Exercised: The Science of Physical Activity, Rest and Health

by Daniel E. Lieberman

As I grumbled about how much money, time, and effort we had wasted getting a treadmill to Pemja, it struck me how these machines encapsulate the main theme of this book: we never evolved to exercise.

Exhorting us to “Just Do It” is about as helpful as telling a drug addict to “Just Say No.”

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.

The problem, of course, is that physical activity helps slow aging and promotes fitness and health. So those of us who no longer engage in physical labor to survive must now weirdly choose to engage in unnecessary physical activity for the sake of health and fitness. In other words, exercise.

In sum, even if you are a highly active person, you probably spend more energy maintaining your body than doing stuff.

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.

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.

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. The compromises your body makes among these functions depend on your age and energetic circumstances.

Studies that compare the energy spent standing with that spent sitting report that standing costs about 8 to 10 percent more calories than sitting quietly in a simple desk chair.

“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

According to this extraordinary database, adult male and female chimpanzees spend on average 87 percent of every day in sedentary activities such as resting, grooming, feeding quietly, and nesting. Over a twelve-hour day, chimps are physically inactive for almost ten and a half hours. On their most active days, chimpanzees rest for almost eight hours; on their least active days, they rest for more than eleven hours.

The answer has recently become apparent thanks to new technologies that accurately measure minuscule quantities of the more than one thousand tiny proteins that cells pump into our bloodstreams. Several dozens of these proteins, termed cytokines (from the Greek cyto for “cell” and kine for “movement”), regulate inflammation.

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.

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.

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.

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.

In addition to moving our bodies, muscles function as glands, synthesizing and releasing dozens of messenger proteins (termed myokines) with important roles.

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.

quench inflammation and reduce physiological stress.53 Finally, 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.

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.

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.

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.

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.

sleeping not only helps mothers and infants sleep better; it helps mothers and infants coordinate their sleeping and feeding and provides a wealth of positive, nurturing interactions.46 Although bed sharing with a parent who smokes, drinks, or takes drugs involves risks to the infant, most especially sudden infant death syndrome, misinformation has scared many parents from co-sleeping.47

The first system is our nearly twenty-four-hour circadian cycle regulated by a specialized group of cells within a region of the brain known as the hypothalamus.52 (The sleep-inducing name for this cluster of cells is the suprachiasmatic nucleus.)

Sugar is synonymous with sweetness, but it’s first and foremost a fuel used to recharge ATPs through a process termed glycolysis (from glyco for “sugar” and lysis for “break down”). During glycolysis, enzymes swiftly snip sugar molecules in half, liberating the energy from those bonds to charge two ATPs.19 Restoring ATPs from sugar doesn’t require oxygen and is rapid enough to provide almost half the energy used during a thirty-second sprint.

The benefits of regular HIIT go well beyond its effect on muscles. Among other payoffs, HIIT increases the heart’s ability to pump blood efficiently by making its chambers larger and more elastic. HIIT also augments the number, size, and elasticity of arteries and increases the number of tiny capillaries that infuse muscles. HIIT further improves muscles’ ability to transport glucose from the bloodstream and increases the number of mitochondria within each muscle, thus supplying more energy.

Concentric contractions are critical for movement, but as Charles Atlas supposedly intuited in the Brooklyn Zoo, they are generally less potent for building muscle than eccentric and isometric muscle actions.

A good place to get carefully reviewed, consensus advice is the American College of Sports Medicine.

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.

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.

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

There are several ways to persistence-hunt. One is to take advantage of the distinctive human ability to not overheat while running.

Another hypothesis is that our bodies are actually marvelously adapted to running and that its dangers have been exaggerated. If something like 80 percent of the world’s millions of runners were dropping like flies from injuries, then doctor’s offices would be overflowing with injured runners, and joggers would eventually be as rare as hen’s teeth.

Many experts thus advocate increasing mileage only 10 percent a week.

I think most experienced runners and coaches agree on four key, related elements illustrated in figure 25: (1) not overstriding, which means landing with your feet too far in front of your body; (2) taking about 170–180 steps a minute; (3) not leaning too much, especially at the waist; (4) landing with a nearly horizontal foot, thus avoiding a large, rapid impact force with the ground.