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

by Daniel E. Lieberman

Expert consensus is that we need 150 minutes of exercise a week, but we also read that just a few minutes of high-intensity exercise a day is enough to make us fit.

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.6

studies of professional athletes who push the limits of endurance reveal that the barriers they must overcome include physiological challenges like generating muscle force effectively, fueling themselves efficiently, and controlling their body temperature, but these competitors are even more challenged by psychological hurdles. To keep going, great athletes must learn to cope with pain, be strategic, and above all believe they can do it.5

scientists decided to measure people’s energy expenditures using the simplest metric possible, the physical activity level, or PAL.20 Your PAL is calculated as the ratio of how much energy you spend in a twenty-four-hour period divided by the amount of energy you would use to sustain your body if you never left your bed.

If you are a sedentary office worker who gets no exercise apart from generally shuffling about, your PAL is probably between 1.4 and 1.6. If you are moderately active and exercise an hour a day or have a physically demanding job like being a construction worker, your PAL is likely between 1.7 and 2.0. If your PAL is above 2.0, you are vigorously active for several hours a day.

The U.S. government recommends I engage in at least 150 minutes of moderate or 75 minutes of vigorous exercise a week and weight train at least twice a week.31 Epidemiologists have calculated that this level of activity will reduce my risk of dying prematurely by 50 percent and lower my chances of getting heart disease, Alzheimer’s, and certain cancers by

roughly 30 to 50 percent.

If you are an average adult American male who weighs 180 pounds (82 kilograms), your rate of energy expenditure while resting quietly in a chair is approximately seventy calories per hour. This is your resting metabolic rate (RMR),

Approximately 15 percent of a typical thin man’s body is fat (thin women average 25 percent body fat). That fat has several functions, but the most essential is to be an enormous reservoir of calories that can be burned when needed.

Technically, “inflammation” describes how the immune system first reacts after it detects a harmful pathogen, something noxious, or a damaged tissue. In most cases, inflammation is rapid and vigorous. Whether the offenders are viruses, bacteria, or sunburns, the immune system quickly launches an armada of cells into battle. These cells discharge a barrage of compounds that cause blood vessels to dilate and become more permeable to white blood cells that swoop in to destroy any invaders. This extra blood flow brings critically needed immune cells and fluids, but the swelling compresses nerves and causes the four cardinal symptoms of inflammation (which literally means “to set on fire”): redness, heat,

swelling, and pain. Later, if necessary, the immune system activates additional lines of defense by making antibodies that target and then kill specific pathogens.

cytokines (from the Greek cyto for “cell”

and kine for “movement”), regulate inflammation. As scientists started to study when and how cytokines 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. In a way, chronic, low-grade inflammation is like having a never-ending cold so mild you never notice its existence. But the inflammation is nonetheless there, and mounting evidence indicates that this slow burn steadily and surreptitiously damages tissues in our arteries, muscles, liver, brain, and other organs.

The discovery of low-grade inflammation and its effects has simultaneously created new opportunities to combat disease and unleashed new worries. In the last decade, chronic inflammation has been strongly implicated as a major cause of dozens of noninfectious diseases associated with aging, including heart disease, type 2 diabetes, and Alzheimer’s. The more we look, the more we find the fingerprints of chronic inflammation on yet more diseases including colon cancer, lupus, multiple sclerosis, and just about every medical condition with the suffix “-itis” including arthritis.31

Although most fat is healthy, obesity can turn fat from friend into inflammatory foe. The biggest danger is when fat cells malfunction from overswelling. The body has a finite number of fat cells that expand like balloons. If we store normal amounts of fat, both subcutaneous and organ fat cells stay reasonably sized and harmless. However, when fat cells grow too large, they distend and become dysfunctional like an overinflated garbage bag, attracting white blood cells that trigger inflammation.35 All bloated fat cells are unhealthy, but swollen organ

fat cells are generally more harmful than subcutaneous fat cells because they are more metabolically active and more directly connected to the body’s blood supply. 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. Disconcertingly, it is also possible to be “skinny fat” with significant deposits of organ fat in and around one’s muscles, heart, and liver without necessarily having a potbelly figure.

Long hours of commuting, a demanding desk job, being sick or disabled, or otherwise being confined to a chair can be stressful situations that elevate the hormone 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. Short bursts of cortisol are natural and normal, but chronic low levels of cortisol are damaging because they promote

obesity and chronic inflammation. Consequently, long hours of stressful sitting while commuting or a high-pressure office job can be a double whammy.

studies suggest that even if you are physically active and fit, the more time you spend sitting in a chair, the higher your risk of chronic illnesses linked to inflammation, including some forms of cancer.47 If these results are correct, then exercise alone doesn’t counter all the negative effects of sitting.

muscles, especially in the calves, act as pumps to prevent blood and other fluids from building up in the legs, not just in veins, but also in the lymph system, which functions like a series of gutters to transport waste throughout the body. It’s good to keep these fluids moving. Sitting for long hours without moving increases the risk of swelling (edema) and developing clots in veins.54 For this reason, squatting and other more active forms of sitting may be healthier than sitting in chairs by requiring intermittent muscle activity, especially in the calves, thus recirculating blood in the legs.

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. The brain apparently also tags and sorts memories, identifying and strengthening ones we may need. And, fantastically, the brain may also analyze certain memories during REM sleep, integrating them and looking for patterns. Critically, however, the brain has limited abilities to multitask and cannot perform these cleaning, organizing, and analytical functions as effectively when we are awake and alert.11

An even more vital function of sleep for the brain is 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 bod...

Some of these garbagy molecules such as beta-amyloid clog up neurons.13 Others such as adenosine make us sleepy as they accumulate (and are counteracted by caffeine).14 Getting rid of these waste products, however, is a challenge. Whereas tissues like liver and muscle wash out metabolites directly into blood, the brain is tightly sealed off from the circulatory system by a blood-brain barrier that prevents blood from coming into direct contact with brain cells.15 To rid itself of waste, the brain evolved a novel plumbing system that relies on sleep. During NREM sleep, specialized cells throughout the brain ex...

spaces also admit enzymes that repair damaged cells and rejuvenate receptors in the brain for neurotransmitters.17 The only catch, however, is that the brain’s interstitial pathways are like single-lane bridges that let cars pass in only one direction at a time. Apparently, we cannot think while cleansing our brains...

there is no single pattern among humans or mammals. Donkeys sleep only two hours a day, but armadillos sleep as much as twenty hours. Some animals like giraffes nap

frequently, but other species sleep in one uninterrupted bout. 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

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.

overly sleepy during the day.56 In turn, getting enough sleep helps people be active and improves athletic performance by allowing the body to have sufficient time to rest and repair.57 Adolescents who sleep less than six hours have twice the injury rate of those who sleep eight or more.58 And finally, adults who are persistently physically inactive are more vulnerable to suffering from insomnia.59 Insomnia, which is a long-term condition and not a night or two of poor sleep in response to an emergency, is especially cruel because it often triggers a vicious cycle. If underlying chronic stress from too much time commuting, social conflicts, or endlessly tough homework assignments elevates stress hormones like cortisol above normal levels,

we become more alert at night when we’d otherwise become drowsy, or we wake up after one or two NREM and REM cycles.60 Then as we become chronically sleep deprived, we produce more cortisol, especially at night, which can then inhibit sleep, keeping the problem going and promoting insomnia.61

bodies are more like battery-powered cars, and instead of having one enormous battery that we recharge occasionally, our cells use millions of tiny organic batteries that we have to recharge constantly. These ubiquitous miniature batteries, which power all life on earth, are called ATPs (adenosine triphosphates). As the name implies, each ATP consists of a tiny molecule (an adenosine) attached to three molecules of phosphate (a phosphorus

atom surrounded by oxygen atoms). These three phosphates are bound to each other in a chain, one on top of the other, storing energy in the chemical bonds between each phosphate. When the last of these phosphates is broken off using water, the tiny quantity of energy that binds it to the second phosphate is liberated along with one hydrogen ion (H+), leaving behind an ADP (adenosine diphosphate). This liberated energy powers almost everything done by every cell in the body like firing nerves, making proteins, and contracting muscles. And, critically, ATPs are rechargeable. By breaking down chemical bonds in sugar and fat molecules, cells acquire the energy to restore ADPs to ATPs by adding back the lost phosphate.14 The problem is, however, that regardless

of whether we are hyenas or humans, the faster we run, the more our bodies struggle to recharge these ATPs, thus curtail...

muscles evolved more than 600 million years ago to generate force by contracting, and their basic structure

muscles are bundles of long, thin cells, called fibers. Each fiber, in turn, is made up of thousands of strands, fibrils, that in turn contain thousands of banded structures called sarcomeres (Greek for “flesh component”). Sarcomeres

generate pulling forces because they are made of two key proteins—one thin, the other thick—that try to slide past each other like interlacing the fingers of your two hands. This contractile action occurs whenever a nerve sends an electrical signal to the muscle, causing tiny projections on the thick filaments to pull against the thin filaments much like a tug-of-war team pulls on a rope. The ratcheting action of each projection exerts a minuscule tug at the cost of a single ATP. Because there are billions of these projections in every muscle, and they keep ratcheting repeatedly, the many little tugs swiftly add up.27

fibers of skeletal muscles that move our bones come in several varieties. At one extreme are slow-twitch fibers that do not contract rapidly or powerfully but use energy aerobically and don’t fatigue easily. These type I fibers are colloquially known as red muscle because of their darker tinge.28 At the other extreme are fast-twitch (type II) fibers, which come in two types: white and pink. White muscle (type IIX) fibers burn sugar to generate powerful and rapid forces but fatigue rapidly. Pink muscle (type IIA) fibers produce moderately powerful forces aerobically and thus fatigue at an intermediate rate. Altogether, red fibers are ideal for sustained low-intensity activities like walking or jogging a marathon, pink fibers are best

for medium-intensity activities like racing a mile, and white fibers are essential for bursts of extreme power but short duration like sprinting a hundred meters.

muscular atrophy—the gruesome technical term is “sarcopenia,” Greek for “loss of flesh”—is a major cause of disability and disease among the elderly. As we age, muscle fibers typically dwindle in size and number, and nerves degenerate.50 The result is a loss of strength and power.

moderate, non-strenuous levels of weight training helps older individuals increase their muscle mass and strength and thus improve their ability to function normally and stay active without requiring assistance.54

as muscle mass declines, people load their bones less, contributing to osteoporosis. This furtive disease occurs when bones become too frail to sustain the loads they incur, causing them to snap or collapse. Because weakened muscles lead to less physical activity, sarcopenia is also a risk factor for other conditions associated with inactivity, including heart disease and type 2 diabetes.

Violence is woven into every culture, including hunter-gatherer societies, calling into question assumptions that we are naturally benign and unaggressive.7

How, then, do we reconcile our extraordinary capacities for cooperation and conflict avoidance (Rousseau) with our capacities for aggression (Hobbes)? A persuasive resolution to this age-old debate was proposed by Richard Wrangham, who points out that we wrongly conflate two profoundly different kinds of aggression: proactive and reactive.10 According to Wrangham, humans differ from

other animals, especially our ape cousins, in having exceedingly low levels of reactive aggression but much higher levels of proactive aggression. We correspond to Rousseau in terms of reactive aggression and to Hobbes in terms of proactive aggression.

The most widely discussed and audacious hypothesis, proposed by Owen Lovejoy, was that the first hominins were selected to become bipeds to be more cooperative and less aggressive.21 According to Lovejoy, early hominin females favored males who were better at walking upright and

thus better able to carry food with which to provision them. To entice these tottering males to keep coming back with food, females encouraged exclusive long-term monogamous relationships by concealing their menstrual cycles and having permanently large breasts (female chimps advertise when they ovulate with eye-catching swellings, and their breasts shrink when they are not nursing). Put crudely, females selected for cooperative males by exchanging sex for food. If so, then selection against reactive aggression and frequent fighting is as old as the hominin lineage.22

Aerobic exercise is sustained physical activity fueled by burning oxygen. The key metrics are heart rate and oxygen use. By convention, aerobic exercise elevates your pulse to between 50 and 70 percent of maximum (most people’s maximum heart rate is between 150 and 200 beats per minute depending on fitness and age).34 Another way to measure exercise intensity is the percentage of the maximum rate of oxygen use, VO2 max. Regardless of how we measure it, aerobic exercise causes breathing that is fast and deep enough to make singing impossible but not hard enough to prevent conversing in normal sentences.

Because the fundamental challenge of aerobic activity is to deliver more oxygen at a faster rate to muscles and other organs, this demand stimulates the chambers of the heart to grow stronger, more capacious, and more elastic. These adaptations in turn increase the heart’s cardiac output, the product of heart rate and the volume of blood pumped per contraction. In the blood, aerobic exercise augments the red blood cell count but also increases the volume of plasma, reducing viscosity so the heart can pump blood more easily. Sustained increased cardiac output also stimulates the expansion of the many small arteries and capillaries where oxygen exchange occurs in muscles everywhere including the heart’s muscle itself. And aerobic exercise raises so-called good cholesterol (HDL) and lowers so-called bad cholesterol (LDL) and circulating fats (triglycerides). Altogether, these many effects help keep hearts strong; arteries clear, supple, and unclogged; and resting blood pressure low.

it burns harmful organ fat, improves the body’s ability to use sugar, lowers levels of inflammation, and beneficially adjusts the levels of many hormones including estrogen, testosterone, cortisol, and growth hormone.

aerobic exercise increases blood flow to the brain and elevates the production of molecules that stimulate brain cell growth, maintenance, and function. A good cardio workout really does improve cognition and mood.