Exercised: The Science of Physical Activity, Rest and Health

by Daniel E. Lieberman

Because natural selection ultimately favors those who can allocate as much energy as possible to reproduction, our physiology has been tuned over millions of generations to hoard energy, especially fat.

Further, because almost no one until recently was able to become overweight or obese, our bodies primarily sense if we are gaining or losing weight rather than how much excess fat we have.

Whether you are skinny or stout, negative energy balance—including dieting—causes a starvation response that helps us restore energetic equilibrium or, better yet, gain weight s...

It’s unfair, but losing ten pounds elicits food cravings and the desire to be inactive regardless of w...

And therein lies another key difference between walking today and in ancient times. If I walk ten thousand extra steps to place my body in negative energy balance, it is literally a piece of cake for me to wipe out the extra cost of such a walk. The ease of refueling with a donut or a Gatorade or just by sitting at my desk for the rest of the day helps explain the counterintuitive result we just saw from ...

Happily, more than a dozen studies on the effects of exercise, food intake, and non-exercise physical activity on weight loss found that modest doses of prescribed exercise rarely cause people to spend the rest of the day ...

When the body regulates energy balance like a thermostat, it apparently does so more through diet than through physical activity.

If I follow a standard prescription of briskly walking thirty minutes a day, almost two extra miles, I’ll spend about a hundred extra calories per day, theoretically allowing me to shed approximately five pounds in half a year—about the reductions most studies report.

Although far from easy, dieting is unquestionably more effective for shedding many pounds.

Than walking.

It bears repeating that the standard public health recommendation is 150 minutes of moderate exercise every week. This amounts to a paltry 21 minutes a day, one-sixth the level of physical activity among nonindustrial people like the Hadza.

An even more demanding study compared obese men prescribed seven hundred calories of exercise a day (about five miles of jogging) with men asked simply to cut back their diets by the same number of calories. Over three months, both groups lost almost seventeen pounds (seven and a half kilograms), but the ones who exercised lost more unhealthy organ fat, even though they also ate more.43

Short-term studies pose a problem, however: because walking is so energy efficient, it takes months or years for small doses of exercise to add up to substantial weight losses.

they observed that more active people spent only slightly more calories per day than more sedentary people who weighed the same.

The proposed explanation is that people’s total energy budgets are constrained: if I use five hundred extra calories walking, I’ll spend less energy on my resting metabolism to help pay for my exertions.46

This controversial idea (termed the constrained energy expenditure hypothesis) is still being tested, as is its relevance to weight loss. If correct, then contrary to many people’s expectations, exercisers might spend almost the same number of total calories per day as similar-sized but more sedentary individuals despite devoting more energy to being active.

In addition, as we will see later, exercise can stimulate repair and maintenance mechanisms that elevate people’s resting metabolic rates—an “afterburn”—for a few hours to as much as two days afterward.

Finally, it truly is faster and it’s often easier to lose weight by dieting because everyone needs to eat but no one has to exercise, and not eating five hundred calories of energy-rich food (four slices of bacon) requires less time and effort than walking five miles a day.

The majority of dieters who do not exercise regain about half their lost pounds within a year, and thereafter the rest typically creeps back slowly but surely. Exercise, however, vastly increases the chances of maintaining weight loss.

In the mid-1960s, a Japanese company, Yamasa Tokei, invented a simple, inexpensive pedometer that measures how many steps you take. The company decided to call the gadget Manpo-kei, which means “ten-thousand-step meter,” because it sounded auspicious and catchy. And it was. The pedometer sold like hotcakes, and ten thousand steps has since been adopted worldwide as a benchmark for minimal daily physical activity.

Many experts, including some who study the Hadza, thus blame the obesity epidemic squarely on industrial diets, not activity levels.

With few exceptions, most organs and functions expend a small percentage of one’s total energy budget. But these many vital expenses add up quickly. Skimping on thermoregulation, digestion, circulation, repairing the body’s tissues, and sustaining the immune system can quickly land us in hot water.

Of the many special qualities that make us human including big brains, language, cooperation, making sophisticated tools, and cooking, efficient bipedal walking was apparently the first and remains one of the most important. We wouldn’t be here if our ancestors didn’t have to walk at least ten thousand steps a day. But that legacy has not remained a necessity. Until recently, walking wasn’t exercise, and despite its being economical, we evolved to do it as little as possible.

Our conversation quickly turned to the importance of stabilizing one’s gaze and the hypothesis that animals adapted for running (“cursors” in biological parlance) have a special rubber-band-like structure, the nuchal ligament, at the back of their heads that acts like a spring to keep their heads still.

In fact, bipedal running is equivalent to quadrupedal trotting.

The horse, however, can do something we bipeds cannot: gallop. When quadrupeds gallop, they alternate landing with their forelimbs and hind limbs, using not just their legs but also their spines as springs.

So while horses, dogs, zebras, and antelopes can gallop faster than any human can sprint (the gray bars), they cannot gallop for more than a few miles before having to slow down to a walk or a trot, especially when it is hot.

Beyond our high-performance legs, the most vital and unique adaptation that enables humans to go the extra mile is our ability to perspire profusely.

If we can’t dump this heat, we must stop running or suffer from heatstroke because body temperatures above 41°C cook cells in the brain and elsewhere.

Most animals take advantage of this natural refrigeration by panting—taking short, shallow breaths to evaporate saliva in their throats and on their tongues.

At some point, humans evolved a magnificent cooling system by taking advantage of special water-secreting glands that most animals have only on their paws.

Sweating effectively turns the entire body into a giant, wet tongue.

We also lost our fur, which helps air move along the skin’s surface without any barrier, thus enabling us to rapidly dump prodigious quantities of heat.

When running in the heat, humans can sweat one liter per hour (sometimes even more), enough to keep cool while racing a marathon in 90°F—something no other animal can do.

At rest, the heart pumps about four to six liters of blood each minute, but during running it must pump as much as five times more to supply hardworking muscles and cool the body.

A typical runner’s heart pumps twenty to twenty-four liters a minute, and an elite runner’s can reach an impressive thirty-five liters a minute.

We also have an elaborated blood supply to the brain to help cool this vital organ during exercise.

And the leg muscles of ordinary humans usually have 50 to 70 percent fatigue-resistant slow-twitch fibers, far more than chimpanzees, which range from 11 to 32 percent.

Humans who train for speed can increase the size of their fast-twitch fibers, but ordinary humans from every population are still slow-twitch dominated, and thus capable of more endurance than apes.

Even if you dislike running, your body is loaded with features from head to toe that help you run long distances efficiently and effectively.

Assuming the hunters resume the chase before the animal fully cools, its body temperature will gradually keep rising until, eventually, it reaches a state of heatstroke and collapses. A hunter can then walk right up to the animal and dispatch it safely without sophisticated weapons (sometimes using just a rock).

Like many runners I had developed plantar fasciitis, inflammation of the thick band of connective tissue that runs like the string on a bow below the arch of the foot.

As a shoe’s elasticity deteriorates, the built-in arch support loses effectiveness, putting extra strain on the plantar fascia.

Unlike me and most of the runners I had measured, Jeffrey landed as light as a feather on the balls of his feet (a “forefoot strike”) thus avoiding the impact peak and resulting shock wave normally caused by landing on the heel.

We speculated that humans evolved primarily to forefoot strike when running, and called for research to test if this running style, common among elite runners, might prevent injuries.

The resulting damage is often termed an overuse injury. Studies claim such injuries afflict between 20 and 90 percent of runners in a given year, suggesting that millions of runners must be overdoing it.

Is running really so injurious, and if we evolved to run long distances, why aren’t our bodies better adapted?

One hypothesis out there is that running injuries are mismatch conditions like type 2 diabetes and myopia caused by our bodies being poorly adapted to the modern environments in which we now live.

Analyses of the combined evidence from hundreds of small studies show that running injury rates follow a U-shaped curve:

the biggest of which is that wear and tear from hoofing too many miles will erode the cartilage in your knees and hips and give you osteoarthritis.

Despite what many doctors and others assume, more than a dozen careful studies show that nonprofessional runners are no more likely to develop osteoarthritis than non-runners.