend daily walking, some experts argue that walking alone is insufficient for weight loss since it burns few calories and may even stimulate hunger. In fact, a popular Time magazine article in 2009 called “The Myth About Exercise” claimed that exercise, including walking, does not lead to weight loss. However, before delving further into these conflicting opinions, it is essential to understand how humans walk on two legs, which is quite peculiar.


Why do we walk on two legs?

To understand the benefits of walking upright versus knuckle walking like an ape, let’s take a closer look at the energy expenditure. A 7.4-mile walk would burn around 325 calories for a human who walks upright. However, if the same distance were covered using the inefficient knuckle walking technique of a chimpanzee, it would burn roughly 700 calories. By walking upright, hunter-gatherers save over 2,400 calories per week compared to knuckle walking. This adds up to 125,000 calories per year, which is equivalent to the energy needed to run about forty-five marathons.

The reasons that compelled our ancient ancestors to walk upright ages ago may appear obsolete now, but they still hold significance. For countless generations until the post-industrial age, our forebears had to walk roughly five to nine miles daily just to stay alive. Endurance walking became an intrinsic part of our evolution. However, like our predecessors, many of us have an innate tendency to expend minimal energy by walking only when indispensable. This impulse to conserve calories highlights another crucial contrast between walking today and in earlier times: the amount of weight we carry, such as infants, nourishment, firewood, and water.


The constrained energy expenditure hypothesis:

Although metabolism is a complex process, there is one more factor that complicates efforts to lose weight by walking – a phenomenon known as metabolic compensation, which is still not fully understood.

Studies of the Hadza population shed some light on this phenomenon. Herman Pontzer and his team found that despite the Hadza’s high levels of physical activity, they burn the same amount of calories per day as sedentary individuals with the same lean body weight. Similarly, when Pontzer and his colleagues collected energetic data from adults in various countries, they discovered that people who were more physically active did not have total energy budgets as high as their exertions would suggest.

This phenomenon is explained by the constrained energy expenditure hypothesis, which proposes that people’s total energy budgets are limited. In other words, if someone uses extra calories for physical activity, they will compensate by reducing their resting metabolism to balance their overall energy budget. While this idea is still being tested, it could mean that exercisers may burn almost the same number of total calories per day as sedentary individuals of similar size, despite being more active.


CHAPTER NINE – Running and Dancing: Jumping from One Leg to the Other

Myth #9 Running is Bad for your Knees


What pigs head tell us about running:

During a conversation about pigs, Lieberman was pointed out the importance of stabilising one’s gaze while running. Something that pigs can’t do. This led to the discovery that animals adapted for running have a nuchal ligament at the back of their heads that acts like a spring to keep their heads still. Humans and fossil species from the genus Homo have this ligament, indicating that humans were selected to run millions of years ago. The basic argument is that by two million years ago, Homo erectus had evolved the necessary anatomy to run long distances in the heat in order to hunt and scavenge. This is supported by evidence that our ancestors sometimes hunted by outrunning fleet-footed animals. Despite this, most humans we see today are walking rather than running, and even the fastest among us are slower and more awkward than most animals.


How do Humans outrun horses?

Let’s examine how and why humans can outrun horses, given our basic understanding of running. We can start by comparing the speeds at which humans can run marathon-length distances with the trotting speeds of horses, greyhounds, and ponies. This comparison is important because quadrupeds such as horses can only run long distances at a trot. Although horses, dogs, zebras, and antelopes can sprint faster than any human, they cannot maintain their pace for more than a few miles, particularly in hot weather.

Humans have an unusual habit of running long distances regularly, in addition to being able to run long distances relatively fast. It’s rare to see a wild animal running several miles on its own without any apparent reason. Wolves, dogs, and hyenas are social carnivores that run up to ten miles to hunt, but few other animals willingly run more than a hundred yards or so without being forced to. On the savanna, antelopes and other prey sprint to escape lions and cheetahs that are chasing them, but these frantic sprints never last longer than a few minutes. Although animals like dogs and horses can run many miles, they only do so when we coerce them with whips and spurs.


Why are we good at endurance running?

What enables ordinary humans who descended from flat-footed apes to be so good at endurance running?

High performance legs:

Human legs are noticeably long and springy compared to other animals of similar weight and size, such as chimpanzees, sheep, or greyhounds. This is due to the lengthy elastic tendons, including the Achilles, which serve as springs during running. While not essential for walking, these tendons stretch as the hips, knees, and ankles bend, and the arch flattens when the feet land on the ground during a run. When the tendons recoil, the energy they store is released, propelling the runner back into the air. Long, springy tendons are common among animals adapted for running, such as kangaroos and deer, but they are shorter in our closest relatives, the African apes. Humans evolved these longer tendons independently to improve our running ability. The Achilles tendon and the spring in the arch of the human foot alone can return up to half the mechanical energy generated when the body hits the ground during running.


Humans have a unique ability to run for extended periods of time, thanks in large part to their remarkable sweating capacity. As we run, our bodies generate a considerable amount of heat, which can be dangerous if it is not dissipated. Body temperatures above 41°C can damage brain cells and other vital tissues, leading to heatstroke. To cool off, we use the process of evaporation, which occurs when heat causes water to turn into steam, thus losing energy and cooling the skin underneath.

Humans possess a unique cooling system that utilises special sweat glands, which are usually found only on the paws of other animals. Although primates have small numbers of these glands elsewhere on their bodies, humans alone have between five and ten million sweat glands located throughout their skin, especially on their limbs, heads, and chests. This profuse sweating essentially turns the entire body into a giant, wet tongue. Humans also lost their fur, which facilitates the movement of air over the skin, thus allowing us to dissipate heat rapidly.

The panting disadvantage: Most animals use panting as a way to cool themselves down by evaporating saliva in their throats and tongues. This process cools the skin, which in turn cools the blood in the veins beneath it, and ultimately cools the entire body. However, panting has limitations. The surface area of tongues, mouths, and noses is relatively small for cooling. Additionally, quadrupeds such as dogs are unable to pant when galloping due to the seesaw motion that causes the guts to slam into the diaphragm with each stride. When these animals transition from trotting to galloping, they must synchronise each breath with each stride, thereby losing the ability to pant. It is important not to make a dog gallop for too long on a hot day as it may overheat.

Elastic hearts:

Humans possess additional adaptations that aid in their endurance. During rest, the heart pumps around four to six liters of blood per minute, but during running, it may need to pump up to five times more to fuel working muscles and maintain body temperature. An average runner’s heart can pump up to twenty-four liters per minute, while elite runners can achieve a remarkable thirty-five liters per minute. Researchers, including Rob Shave, Aaron Baggish, and myself, have shown that humans, like other endurance-adapted animals such as horses, have voluminous and elastic heart chambers that differ significantly from the smaller, thicker, and stiffer hearts of apes. These adaptations enable humans to efficiently pump large volumes of blood with each heartbeat. Additionally, humans have a highly developed blood supply to the brain, which aids in cooling this vital organ during exercise.

Slow twitch leg muscles:

In ordinary humans, leg muscles typically consist of 50 to 70 percent fatigue-resistant slow-twitch fibres, which is considerably more than the range of 11 to 32 percent in chimpanzees. Although humans who train for speed can increase their fast-twitch fibre size, ordinary humans from all populations are still slow-twitch dominated, which allows for greater endurance than apes.

In summary, despite one’s personal dislike for running, the human body possesses various features that aid in running long distances with efficiency and effectiveness, from head to toe. Since many of these features are not beneficial for other activities such as walking, it suggests that they evolved specifically as adaptations for running. This is why ordinary runners can even compete with horses in marathons. However, the question remains: why?


Power Scavenging and Persistence Hunting:

After years of contemplation, Lieberman has come to the conclusion that the reason why our ancestors evolved numerous adaptations for running long distances is to acquire meat. Although in today’s farming and supermarket culture, you may choose to be a vegetarian or acquire meat through other means, this is a very modern attitude. For hunter-gatherers, obtaining free meat is highly valued, regardless of its freshness, and hunting is a way to obtain nutrient-rich food and achieve status. Historically, scavenging and hunting were extremely challenging, perilous, and almost unfeasible without running.

There are multiple methods for persistence-hunting, but one involves taking advantage of the unique ability of humans to run without overheating. During the hottest part of the day, a group of hunters will pursue a large animal because they overheat and tire more quickly than smaller animals. Initially, the prey will run faster than the hunters, who typically maintain a steady jog. As the animal pants to cool down, the hunters track it, often by walking, so they can resume the chase before the animal fully cools. This process of chasing and tracking is repeated until the animal eventually collapses due to heatstroke, assuming the hunters can catch up with it before it fully cools down.


If humans evolved to run, why do so many runners get injured?

One hypothesis suggests that our bodies are well-suited to running and that the risks of running have been overstated. If running was truly as dangerous as some people claim, we would see a significant number of runners suffering from injuries. However, this is not the case. Analyses of numerous studies suggest that the risk of injury from running follows a U-shaped curve. The highest risk of injury occurs among beginners who rapidly increase their mileage, competitive runners who prioritise speed, and marathoners. Everyday runners who fall between these extremes are less likely to experience problems.

Concerns regarding the potential damage caused by running are common among both current and aspiring runners. In addition to the inherent risks of falls and other traumatic incidents, the act of repeatedly running on hard surfaces is widely believed to cause excess wear and tear, much like a car with too many miles on it. This type of damage is frequently referred to as an overuse injury, and research indicates that anywhere from 20 to 90 percent of runners suffer from such injuries in any given year, leading to concerns that a significant number of runners may be pushing themselves too hard.

Preventing running injuries should be a top priority, but it’s also important to dispel common misconceptions about the potential harm caused by running. The most pervasive myth is that running too many miles leads to the erosion of cartilage in the knees and hips and causes osteoarthritis. However, over a dozen careful studies have shown that nonprofessional runners are no more likely to develop osteoarthritis than non-runners.

In reality, physical activity such as running can actually promote healthy cartilage and protect against osteoarthritis. In fact, research conducted by my lab suggests that the risk of knee osteoarthritis has doubled over the past two generations due to decreased physical activity, not increased activity. Nonetheless, injuries still do happen, with knee injuries being the most common.


CHAPTER ΤΕΝ Endurance and Aging: The Active Grandparent and Costly Repair Hypotheses

MYTH #10 It’s Normal to Be Less Active as We Age


Fitness and Chronic Diseases:

Over the course of several decades, researchers at the Cooper Center monitored the health of more than 18,000 middle-aged individuals who were initially healthy to determine who developed chronic conditions like Alzheimer’s and diabetes. They found that those who were more physically fit were approximately half as likely to experience chronic disease and, if they did become ill, they did so at a later age. These findings, along with others, support the adage that “Men do not stop playing because they grow old; they grow old because they stop playing.”


The active grandparent hypothesis:

To shed light on the connection between exercise and aging, Lieberman suggests a new idea, the active grandparent hypothesis, as an extension of the grandmother hypothesis. This concept proposes that humans evolved to live longer not only by having genes that promote survival past the age of fifty but also by requiring older individuals to engage in moderate physical activity to support the survival and well-being of younger relatives, including children and grandchildren. Consequently, genes that repair and maintain our bodies during physical activity were also selected for, contributing to mechanisms that slow aging and prolong life. Physical activity, particularly as we age, triggers many of these mechanisms that promote longevity and good health, which are both sustained by and beneficial for physical activity.

The active grandparent hypothesis posits that human longevity did not evolve to facilitate leisurely retirement in warm climates, where elderly people can idle by the pool or ride golf carts. Rather, in the Stone Age, old age meant physical activity, such as walking, digging, and carrying. Natural selection favored older individuals whose bodies stimulated repair and maintenance mechanisms in response to the stresses caused by these activities. Since middle-aged and elderly humans did not have the luxury of retiring and relaxing, there was no strong selection to turn on these mechanisms to the same degree without the stresses caused by physical activity.

Let’s compare the walking habits of Americans and Hadza people. According to a study of thousands, the average American woman aged 18-40 walks about 5,756 steps per day (approximately two to three miles), but this number decreases significantly with age. By the time American women reach their seventies, they take approximately half as many steps. In contrast, Hadza women walk twice as much per day as Americans, with only slight reductions in activity as they age. Additionally, heart rate monitors revealed that elderly Hadza women engaged in more moderate to vigorous activity than younger women who were still bearing children. If elderly American women had to walk five miles a day to shop for their families and dig through hard soil for food items, like cereal, frozen peas, and Fruit Roll-Ups, they would likely be in better shape. Hard work helps to maintain the fitness of elderly hunter-gatherers, and walking speed is a reliable indicator of age-related fitness that correlates positively with life expectancy.

Furthermore, how did our hunter-gatherer ancestors cope with the unavoidable consequences of natural selection when they were no longer capable of hunting and gathering? Some modern societies offer nursing homes, pensions, and publicly funded healthcare to support the elderly. While elderly hunter-gatherers are respected, those who are unable to perform physically demanding tasks such as walking long distances, digging for tubers, collecting honey, and carrying heavy loads are likely to become a burden when food is scarce. Thus, if human beings evolved to live long after they stopped having children, they were likely not selected to spend those years in a state of chronic disability. From a Darwinian viewpoint, the optimal approach is to live long and actively and then die rapidly once physical activity declines. A superior approach, however, would be to prevent age-related deterioration altogether.


Why do we age?

At a cellular level, there are various detrimental processes that lead to senescence (The process of growing old) by damaging cells, tissues, and organs. One such process arises from the chemical reactions that occur within our bodies to sustain life.


When we breathe in oxygen, our cells use it to produce energy, but in the process, they also create unstable oxygen molecules with unpaired electrons, known as reactive oxygen species or free radicals. These molecules steal electrons from other molecules in an uncontrolled manner, leading to oxidation.

This chain reaction gradually damages the body by creating other unstable molecules that steal electrons from yet more molecules. Oxidation affects the body in various ways, damaging DNA, scarring artery walls, inactivating enzymes, and altering proteins. Surprisingly, the more oxygen we consume, the more reactive oxygen species we generate, which means that physically demanding activities that require more oxygen could potentially accelerate the aging process.

Mitochondrial dysfunction:

Another contributor to senescence is malfunctioning mitochondria. Mitochondria are small organelles in cells responsible for generating energy (ATP) by burning fuel with oxygen. Cells in energy-demanding organs like the brain, muscles, and liver contain thousands of mitochondria. These organelles also have their own DNA and are involved in regulating cell function, producing proteins that protect against diseases like diabetes and cancer. However, because mitochondria burn oxygen, they produce reactive oxygen species that can cause damage to cells if not controlled. When mitochondria stop functioning correctly or decrease in number, they can lead to senescence and disease.


Another detrimental process that results from metabolic activities is glycation, also known as browning. Glycation occurs when sugar and protein combine in the presence of heat. This reaction gives foods like baked bread and roasted meat their savory exteriors, but it can be harmful to organs like the kidneys. Glycation reactions can cause tissue damage and generate advanced glycation end products (AGEs), which harden blood vessels, wrinkle skin, clog kidneys, and trigger inflammation, among other harmful effects.

In addition to oxidation, mitochondrial dysfunction, mutations, glycation, and inflammation, there are other processes that contribute to senescence by damaging and breaking down cells. One of these processes is the accumulation of tiny molecules that attach themselves to DNA in cells, known as epigenetic modifications. These modifications can impact which genes are expressed in specific cells and are partly influenced by environmental factors such as diet, stress, and exercise. As we age, we accumulate more epigenetic modifications, which can increase the risk of mortality. Other forms of senescence include the inability of cells to properly recycle damaged proteins, insufficient nutrient sensing and acquisition, and, in rare cases, an inability to divide due to shortened telomeres – the protective caps on the ends of chromosomes.

If you find this list of ageing mechanisms concerning, you are right to do so. Collectively, these mechanisms gradually cause chaos in our bodies. Plaque accumulates in our blood vessels, causing them to harden and become blocked. Cell receptors become congested, and muscles lose their strength. Debris accumulates around critical cells and neurons, which eventually die. Membranes break, bones become fragile, and tendons and ligaments become weakened. Our immune systems become less effective at warding off infections.


Why does exercise slow down ageing?

Numerous studies have validated the anti-aging advantages of exercise, yet few have expounded on the underlying reasons. The most prevalent justification for the impact of exercise in hindering and occasionally reversing the gradual deterioration towards ill health is its potential to prevent or mitigate factors that hasten senescence.

At the forefront of this list is fat. Exercise wards off and, on occasion, reverses the build-up of excessive fat, particularly visceral fat, which is a key trigger of inflammation and other ailments. Exercise is also effective in decreasing the levels of sugar, fat, and unhealthy cholesterol in the bloodstream that gradually contribute to arterial stiffening, protein damage, and other disruptions in the body.


The costly repair hypothesis:

If exercise causes damage to the body, then why is it considered healthy? One possible explanation is that when you stop exercising, your body responds by repairing not only the damage caused by the exercise, but also some of the damage that was present before you began exercising. As a result, many of your tissues are restored to their previous state.

Your body has various repair and maintenance responses to your workout. These include activating your “rest and digest” system, which slows your heart rate, reduces your cortisol levels, and directs unused energy back into your muscle and fat cells to replenish your energy reserves. To repair tissue damage caused by your workout, your body will initiate an initial inflammatory response followed by a later anti-inflammatory response. Additionally, your body will produce potent antioxidants to neutralise the reactive oxygen species generated by your mitochondria. Your body will also activate other processes to eliminate waste products and repair DNA mutations, damaged proteins, epigenetic modifications, cracks in your bones, and to add and replace mitochondria. These maintenance and repair processes are less costly than your workout, but they still require calories, which slightly elevates your resting metabolic rate for some time.

Recall from earlier that organisms with limited energy supplies (which was the case for almost everyone until recently) must allocate their calories towards reproducing, moving, or maintaining their bodies. However, natural selection is primarily concerned with reproduction. As a result, our bodies evolved to spend as little energy as possible on expensive maintenance and repair tasks. Therefore, while physical activity triggers cycles of damage and restoration, individuals who allocate just enough energy to produce antioxidants, strengthen the immune system, repair and enlarge muscles, mend bones, and so on are favoured by natural selection. The challenge is to maintain and repair any damage caused by physical activity just enough, in the right place, and at the right time.

Evolution’s frugal approach to this challenge involves matching the body’s capacity to respond to the demands imposed by physical activity. These demands include stress caused by reactive oxygen species and other harmful processes that can lead to damage such as stiffening arteries, gene mutations, and cellular congestion. The capacity is the ability to maintain a stable internal environment by repairing the damage caused by these stressors, allowing us to function effectively for survival and reproduction. Importantly, the maintenance and repair mechanisms activated by exercise don’t cease to function as we age, although they may become less responsive. Thus, physically active individuals, even in post-reproductive years, can slow or delay the effects of ageing.


Mortality and morbidity:

We must acknowledge that death is inevitable, even for physically active individuals who maintain a sensible diet and adhere to other healthy practices. If you’re doubtful about this claim, which is reasonable, let’s examine the probabilistic connection between death, or mortality, and illness, or morbidity. Often, ageing statistics concentrate on life expectancy, without accounting for health expectancy, which is the period spent in good health without illness.

The advancements in public health and medical science have brought both positive and negative changes to health span and life span, as shown in the lower graph of Figure [not provided]. On the one hand, there have been significant successes in preventing and treating infectious diseases. However, on the other hand, many individuals today experience chronic noninfectious diseases that result in years of illness prior to death. This extended period of morbidity is referred to as the “extension of morbidity” in medical terminology. Westernised populations, in particular, are affected by various illnesses such as heart disease, type 2 diabetes, Alzheimer’s, and chronic respiratory disease, along with conditions like osteoarthritis, osteoporosis, and an increasing number of autoimmune diseases.

Roughly 20% of Americans aged sixty-five or older report fair or poor health. However, despite this high level of morbidity, we still have a longer lifespan than our agricultural predecessors and slightly longer than hunter-gatherers. The average lifespan of an American in 2018 was seventy-eight years, which is almost twice as long as it was one hundred years ago.

James Fries, a medical professor at Stanford, conducted a study that demonstrated the potential for preventive medicine to extend healthy lifespan by compressing morbidity. Fries’ study was based on the analysis of data from over 2,300 University of Pennsylvania alumni, measuring life span, disability, and three risk factors for disease: high body weight, smoking, and lack of exercise. The study found that individuals with two or more risk factors died 3.6 to 3.9 years earlier than those with one or no risk factors. Furthermore, they experienced an extended period of disability before death, ranging from 5.8 to 8.3 years. This implies that an unhealthy lifestyle has twice the impact on morbidity than on mortality.

To summarise, the primary causes of major illnesses leading to death in industrialised western societies are persistent physical inactivity, smoking, and excess body fat. While the death certificates of two-thirds of Americans list heart disease, cancer, or stroke as the cause of death, these conditions are likely rooted in smoking, obesity, and physical inactivity.


What does this all mean?

Similar findings on the effects of physical activity on morbidity and mortality are produced by many other studies, as Hippocrates would have foreseen. However, this does not guarantee that physical activity is a reliable source of eternal youth, and it does not delay mortality by preventing the aging process itself. Rather, physical activity stimulates a range of mechanisms that raise the likelihood of maintaining good health with age by delaying aging and preventing numerous chronic diseases that contribute to mortality over time. This reasoning highlights three crucial insights.

To begin with, it’s important to understand that the statistics regarding mortality and morbidity discussed earlier are based on probabilities. Engaging in healthy habits like sensible eating and regular exercise doesn’t guarantee a long and healthy life, but it does reduce the risk of falling ill. On the flip side, smokers are more likely to develop lung cancer, and those who are overweight or physically unfit have a higher chance of developing heart disease or diabetes, but not everyone in these categories will necessarily experience these outcomes.

Second, the relationship between morbidity and mortality is being reshaped by advancements in medical care. Diseases like diabetes, heart disease, and certain types of cancer are no longer considered immediate death sentences, as they can now be treated or controlled with drugs that regulate blood sugar levels, lower harmful cholesterol levels, reduce blood pressure, and combat abnormal cells.

Lastly, the likelihood of contracting a disease is influenced by a multitude of intricate environmental and genetic factors, making it challenging to establish causation. According to twin studies, only around 20 percent of the differences in lifespan up to the age of eighty can be attributed to genetics. Nevertheless, if someone reaches that age, genetics may have a more significant impact on whether they become a centenarian. However, this doesn’t imply that genes aren’t a significant factor in the development of diseases.

In summary, physical activity is not only beneficial for the young. Our bodies have evolved to remain active as we age, and in turn, physical activity helps us age gracefully. Additionally, the longer we remain physically active, the greater the benefits, and it is never too late to start. People who choose to adopt a healthy lifestyle and become physically active after the age of sixty can significantly decrease their mortality rate compared to those who remain inactive.


PART IV Exercise in the Modern World


CHAPTER ELEVEN To Move or Not to Move: How to Make Exercise Happen

MYTH #11 “Just Do It” Works


Lifelong physical activity:

Our ancestors, both young and old, relied on physical activity each day to survive, spending hours walking, digging, and performing other essential tasks. Occasionally, they participated in activities like dancing and playing for enjoyment and socialisation. However, they generally avoided nonessential physical activities that detracted from their ability to reproduce. This presents a paradox, as our bodies require lifelong physical activity to function optimally, but our minds lack the motivation to engage in exercise unless it is necessary, pleasurable, or rewarding.

In the modern world, we struggle to replace physical activity with exercise, which is often optional and unenjoyable. Despite encouragement from doctors, trainers, and other fitness professionals, many of us still avoid exercise.

The impulse to postpone or avoid exercise is universal, and environments that do not necessitate or support physical activity are likely to promote a sedentary lifestyle. When faced with the choice between lounging in a comfortable chair or engaging in a sweaty workout, the chair is typically more appealing. Even though my logical mind acknowledges the importance of exercise, my instincts resist and suggest, “I’d rather not.” An alluring voice whispers, “Why not do it tomorrow?” Perhaps I am short on time or energy, or I must go out of my way to be physically active due to factors such as time constraints, a lack of sidewalks in my neighbourhood, or an inconvenient or unappealing staircase in my building.


How to promote exercise:

In Liebermans view, to effectively promote exercise, we must confront the issue that engaging in physical activity voluntarily for the sake of health and fitness is a modern and optional behaviour, which our brains may resist. Therefore, it is essential to reconsider the evolutionary and anthropological perspective of two factors: necessity and pleasure.

First, let’s talk about necessity. Everyone knows that exercise is good for their health, including the billion or so humans who regularly do not get enough exercise. Despite this, many non-exercisers feel frustrated or bad about themselves, and it rarely helps when exercisers nag or brag about their routines. The problem lies in the difference between “should” and “need.” While I know I should exercise to increase the probability of being healthier, happier, and living longer with fewer disabilities, there are many legitimate reasons why I do not need to exercise. In fact, it is evident that one can lead a reasonably healthy life without exercising.

Exercise, being inherently unnecessary, is further exacerbated by the modern, mechanised world that has eliminated many forms of non-exercise physical activity that were once necessary for our ancestors. In today’s world, it is possible to spend an entire day without elevating the heart rate or breaking a sweat. Commuting to work, taking the elevator to the office floor, and sitting for hours on end has become the norm. Even routine chores such as getting water, making dinner, and washing clothes have become effortless with the advent of modern technology.

Exercise not only seems unnecessary, but it also takes up valuable time that could be spent on more pressing matters. Personally, I am fortunate enough to have a brief commute and a job with flexible hours, allowing me to squeeze in a jog or walk my dog as needed. However, many of my acquaintances face a longer commute and must work rigid hours at sedentary office jobs. They may also have additional responsibilities, such as caring for children or elderly relatives, that consume significant time. Oddly enough, wealthier individuals today engage in more physical activity than their less affluent counterparts, marking a first in history.

With all this being said, an important lesson about why we exercise lies in the fact that it is primarily done for emotional or physical rewards, given that exercise is not a necessity.


Chapter Twelve How Much and What Type?

MYTH #12 There Is an Optimal Dose and Type of Exercise


The ‘perfect dose’:

It is difficult to determine a single “best” or “optimal” amount and type of exercise, despite claims to the contrary. The meaning of “best” is subjective and can vary depending on the desired outcome. For instance, one may define the best exercise in terms of increasing lifespan, time efficiency, preventing heart disease, weight loss, injury prevention, or avoiding Alzheimer’s. However, even if there were a way to identify the best plan for one of these goals, it might not be suitable for everyone, considering factors such as age, gender, weight, fitness level, and injury history.

While there may not be an ideal exercise plan, physical activity still triggers growth, maintenance, and repair processes that enhance our abilities and decelerate aging. Therefore, we have medicalised exercise and have set certain doses and types of exercise. However, determining the appropriate amount and type of exercise is still a complex issue. Whether we exercise for enjoyment or fitness, certain amounts and forms of exercise are undoubtedly more beneficial or detrimental to our health based on our objectives and conditions.


CHAPTER THIRTEEN Exercise and Disease

Even the most skeptical among us acknowledges that exercise promotes good health. However, it’s important to keep in mind that exercise is an unusual kind of medicine. It’s medicinal because a lack of physical activity is detrimental to our health. Moreover, exercise never evolved to be therapeutic; rather, we evolved to spend energy on physical activity primarily out of necessity and for social reasons. We conserve calories for reproductive success and many of the genes that maintain our bodies depend on the stresses caused by being active. When we are young, physical activity helps us develop capacities such as strong bones and improved memory, while as we age, it triggers maintenance and repair mechanisms that help us remain vigorous. For generations, our ancestors led challenging lives, spending many hours a day walking, carrying, digging, and occasionally running, climbing, throwing, dancing, and fighting. Although their lives were tough and many did not survive childhood, physical activity helped many of those who did live to become active and productive grandparents.

But how much and in what way will exercise help ward off disease?

In the previous twelve chapters, we explored the impact of exercise on various aspects of health, such as aging, metabolism, weight, and muscle function. However, we have yet to discuss how exercise affects the diseases that are most likely to cause significant disability or mortality. Using an evolutionary anthropological perspective, we can examine why and how different types and amounts of physical activity impact our vulnerability to major health conditions, both physical and mental. To conclude practically, let’s address some important questions related to each major health condition: Is the condition more prevalent now than in the past due to reduced physical activity? How does physical activity help to prevent or treat the condition? What type and dosage of exercise are most effective in combating the condition?



In 2013, the American Medical Association generated controversy by designating obesity as a disease. This classification aimed to raise awareness of the various health hazards associated with obesity, alter the way medical institutions pay for its treatment, and eliminate the stigma surrounding the condition. Nonetheless, the classification remains a subject of debate. Although obesity is a risk factor for several diseases, not all obese individuals experience health problems. Additionally, labeling one-third of Americans as diseased is staggering, and classifying obesity as a disease may imply that it is a permanent, unmodifiable state.

What is the Hypothesised Mismatch?

If we consider mismatches as harmful interactions between genes and environments, where environments have changed more than genes, then there are few examples bigger than obesity. While some people may carry genes that increase their susceptibility to becoming obese, the influence of the environment cannot be denied. Obesity is almost non-existent among foraging populations and was far less prevalent in previous generations, yet today nearly two billion people are classified as overweight or obese.

Although diet is considered the main culprit for obesity, particularly the consumption of processed foods that are high in sugar and low in fibre, the role of exercise in preventing or treating obesity remains a topic of debate. Some experts and individuals argue that exercise has little impact on weight loss. The most common objections to using exercise as a means of controlling weight are that calories from diet vastly exceed those burned through physical activity, and that exercise increases hunger and fatigue, leading to increased food intake and inactivity after exercise.

How Does Physical Activity Help?

There is considerable debate over how much exercise can protect against the negative effects of being overweight or obese, and whether it is acceptable to be “fat but fit.” Many studies suggest that overweight individuals who exercise are healthier and live longer than those who do not, but not all overweight or obese individuals suffer from health problems or premature death. In fact, some studies indicate that individuals who become slightly overweight in old age may live slightly longer due to having more energy reserves to fight serious illnesses such as pneumonia.

How Much and What Kind of Exercise Are Best?

Later on, we will discover that weightlifting helps alleviate some of the metabolic impacts of obesity, but when it comes to preventing and reversing excess weight, cardio is superior. So, it’s simple: cardio is better than weights for obesity.


Metabolic Syndrome and Type 2 Diabetes:

According to convention, an individual is considered to have metabolic syndrome when they exhibit most of the following characteristics: elevated blood sugar levels, high cholesterol levels, high blood pressure, and a large waist. These characteristics frequently occur together with a fatty liver and other forms of obesity, indicating metabolic disturbances. Metabolic syndrome is a common precursor to type 2 diabetes.

What is the Hypothesised Mismatch?

Metabolic syndrome and type 2 diabetes are indisputable mismatch conditions. Hunter-gatherers have practically no record of these diseases, while they are rare among subsistence farmers and have only recently become widespread. About 20 to 25 percent of adults globally have metabolic syndrome, and this percentage is expected to double in the coming decades. Metabolic syndrome is a major risk factor for various alarming conditions, including cardiovascular disease, strokes, and dementia. Nonetheless, type 2 diabetes (also known as adult-onset diabetes) is the most well-known disease associated with metabolic syndrome.

How Does Physical Activity Help?

Type 2 diabetes increases the risk of mortality to varying degrees, and it can be managed through medication, dietary changes, and physical activity. While medication can be helpful, it may not always be required. The body can sometimes repair itself through diet and exercise. An impressive example of this was observed when ten overweight Australian aborigines with type 2 diabetes were able to reverse their condition in just seven weeks by adopting an active hunting and gathering lifestyle.

How Much and What Kind of Exercise Are Best?

While exercise alone may not be sufficient to treat metabolic syndrome and type 2 diabetes as they have multiple causes, it can be a valuable complement to medication and dieting. Moderate to high levels of physical activity have been shown to be effective in managing these conditions.


Cardiovascular Disease

Based on statistics, it is highly probable that both you and I will succumb to some kind of cardiovascular illness. However, there is good news: the pioneering research of Dr. Jeremy Morris has provided us with compelling proof that modifying our lifestyle can greatly reduce this risk.

What Is the Hypothesised Mismatch?

The heart functions as a muscular pump connected to an intricate network of branching tubes. The majority of cardiovascular diseases arise from malfunctions in either the tubes or the pump. Typically, the tubes, specifically the arteries that transport blood from the heart to every part of the body, are the starting point for most problems. Arteries, like building pipes, are susceptible to clogging with unwanted deposits.

While coronary artery disease has existed for centuries and has even been identified in mummies, research on non-industrialised populations provides compelling evidence that it and hypertension are primarily evolutionary mismatches. Although medical textbooks commonly teach that blood pressure increases with age, studies on hunter-gatherer groups like the San and the Hadza reveal that this is not the case. The average blood pressure in a seventy-year-old San hunter-gatherer is no different from that of a twenty-year-old, measuring 120/67.

Since Jeremy Morris’s groundbreaking study on London bus conductors, it has been unequivocal that coronary artery disease is mostly preventable and caused by a combination of previously rare risk factors: high cholesterol, high blood pressure, and chronic inflammation. These precursors to disease are influenced by genes but are primarily caused by the same interrelated behavioural risk factors we frequently encounter, such as smoking, obesity, poor diets, stress, and lack of exercise.

How Does Physical Activity Help?

As we have previously discussed, physical activity has been shown to decrease inflammation, which is often caused by factors such as obesity, unhealthy diets, excessive alcohol consumption, and smoking.

Furthermore, chronically high blood pressure can put a strain on the heart, leading to abnormal thickening and weakening. However, physical activity can help generate new arteries and keep existing ones supple, protecting against high blood pressure by increasing blood flow through arteries.

Much and What Kind of Exercise Are Best?

Although some individuals inherit genes that increase their risk for cardiovascular disease, genetics alone do not determine their fate. Preventing hypertension, coronary artery disease, and related problems involves avoiding smoking and limiting consumption of processed foods high in sugar, saturated fats, and salt. Additionally, physical activity is critical since the cardiovascular system requires regular stimulation to develop and maintain capacity. Lack of physical activity leaves individuals vulnerable to high blood pressure and heart disease.

It is commonly recognised that cardio exercise is particularly beneficial for the cardiovascular system. Extended periods of aerobic physical activity promote high blood flow throughout the body, which stimulates favorable responses that help to maintain low blood pressure and a healthy heart.



Cancer currently ranks as the second leading cause of death globally, claiming approximately one in four lives. Cancer is like a game of chance at the cellular level that appears to affect people regardless of age, but more commonly after age fifty. Although remarkable progress has been made in comprehending and treating different types of cancer, a diagnosis often still carries a grim prognosis.

What is the Hypothesised Mismatch?

Cancer is not a singular disease, but rather an overarching term for the outcome of cells competing with each other in a distorted form of natural selection within the body. Visualise your body as a vast ecosystem of nearly forty trillion cells from over two hundred diverse cell lines. Usually, these cells work together amicably, even as they acquire random mutations, almost all of which are harmless. On rare occasions, cells acquire mutations that disrupt their function, and a minuscule percentage of those mutations prompt cells to compete with one another. Once these mutations happen, the cells become cancerous.

Subsequently, they proliferate uncontrollably, move throughout the body, and avidly consume calories. If your immune system cannot eliminate these cancerous cells rapidly enough, they take over organs, disrupt their function, and deprive other cells of resources.

How Does Physical Activity Help?

The mechanisms behind how and why physical activity helps prevent cancer are not fully understood, but some evidence suggests that the mechanisms are linked to energy, as predicted by an evolutionary perspective. High levels of physical activity may divert energy from cancerous cells in several ways.

Firstly, moderate physical activity can affect reproductive hormones, as energy spent on physical activity is energy not spent on reproduction, which is modulated by hormones like estrogen. Sedentary women naturally allocate more energy towards reproduction than women who exercise moderately.

Secondly, exercise may prevent and fight cancer by depriving cancer cells of ready energy from sugars, which cancer cells tend to burn anaerobically. High levels of blood sugar from metabolic syndrome are associated with increased cancer rates. High-intensity exercise, which inhibits anaerobic sugar metabolism, may be especially effective for preventing and fighting certain cancers.

Thirdly, physical activity can help prevent or reduce inflammation, which is a risk factor for many cancers and is associated with chronic positive energy balance and obesity. Inflammation causes cellular damage and mutations that can lead to cancer. Exercise may counteract cancer indirectly by reducing levels of inflammation.

Lastly, physical activity stimulates the body to invest energy in repair and maintenance systems, such as antioxidant production and immune function. Antioxidants counteract reactive atoms that can cause cancerous mutations. Non-extreme levels of exercise also boost the immune system, which plays a vital role in fighting cancer.


Final Exercise Advice

The advice from this book can be boiled down concisely and simply. Make exercise necessary and fun. Do mostly cardio, but also some weights. Some is better than none. Keep it up as you age.