Exercised: The Science of Physical Activity, Rest and Health

by Daniel E. Lieberman

by measuring the rate at which these heavy atoms become less abundant in urine, we can calculate the rates at which both the hydrogen and the oxygen atoms leave the body from sweating, urinating, and breathing.

Because hydrogen exits the body only in water but oxygen leaves in both water and carbon dioxide, the difference in the concentration of these two atoms in urine allows us to compute exactly how much carbon dioxide someone produced from breathing, hence how much energy he or she used.9

When we make this correction, it becomes evident that adult humans spend about 30 calories every day for each kilogram of fat-free body mass just to maintain their bodies regardless of whether they spend the day staring into computer screens in New York, manufacturing shoes in a Chinese factory, growing corn in rural Mexico, or hunting and gathering in Tanzania.

A more demanding, hence revealing, stress test for your metabolism requires a more extreme reduction in energy intake: starvation. For good reason, it is unethical and illegal to starve people in the lab for the sake of science.

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.

One of the main ways the starving volunteers spent less energy was to skimp on maintenance. Basically, their metabolisms slowed down and cut back on costly physiological processes that keep the body in a state of balance.

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.

If you are sitting while reading this, for every five breaths you take, one pays for your brain, another for your liver, a third for your muscles, and the last two pay for the rest of your body.

Keys’s data showed that the bodies of the men he starved saved calories in a manner similar to the way most people economize when they confront a severely reduced income: they prioritized “essential” organs like the brain but abandoned “expendable” costs like reproduction and drastically cut back on “reducible” functions like staying warm, active, and strong.

However, one of the principal insights we learned from those courageous volunteers is that resting is not just a state of physical inactivity.

In the grand scheme of things, however, the way our bodies allocate resources has been molded by a much larger process: evolution by natural selection. We use energy as we do in large part because of the way Darwinian evolution acted on millions of generations of our ancestors.

Selection will inevitably favor whichever alternative or compromise most improves your reproductive success in your environment.

But without knowing it, our bodies are constantly making plenty of other consequential trade-offs—many involving energy—that have been selected over millions of generations.

you can spend a given calorie in just five ways: growing your body, maintaining your body (resting metabolism), storing energy (as fat), being active, or reproducing.

you climb a mountain today, you will have less energy to spend on maintenance, storing fat, and (perhaps) reproducing. If you go on a diet, you will have less energy to be active or reproduce. And so on.

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.

Apart from youthful tendencies to play and other social reasons (topics for later chapters), the instinct to avoid nonessential physical activity has been a pragmatic adaptation for millions of generations.

Perhaps by investing more energy in essential active pursuits, we have less to spare for inessential activities?

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.

Rather than blame and shame each other for taking the escalator, we’d do better to recognize that our tendencies to avoid exertion are ancient instincts that make total sense from an evolutionary perspective.

My colleagues Eric Castillo, Robert Ojiambo, and Paul Okutoyi and I found that rural teenagers in Kenya who rarely sit in chairs with backrests have 21 to 41 percent stronger backs than teenagers from the city who regularly sit in the sorts of chairs you and I usually use.15

It is reasonable to conclude that those of us who regularly sit in chairs with backrests have weak back muscles that lack endurance, making it uncomfortable to sit for long on the ground or on stools. The result is a vicious cycle of chair dependency.

In 1859, Thonet perfected his archetypal café chair, which sold like hotcakes and remains popular in coffeehouses.

As chairs with backrests became less expensive and more common, some experts condemned them. To quote one alarmed physician in 1879: “Of all the machines which civilization has invented for the torture of mankind … there are few which perform their work more pertinaciously, widely, or cruelly than the chair.”18

So while nonindustrial people engage in considerably more physical activity than average industrialized and postindustrialized people, they also sit a lot.

By convention, your heart rate during sedentary activities is between its resting level and 40 percent of maximum;

vigorous activities such as running, jumping jacks, and climbing a mountain demand heart rates of 70 percent or higher.27

In contrast, a typical Hadza adult spends nearly four hours doing light activities, two hours doing moderate-intensity activities, and twenty minutes doing vigorous activities.29

The second concern is that long periods of uninterrupted inactivity harmfully elevate levels of sugar and fat in the bloodstream. Third and most alarmingly, hours of sitting may trigger our immune systems to attack our bodies through a process known as inflammation.

Technically, “inflammation” describes how the immune system first reacts after it detects a harmful pathogen, something noxious, or a damaged tissue.

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.

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 cond...

The most egregious claim is that we can efficaciously prevent or treat almost any disease—from autism to Parkinson’s—by simply avoiding “pro-inflammatory” foods like gluten and sugar or by wolfing down “anti-inflammatory” foods like turmeric and garlic.

The bad news is that chronic inflammation plays a role in many serious diseases. The good news is that the biggest causes of chronic inflammation are largely avoidable, preventable, or addressable: smoking, obesity, overconsumption of certain pro-inflammatory foods (a chief one being red meat), and—surprise, surprise—physical inactivity.

In healthy, normal human adults, including hunter-gatherers, fat constitutes about 10 to 25 percent of body weight in men and about 15 to 30 percent in women. The majority of that fat (about 90 to 95 percent) is subcutaneous, so named because it is stored in billions of cells distributed in buttocks, breasts, cheeks, feet, and other nameless places just below the skin.

The other major type of fat is cached in cells in and around our bellies and other organs including the heart, liver, and muscles. There are many terms for this fat including “visceral,” “abdominal,” “belly,” and “ectopic,” but I will use the term “organ fat.”

Organ fat in moderate quantities (about 1 percent of total body weight) is thus normal and beneficial as a short-term energy depot for times when we need rapid access to a lot of calories such as when we walk or jog a long distance.

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

The mechanisms by which too much fat, especially in and around organs, can ignite low-grade, chronic inflammation suggest that too much sitting may be hazardous simply because it causes weight gain.

So if I sit instead of move, the calories I consumed at lunch are more likely to be converted to fat rather than burned.

A second way lengthy periods of sitting may incite widespread, low-grade inflammation is by slowing the rate we take up fats and sugars from the bloodstream.

If it was within the last four or so hours, you are in a postprandial state, which means your body is still digesting that food and transporting its constituent fats and sugars into your blood. Whatever fat and sugar you don’t use now will eventually get stored as fat, but if you are moving, even moderately, your body’s cells burn these fuels more rapidly.

Put simply, regular movement, including getting up every once in a while, helps prevent chronic inflammation by keeping down postprandial levels of fat and sugar.

Sadly, sitting is not always relaxing. 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...

Last, and perhaps most important, prolonged sitting can kindle chronic inflammation by allowing muscles t...

In addition to moving our bodies, muscles function as glands, synthesizing and releasing dozens of messenger proteins (ter...