Exercised: Why Something We Never Evolved to Do Is Healthy and Rewarding

by Daniel E. Lieberman

Everyone knows they should exercise, but few things are more irritating than being told to exercise, how much, and in what way.

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 behavior makes sense except in the light of anthropology.

Not only is the myth of the athletic savage an example of truthiness—something that feels true because we want it to be true—it trivializes the physical and psychological challenges faced by all athletes everywhere,

Here’s another, startling way of thinking about these numbers: if you are a typical person who barely exercises, it would take you just an hour or two of walking per day to be as physically active as a hunter-gatherer.

Philosophers like Plato, Socrates, and Zeno of Citium preached that to live the best possible life, one should exercise not only one’s mind but also one’s body. This idea is not just Western. Confucius and other prominent Chinese philosophers also taught that exercise was equally essential for physical and mental health and encouraged regular gymnastics and martial arts.

In India, yoga was developed and popularized thousands of years ago to train both body and mind.

Friedrich Jahn, the “Father of Gymnastics.” Following Napoleon’s humiliating string of victories over German armies in the early nineteenth century, Jahn argued that educators had a responsibility to restore the physical and moral strength of his nation’s youth with calisthenics, gymnastics, hiking, running, and more.

A hundred and twenty years later, a comprehensive survey of college students from Harvard and elsewhere found less than half exercised regularly, thus contributing to “poor mental health and increased stress.”30

“athleisure”—workout

calorie. (Confusingly, the “calorie” used on food labels is actually one kilocalorie, the amount of heat required to raise the temperature of a kilogram of water 1°C, and I will follow the same convention here.)

This is your resting metabolic rate (RMR), so named because your resting metabolism comprises

fast. That measurement, your basal metabolic rate (BMR), would be roughly 10 percent lower than your RMR (in our example, 1,530 calories).

In sum, even if you are a highly active person, you probably spend more energy maintaining your body than doing stuff. Understandably, this fact seems counterintuitive. As I sit here writing these words, there is little visible evidence that every one of my body’s systems is working industriously to keep me alive apart from the fifteen to twenty gentle breaths I take every minute.

The key lesson to digest from the starving men’s dramatically lower resting metabolic rates is that human resting metabolisms are flexible. Most critically, resting metabolism is what the body has opted to spend on maintenance, not what it needs to spend.

consume. Such measurements indicate that nearly two-thirds of a person’s resting metabolism is spent on just three very expensive tissues: brain, liver, and muscle. Your brain and liver each consume about 20 percent of your resting metabolism, and if you are a typically strong human, your muscles expend 16 to 22 percent of your resting metabolism.

physical activities? That problem partly depends on our goals,

Which brings us back to physical inactivity. From the perspective of natural selection, when calories are limited, it always makes sense to divert energy from nonessential physical activity toward reproduction or other functions that maximize reproductive success even if these trade-offs lead to ill health and shorter life spans.

Stated simply, we evolved to be as inactive as possible. Or to be more precise, our bodies were selected to spend enough but not too much energy on nonreproductive functions including physical activity.

Of course, this logic applies to all animals. Whether you are a human, ape, dog, or jellyfish, natural selection will select against activities that waste energy at a cost to reproductive success. In this regard, all animals should be as lazy as possible. However, the evidence suggests that humans are more averse to needless physical activity than many other species because we evolved an unusually expensive way of increasing our reproductive success from an unusually low-energy-budget ancestor. When your expenses are high, every penny saved is valuable.

physical activity by parking at the back of the lot. How did something as normal and instinctive as saving energy become associated with the sin of slothfulness?

I might be lazy, but spiritually I am in the clear. The mortal sin of sloth comes from the Latin word acedia, which means “without care.” To early Christian thinkers like Thomas Aquinas, sloth had nothing to do with physical laziness, but instead was a sort of mental apathy, a lack of interest in the world.

Today, many people have jobs that involve little to no manual labor, requiring us to choose to be physically active through exercise. Whether we take the stairs, jog, or go to the gym, we need to override ancient, powerful instincts to avoid unnecessary physical activity, and it should hardly be surprising that most of us—hunter-gatherers included—naturally avoid exercise.

Although not all latter-day Americans sit as much as some alarmists suggest, we are more sedentary than earlier generations. There is evidence that the total time Americans spent sitting increased 43 percent between 1965 and 2009, and slightly more for people in England and other postindustrial countries.

turn inflammation on and off, they discovered that some of the same cytokines that ignite short-lived, intense, and local inflammatory responses following an infection also stimulate lasting, barely detectable levels of inflammation throughout the body. Instead of blazing acutely in one spot for a few days or weeks, as when we fight a cold, inflammation can smolder imperceptibly in many parts of the body for months or years.

So when organ fat cells swell, they ooze into the bloodstream a great many proteins (cytokines) that incite inflammation. Telltale signs of excess organ fat are a paunch or an apple-shaped body.

Alarmingly, as these volunteers gained fat, they started to exhibit the classic signs of chronic inflammation including less ability to take up blood sugar after a meal.

cortisol. This much-misunderstood hormone doesn’t cause stress but instead is produced when we are stressed, and it evolved to help us cope with threatening situations by making energy available. Cortisol shunts sugar and fats into the bloodstream, it makes us crave sugar-rich and fat-rich foods, and it directs us to store organ fat rather than subcutaneous fat.

Last, and perhaps most important, prolonged sitting can kindle chronic inflammation by allowing muscles to remain persistently inactive. In addition to moving our bodies, muscles function as glands, synthesizing and releasing dozens of messenger proteins (termed myokines) with important roles. Among other jobs, myokines influence metabolism, circulation, and bones, and—you guessed it—they also help control inflammation.

Because the anti-inflammatory effects of physical activity are almost always larger and longer than the pro-inflammatory effects, and muscles make up about a third of the body, active muscles have potent anti-inflammatory effects. Even modest levels of physical activity dampen levels of chronic inflammation, including in obese people.42

Sitzfleisch connotes perseverance and endurance. To win a chess game, solve a complex math problem, or write a book requires Sitzfleisch.

Even those who engaged in more than seven hours per week of moderate or vigorous exercise had a 50 percent higher risk of dying from cardiovascular disease if they otherwise sat a lot.

One study even found a 30 percent lower rate of all-cause mortality among people who fidget after adjusting for other forms of physical activity, smoking, diet, and alcohol consumption.

Among the many hyperbolic statements written about sitting, maybe the most extreme is that sitting is the new smoking. While cigarettes are novel, addictive, expensive, smelly, toxic, and the world’s number one killer, sitting is as old as the hills and utterly natural. More truthfully, the problem isn’t sitting itself, but hours upon hours of inactive sitting combined with little to no exercise.

When I sat down to read these papers, I was frankly astonished: nearly all high-quality studies on this topic fail to find consistent evidence linking habitual sitting in flexed or slouched postures with back pain.

70 Instead, the best predictor of avoiding back pain is having a strong lower back with muscles that are more resistant to fatigue; in turn, people with strong, fatigue-resistant backs are more likely to have better posture.71 In other words, we’ve confused cause and effect.

“Good posture is primarily a reflection of environment, habits, and mental state and is not a talisman against back pain.”

Such habits prevent or lessen chronic inflammation that provokes ill health, and it bears repeating that the scary statistics we read about sitting are primarily driven by how much we sit when not at work.

As the day marches on, we store memories in a region of the brain called the hippocampus, which functions as a short-term storage center like a USB drive. Then, during NREM sleep, the brain triages these memories, rejecting the innumerable useless ones (like what color socks the man sitting next to me on the subway wore) and sending the important ones to long-term storage centers near the surface of the brain.

janitorial. The zillions of chemical reactions that make life possible inevitably create waste products known as metabolites, some highly reactive and damaging.12 Because the power-hungry brain uses one-fifth of the body’s calories, it generates abundant and highly concentrated metabolites. Some of these garbagy molecules such as beta-amyloid clog up neurons.

During NREM sleep, specialized cells throughout the brain expand the spaces between neurons by as much as 60 percent, allowing cerebrospinal fluid that bathes the brain to literally flush away this junk.16 These opened spaces also admit enzymes that repair damaged cells and rejuvenate receptors in the brain for neurotransmitters.

Apparently, we cannot think while cleansing our brains. We thus must sleep to flush out the cobwebs left behind by the day’s experiences.

Sleep is therefore a necessary trade-off that improves brain function at the cost of time. For every hour spent awake storing memories and amassing waste, we need approximately fifteen minutes asleep to process those memories and clean up. That

A few large animals like elephants can nap standing up, and, most extraordinarily, marine mammals such as dolphins and whales evolved the ability to put just one half of their brain to sleep at a time while they swim.19

hours. Chimpanzees, our closest relatives, apparently average eleven to twelve hours of rest per night.

New sensor technologies that monitor sleep objectively indicate that the average adult in the United States, Germany, Italy, and Australia tends to sleep about six and a half hours in the summer when it is warm and light and between seven and seven and a half hours in the colder, darker winter months.24 Altogether and despite much variation, most adult Westerners probably average about seven hours a night, a good hour (13 percent) less than the eight hours we supposedly need.

These scholars argued that it was normal prior to the Industrial Revolution for people to wake up for an hour or so in the middle of the night before going back to sleep. In between “first sleep” and “second sleep,” people talk, work, have sex, or pray.

and often one hears animals in the distance. In my opinion, the worst offenders of the night in Africa are not humans or hyenas but tree hyraxes, cat-sized, tree-dwelling ungulates (distant relatives of elephants) whose hair-raising nocturnal calls resemble the screams of someone being throttled.

culture until recently, infants slept with their mothers. Many cultures consider not sleeping with your child a form of child abuse.44 Yet when my wife and I first became parents, many books and strangers advised us against co-sleeping with our daughter. Naively, we followed the advice of Dr. Richard Ferber, whose infamous method

College students are a special breed of humans, in part because so many of them are enjoying their first taste of being grown up without yet shouldering the responsibilities of being adults. Most of my sleep-deprived students will have no choice but to settle down and get more sleep once they leave the ivory tower, but some will stay sleep deprived. According to some studies, about 10 percent of American adults have diagnosable insomnia (that is, repeatedly

taking more than half an hour to fall asleep or persistently being unable to sleep through the night), and almost one-third think they don’t sleep enough.48