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October 16 - December 20, 2022
we never evolved to exercise. What do I mean by that? Well, exercise today is most commonly defined as voluntary physical activity undertaken for the sake of health and fitness. But as such it is a recent phenomenon. Our not-too-distant ancestors who were hunter-gatherers and farmers had to be physically active for hours each day to get enough food, and while they sometimes played or danced for fun or social reasons, no one ever ran or walked several miles just for health.
so exercise is paradoxical: salubrious but abnormal, intrinsically free but highly commodified, a source of pleasure and health but a cause of discomfort, guilt, and opprobrium.
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.
Dozens of anthropologists over the last century have written about the Tarahumara, but in 2009 they gained an extra boost of worldwide fame from the best seller Born to Run. The book portrays them as a “hidden tribe” of barefoot, ultra-healthy, “superathletes” who routinely run unimaginable distances.
myth of the athletic savage. The essential premise of this myth is that people like the Tarahumara whose bodies are untainted by modern decadent lifestyles are natural superathletes, not only capable of amazing physical feats, but also free from laziness.
Tarahumara run rarájiparis because it is a deeply spiritual ceremony that they consider a powerful form of prayer.6 Many Tarahumara I have interviewed say that the ball-game race makes them feel closer to the Creator. To them, chasing that unpredictable ball for mile upon mile is a sacred metaphor for the journey of life, and it induces a spiritual trancelike state. It is also an important communal event that brings money and prestige.
Among the many studies of the Hadza, one asked forty-six Hadza adults to wear lightweight heart rate monitors for several days.14 According to these sensors, the average adult Hadza spends a grand total of three hours and forty minutes a day doing light activities and two hours and fourteen minutes a day doing moderate or vigorous activities.
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.
Although there is much variation, PALs of hunter-gatherers average 1.9 for men and 1.8 for women, slightly below PAL scores for subsistence farmers, which average 2.1 for men and 1.9 for women.
hunter-gatherer PALs are about the same as those of factory workers and farmers in the developed world (1.8), and about 15 percent higher than PALs of people with desk jobs in developed countries (1.6). In other words, typical hunter-gatherers are about as physically active as Americans or Europeans who include about an hour of exercise in their daily routine.
while many animals are driven by deep instincts to move, sometimes causing pleasure, exercise as we define it—discretionary, planned physical activity for the sake of physical improvement—is a uniquely human behavior.
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), so named because your resting metabolism comprises all the chemical reactions going on in your body while you are not being physically active.
That measurement, your basal metabolic rate (BMR), would be roughly 10 percent lower than your RMR (in our example, 1,530 calories). Your BMR is the energy you use to maintain the most basic processes of your body necessary to stay alive in a nearly coma-like state.
we next need to measure your total daily energy expenditure (DEE), the overall number of calories you spend over the course of twenty-four hours doing everything you do including moving, reading, sneezing, talking, and digesting.
If you weigh 180 pounds, your DEE is probably about twenty-seven hundred calories a day. Because we already learned your RMR is about seventeen hundred calories a day, that means nearly two-thirds (63 percent) of the energy you expend each day is spent just on your resting metabolism.
In sum, even if you are a highly active person, you probably spend more energy maintaining your body than doing stuff.
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.
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.
Darwin’s theory of natural selection. As a quick refresher, this theory—among the most thoroughly scrutinized and tested theories ever proposed—is that over generations heritable features that cause organisms to have more surviving offspring will become more prevalent while features that impair reproductive success will become rarer.
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.
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.
Most wild mammals have physical activity levels of 2.0 to 4.0, and the Hadza fall at the bottom of that range (Hadza women average 1.8, and men average 2.3).21 Instead, it is chimpanzees and sedentary Westerners who have unusually low PALs of about 1.5 and 1.6. Other great apes like orangutans also have low PALs.
by walking long distances, digging, sometimes running, processing our food, and sharing, we spend a lot more energy being active every day than chimpanzees, but that effort yields more calories that enable us not only to be more physically active but also to reproduce at about twice the rate. The extra energy also enables us to have bigger brains, store more fat on our bodies, and do other good things.
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.
Compared with the chimpanzee (left), the human lower spine (the lumbar region) has a curvature (a lordosis) that positions our center of mass (circle) above our hips when we stand. When we squat on the ground (the way people often sat for millions of years) or slouch when sitting in a chair with a backrest, we tend to rotate the pelvis backward and flatten the lower spine, reducing this lordosis.
humans also stand more efficiently than apes because we can straighten our hips and knees and our lower spine has a backward curve (a lordosis) that positions our torsos mostly above rather than in front of our hips.
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 We can’t prove that rural Kenyans have stronger backs solely because of their chair habits, but other studies show that backrests demand less sustained muscular effort.
Large samples of Americans asked to wear heart rate monitors indicate that a typical adult engages in about five and a half hours of light activity, just twenty minutes of moderate activity, and less than one minute of vigorous activity.28 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.
Technically, “inflammation” describes how the immune system first reacts after it detects a harmful pathogen, something noxious, or a damaged tissue.
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.
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 cells are dynamic participants in metabolism and, when activated, can quickly dump fat into the bloodstream. 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.
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.
postprandial state, which means your body is still digesting that food and transporting its constituent fats and sugars into your blood.
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.
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.
we have learned that the body first initiates a proactive inflammatory response to moderate- or high-intensity physical activity to prevent or repair damage caused by the physiological stress of exercise and subsequently activates a second, larger anti-inflammatory response to return us to a non-inflamed state.41 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
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These activities aren’t serious exercise, but experiments that ask people to interrupt long periods of sitting even briefly—for example, just a hundred seconds every half hour—result in lower levels of sugar, fat, and so-called bad cholesterol in their blood.
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.
“Good posture is primarily a reflection of environment, habits, and mental state and is not a talisman against back pain.”
At first, we go through several progressive stages of “quiet” NREM (non-rapid eye movement) sleep. With each stage, we become increasingly unconscious, metabolism slows, body temperature falls. The brain’s electrical signals during NREM sleep are mostly characterized by slow waves with high voltages, and our eyes stay still or roll slowly behind our eyelids.
Eventually, we enter a different, more “active” REM (rapid eye movement) stage of sleep. During REM sleep, when we mostly dream, the brain’s electrical output is characterized by fast waves with low voltages, and our eyeballs rotate swiftly. Other characteristics of REM sleep include less regular heart rate and breathing, temporary paralysis, and spontaneous swelling of the clitoris or penis.
During sleep, our metabolic rate drops about 10 to 15 percent; about 80 percent of growth also occurs during NREM sleep.
Effective cognition, however, requires organisms to sort through all the memories they generate every day, throw out the inconsequential ones, store the important ones, and make sense of them.
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.
The strongest correlation is that vulnerable prey animals tend to sleep less than the carnivores that want to eat them.
