Why Something We Never Evolved to Do Is Healthy and Rewarding

On squatting

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Aside from causing me humiliation, my lack of stamina while squatting, not to mention sitting on a sled, highlights how addicted I am to chairs, especially those with backrests. Whenever I sit on the ground or use a stool without a backrest, muscles in my back and abdomen must do a little work to hold up my torso, and when I squat, muscles in my legs, especially my calves, are also sometimes active. To be sure, the muscular effort isn’t great: squatting and standing use about the same degree of muscle activity.14 But over long periods of time those muscles require and develop endurance.

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As we have already discussed, a likely explanation is that short bouts of activity wake up our muscles and thus keep down levels of blood sugar and fat. When we squat, periodically stand, or do light activities like pick up a child or sweep the floor, we contract muscles throughout the body, setting in motion their cellular machinery. Like turning on a car engine without driving anywhere, these light activities stimulate muscle cells to consume energy, turn on and off genes, and perform other functions.

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If you are like me, you rarely squat, but that avoidance is a modern Western peculiarity. Because squatting creates tiny smoothed regions on ankle bones known as squatting facets, we can see that humans for millions of years, including Homo erectus and Neanderthals, regularly squatted.12 Squatting facets also indicate that many Europeans squatted habitually until furniture and stoves became common after the Middle Ages.

On sitting

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3 major health concerns related to sitting:

There are three major, related health concerns about long periods of uninterrupted sitting:

• The first is what we are otherwise not doing. Every hour spent resting comfortably in a chair is an hour not spent exercising or actively doing things
• 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.

Don’t panic, but as you sit comfortably reading this, your body may be on fire.

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Link between sitting and inflammation:

No one is claiming that sitting itself caused these Danish guys to gain weight: it was the combination of physical inactivity plus excess calories that caused them to stockpile excess organ fat, which in turn lit the smoldering fire of chronic inflammation. In addition, these volunteers added mostly organ fat, which suggests they were stressed, and there are plenty of physically inactive people who are not overweight but suffer from inflammation. So what else about sitting might promote chronic inflammation?

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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. When was the last time you had a meal? 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. Light, intermittent activities such as taking short breaks from sitting and perhaps even the muscular effort it takes to squat or kneel reduce levels of fat and sugar in your blood more than if you sit inertly and passively for long.

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Is sitting the new smoking:

More truthfully, the problem isn’t sitting itself, but hours upon hours of inactive sitting combined with little to no exercise. If our ancestors from generations ago behaved like today’s hunter-gatherers and farmers, then they likely sat for five to ten hours a day, as much as some but not all contemporary Americans and Europeans.58 But they also got plenty of physical activity when not sitting, and when these chairless ancestors plunked themselves down, they didn’t rest in supportive chairs with seat backs; instead, they squirmed as they squatted, kneeled, or sat on the ground, using about the same degree of muscle activity in their thighs, calves, and backs as when they stood.

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On the history of chairs:

How often have you been admonished to stop slouching and sit up straight? This old chestnut dates back to the late-nineteenth-century German orthopedic surgeon Franz Staffel. As the Industrial Revolution caused more people to work long hours in chairs, Staffel worried these sitters were ruining their posture by sliding their buttocks forward and straightening their lower backs. Alarmed, Staffel opined that a person’s spine should maintain the same characteristic double-S curve when sitting as when standing normally, and he advocated chairs with lower back supports to force us to sit upright (like the second fellow from the right in figure 5). Decades later, Staffel’s opinions were backed up by the Swedish ergonomics pioneer Bengt Åkerblom and his students, who X-rayed people in chairs while measuring their muscle activity. As a result, most Westerners, including a majority of health-care professionals, think we can avoid back pain by sitting with a curved lower back and an unrounded upper back.

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Scientific evidence discredits this modern cultural norm. A big clue is that while chairs with backrests do facilitate slouching, chairless people worldwide also commonly adopt comfortable postures that straighten the lower back and round the upper back, as evident in figure 5.66 Many biomechanical arguments against slouching have also been disproved.

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FIGURE 5 The spine and pelvis during standing and sitting. 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. (Note that I have shown just a few of the many postures people adopt when sitting.)

Cytokine

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On cytokine and inflammations:

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.

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.

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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.

On sleep

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What happens during sleep:

Elegant experiments using sensors that peer into the brain of people before, during, and after they have slept (or been deprived of sleep) reveal that these functions often occur during sleep.10 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.

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The brain and its waste:

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. Because the power-hungry brain uses one-fifth of the body’s calories, it generates abundant and highly concentrated metabolites. Some of these garbagy molecules such as beta-amyloid clog up neurons. Others such as adenosine make us sleepy as they accumulate (and are counteracted by caffeine).

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For every hour spent awake storing memories and amassing waste, we need approximately fifteen minutes asleep to process those memories and clean up.

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The brains cleaning process:

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. These opened spaces also admit enzymes that repair damaged cells and rejuvenate receptors in the brain for neurotransmitters. 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. We thus must sleep to flush out the cobwebs left behind by the day’s experiences.

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How much are my sleep patterns contaminated by alarm clocks, lights, smartphones, and other enemies of sleep such as jobs, train schedules, and the nightly news? Fortunately, researchers have woken up to these problems, and new technologies have made possible a surge of high-quality data on sleep in nonindustrial populations. The most electrifying study by far was by the UCLA sleep researcher Jerome Siegel and his colleagues, who affixed wearable sensors to ten Hadza hunter-gatherers from Tanzania, thirty San forager-farmers from the Kalahari Desert, and fifty-four hunter-farmers from the Amazon rain forest in Bolivia. None of these populations have electric lights, let alone clocks or internet access. Yet to Siegel’s astonishment, they slept less than industrialized people did. In warmer months, these foragers slept on average 5.7 to 6.5 hours a day, and during colder months they slept on average 6.6 to 7.1 hours a night. In addition, they rarely napped. Studies that monitored Amish farmers who shun electricity as well as other nonindustrial populations such as rural Haitians and subsistence farmers in Madagascar report similar average sleep durations, about 6.5 to 7.0 hours a day.

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What regulates sleep:

For this reason, our sleep-wake states are modulated by a second system that is tightly linked to activity levels. This homeostatic system functions like an hourglass that counts how long we’ve been awake, slowly building up pressure for us to sleep. The longer we stay awake, the more sleep pressure we accrue from the accumulation of molecules such as adenosine left behind when the brain expends energy. Then by sleeping, we reset the hourglass, primarily through NREM sleep. Overall, the homeostatic system helps balance the time we spend awake versus asleep, and if we are up too long, it will eventually override our circadian systems and help us recover lost sleeping time.

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The effects of the fight-and-flight response (technically, the Sympathetic Nervous System ) on sleep explain how and why exercise has such important, well-known effects on sleep. If you run a mile at top speed or lift heavy weights just before going to bed, you’ll probably have a hard time falling asleep because vigorous physical activity turns on this system, stimulating arousal. In contrast, a good dose of physical activity earlier in the day like a game of soccer, an hour or two of gardening, or a long walk helps sleep come more easily. These activities increase sleep pressure, and they stimulate the body to counter the initial fight-and-flight response with a deeper “rest and digest” response (technically the parasympathetic nervous system). Among other benefits, recovery from exercise gradually lowers basal cortisol and epinephrine levels, depresses body temperature, and even helps re-sync the circadian clock.

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Link between insomnia and physical activity:

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.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.

Energy levels

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About ATP and ADP:

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. 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 curtailing our speed after a short while.

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But there is a consequential catch: during glycolysis the leftover halves of each sugar, molecules known as pyruvates, accumulate faster than cells can handle. As pyruvates pile up to intolerable levels, enzymes convert each pyruvate into a molecule called lactate along with a hydrogen ion (H+). Although lactate is harmless and eventually used to recharge ATPs, those hydrogen ions make muscle cells increasingly acidic, causing fatigue, pain, and decreased function. Within about thirty seconds, a sprinter’s legs feel as if they are burning. It then takes a lengthy period of time to slowly neutralize the acid and shuttle the surplus lactate into the third, final, but long-term aerobic energy process.

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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. Restoring ATPs from sugar doesn’t require oxygen and is rapid enough to provide almost half the energy used during a thirty-second sprint. In fact, a fit human can store enough sugar to run nearly fifteen miles.

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your VO2 max, you need glycolysis to supply additional fuel to your muscles. Speeds above this range cannot be sustained, because muscles become acidic. Fortunately, your VO2 max has little effect on speed during short bursts of maximum intensity, such as a thirty-second sprint, but the longer the distance, the more it matters. For a hundred-meter dash, only 10 percent of your energy comes from aerobic respiration, but that percentage increases to 30 percent over four hundred meters, 60 percent for eight hundred meters, and 80 percent for a mile. The farther you go, the more your maximum speed benefits from a high VO2 max (which, as we will see, you can increase by training)

About exercise in general

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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.

Even the salubrious meaning of the word “exercise” is recent. Adapted from the Latin verb exerceo (to work, train, or practice), the English word “exercise” was first used in the Middle Ages to connote arduous labor like plowing a field.

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Exercise is big business. Walking, jogging, and many other forms of exercise are inherently free, but giant multinational companies entice us to spend lots of money to work out in special clothes, with special equipment, and in special places like fitness clubs. We also pay money to watch other people exercise, and a handful of us even pay for the privilege of suffering through marathons, ultramarathons, triathlons, and other extreme, grueling, or potentially dangerous sporting events.

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the majority of us struggle to exercise enough, safely, or enjoyably. We are exercised about exercise.

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No consens how we should train and for how long:

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. Some fitness professionals recommend free weights, others prescribe weight machines, yet others reproach us for not doing enough cardio. While some authorities urge us to jog, others warn that running will ruin our knees and promote arthritis. One week we read how too much exercise may damage the heart and that we need comfortable sneakers, but the next we read it is almost impossible to exercise too much and that minimal shoes are best.

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On triathlons and obsession about this sport:

Full triathlons require extreme obsession and money. If you consider airfare, hotel bills, and gear, many Ironmen spend tens of thousands of dollars a year on their sport. Although Ironman attracts diverse participants, including cancer survivors, nuns, and retirees, a large percentage are wealthy Type A personalities who apply the same fanatical devotion to exercise they previously dedicated to their careers.

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but subsequent dictionaries generally define “*exercise*” as a “*planned, structured physical activity to improve health, fitness, or physical skills*,” and “*play*” as “*an activity undertaken for no serious practical purpose*.”

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To quote Rousseau: “Do you, then, want to cultivate your pupil’s intelligence? Cultivate the strengths it ought to govern. Exercise his body continually; make him robust and healthy in order to make him wise and reasonable. Let him work, be active, run, yell, and always be in motion. Let him be a man in his vigor, and soon he will be one in his reason.

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A veritable who’s who of medical organizations agree that adults should get at least 30 minutes of “ moderate to vigorous ” aerobic exercise at least five days a week for a minimum of 150 minutes per week.

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Ten thousand steps:

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.

Further resources

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the anthropologist Gordon Hewes documented more than a hundred postures that humans from 480 different cultures adopt when they sit without a chair.

Longetivity

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A related driver of senescence is mitochondrial dysfunction. Mitochondria are the tiny power plants in cells that burn fuel with oxygen to generate energy (ATP). Cells in energy-hungry organs like muscles, the liver, and the brain can have thousands of mitochondria. Because mitochondria have their own DNA, they also play a role in regulating cell function, and they produce proteins that help protect against diseases like diabetes and cancer.

Mitochondria, however, burn oxygen, creating reactive oxygen species that, unchecked, cause self-inflicted damage. When mitochondria cease to function properly or dwindle in number, they cause senescence and illness.

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On epigenetic modifications:

Over time, tiny molecules glue themselves to the DNA in cells. These so-called epigenetic (on top of the genome) modifications can affect which genes are expressed in particular cells.

Because environmental factors like diet, stress, and exercise partly influence epigenetic modifications, the older we are, the more of them we accumulate.

Most epigenetic modifications are harmless, but the more you have for a given age, the higher your risk of dying.

Other forms of senescence include cells losing the ability to recycle damaged proteins, inadequately sensing and acquiring nutrients, and (less likely) being unable to divide because the little caps (telomeres) that protect the ends of chromosomes from unraveling have become too short.

TODO Notes

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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.

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Ernesto told me he was a champion runner in his youth and that he still competed in several races a year. But when I asked him how he trained, he didn’t understand the question. When I described how Americans like me keep fit and prepare for races by running several times a week, he seemed incredulous. As I asked more questions, he made it pretty clear he thought the concept of needless running was preposterous. *“Why,” he asked me with evident disbelief, “would anyone run when they didn’t have to?*”

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had traveled to the Sierra to do research on Tarahumara Native Americans, famous for their long-distance running. 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.

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Even though Ernesto never trained, and I hadn’t seen any Tarahumara running on their own, I had heard and read numerous accounts about how Tarahumara men and women have their own Ironman-like competitions. In the women’s race, known as ariwete, teams of teenage girls and young women run about twenty-five miles while chasing a cloth hoop. In the men’s race, the rarájipari, teams of men run up to eighty miles while kicking an orange-sized wooden ball. If the Tarahumara think needless exercise is foolish, why do some of them sometimes run insanely long distances like Ironmen? Just as important, how do they accomplish these feats without training?

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Or to be more precise, our bodies were selected to spend enough but not too much energy on nonreproductive functions including physical activity.

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One critique of these statistics is that they classify activity levels rather coarsely as either sitting or not sitting. Standing isn’t exercise, and sitting isn’t always totally inactive. What if I am playing a violin or making an arrow while sitting? Or standing while listening to a lecture? A solution to this problem is to classify activity levels based on percentage of maximum heart rate. By convention, your heart rate during sedentary activities is between its resting level and 40 percent of maximum; light activities such as cooking and slow walking boost your heart rate to between 40 and 54 percent of maximum; moderate activities like rapid walking, yoga, and working in the garden speed your heart rate to 55 to 69 percent of maximum; vigorous activities such as running, jumping jacks, and climbing a mountain demand heart rates of 70 percent or higher.

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About fat in the organs:

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.

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About fat subcutaneos:

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.

These fat-filled cells are efficient storehouses of energy that help us cope with long-term shortages of calories (as we saw from the Minnesota Starvation Experiment). Subcutaneous fat cells have other functions too, especially as glands that produce hormones regulating appetite and reproduction.

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Burn more calories by doing nonphysical activities:

By merely engaging in low-intensity, “non-exercise” physical activities for five hours a day, I could spend as much energy as if I ran for an hour.

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Cortisol and sugar absorption:

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.

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About caloric excess:

Another drawback of being superstrong that mattered in the Stone Age is its caloric cost. Bodybuilders who can lift a cow must also eat as much as a cow. Well, almost. Recall that muscle is an expensive tissue, accounting for about one-third of a typical person’s body mass and one-fifth of her or his energy budget. I need about three hundred calories a day to sustain my unmuscular frame. Beefed-up weight lifters, however, can be more than 40 percent muscle mass, which means they carry as much as twenty added kilograms of costly flesh. If I ever decide to bulk up like that, I’ll have to eat two hundred to three hundred more calories a day to pay for my new physique. While obtaining an extra three hundred calories is a trivial task today (accomplished by wolfing down a milk shake), the challenge of foraging daily for those additional calories in the Stone Age would have compromised one’s reproductive success.

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If there is one physical activity that most fundamentally illustrates the central point of this book—that we didn’t evolve to exercise but instead to be physically active when necessary*—it is walking. *Average hunter-gatherer men and women (Hadza included) walk about nine and six miles a day, respectively, not for health or fitness but to survive. Every year, the average hunter-gatherer walks the distance from New York to Los Angeles. Humans are endurance walkers.

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The big difference between you and Bagayo and Hasani is that their survival demands up to 20,000 steps per day, whereas data culled from millions of cell phones indicate the average American takes 4,774 steps (about 1.7 miles), the average Englishman takes 5,444 steps, and the average Japanese 6,010.3 Consider also that these numbers are averages.

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Walking as a complex movement:

Unless you are like Mary, you have probably given little thought to the act of walking since you started toddling at about one year old.That effortlessness is a remarkable achievement of your amazing nervous system, which dynamically controls the many dozens of muscles needed to put one foot in front of the other in varied and sometimes treacherous conditions including rocky mountain paths and icy sidewalks. Sadly, it often takes an accident or a stroke for you to appreciate these patterned movements and reflexes, which must accomplish two major things: move you efficiently and keep you from falling over.

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On carrying stuff:

In theory, the cost of carrying something should be approximately proportional to its weight. Carrying an infant who weighs 10 percent of your body weight should be like being 10 percent heavier and thus cost you 10 percent more calories when you walk. If only it were that easy. Dozens of studies have found that carrying loads less than half one’s body weight typically costs an extra 20 percent of the added weight, and when loads get really heavy, the costs increase exponentially.

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Hippocrates wrote twenty-five hundred years ago, “Eating alone will not make a man well; he must also take exercise.”

quote

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Donald Trump needs little introduction. Born in 1946 to wealthy parents, he was sent to a military academy where presumably he had to participate in sports. Although a teetotaler and nonsmoker, Trump famously enjoyed eating abundant junk food and large steaks, drinking Diet Coke, getting little sleep, and avoiding any form of exercise apart from golf. According to biographers, “*Trump believed the human body was like a battery, with a finite amount of energy, which exercise only depleted. So he didn’t work out*.

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Among those who lend her a hand are middle-aged and elderly folks. Anthropologists have shown that grandmothers, grandfathers, aunts, uncles, and other older individuals in foraging populations from Australia to South America remain active throughout life, gathering and hunting more calories every day than they consume, which they provide to younger generations.13 This surplus food helps provide adequate calories to children, grandchildren, nieces, and nephews and reduces how much work mothers have to do. Elderly hunter-gatherers also help younger generations by contributing knowledge, wisdom, and skills for about two to three decades beyond childbearing years.

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Chimpanzee mothers, for example, cannot give birth to babies faster than once every five to six years because they forage only enough food every day to sustain their caloric needs plus those of one hungry youngster. Not until her juvenile is old enough to be fully weaned and forage for itself can she muster enough calories to become fertile again. Human hunter-gatherers, in contrast, typically wean their offspring after three years and become pregnant again long before their little ones are able to feed or fend for themselves, let alone stay out of danger. A typical hunter-gatherer mother, for example, might have a six-month-old infant, a four-year-old child, and an eight-year-old juvenile. Because she is usually capable of gathering only about two thousand calories a day, she cannot get enough food to provide for her own substantial caloric needs, which exceed two thousand calories, as well as the needs of her several offspring, none of whom are old enough to forage on their own.12 She needs help.

animals humans pregnancy

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Grandparent Hypothesis

I propose a corollary to the grandmother hypothesis, which I call the active grandparent hypothesis. According to this idea, human longevity was not only selected for but also made possible by having to work moderately during old age to help as many children, grandchildren, and other younger relatives as possible to survive and thrive. That is, while there might have been selection for genes (as yet unidentified) that help humans live past the age of fifty, there was also selection for genes that repair and maintain our bodies when we are physically active. As a result, many of the mechanisms that slow aging and extend life are turned on by physical activity, especially as we get older. Human health and longevity are thus extended both by and for physical activity.

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The active grandparent hypothesis raises a classic chicken-or-egg question. How much do humans live to old age so they can be active grandparents helping younger generations, or how much does their hard work cause them to live long lives in the first place? Is human longevity a result of physical activity or an adaptation to stay physically active?

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Costly repair hypothesis. Representation of total energy expenditure (TEE), resting metabolic rate (RMR), and active energy expenditure (AEE) over the course of a day showing how energy use changes before, during, and after a bout of exercise. AEE is low before exercise, goes up during exercise, and then falls again. However, RMR can remain elevated for several hours after exercise as the body recovers, replenishes energy stores, and repairs damage.

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While exercise restores most structures (what biologists term homeostasis), in some cases it may make things even better than before (this is termed allostasis). For example, demanding physical activities can increase the strength of bones and muscles, increase cells’ abilities to take up glucose from the blood, and both augment and replace mitochondria in muscles. In addition, repair mechanisms sometimes overshoot the damage induced by exercise, leading to a net benefit. It’s like scrubbing the kitchen floor so well after a spill that the whole floor ends up being cleaner. Among other effects, while physical activity initially stimulates inflammation, especially via muscles, it subsequently causes muscles to produce an even stronger, more lasting, and more widespread anti-inflammatory response whose long-term effect is less inflammation not just in the affected muscle but elsewhere.45 As a result, physically active people tend to have lower baseline levels of inflammation. In addition, exercise causes the body to produce more antioxidants than necessary, decreasing overall levels of oxidative stress.46 Exercise also causes cells to clean out damaged proteins, lengthen telomeres, repair DNA, and more. All in all, the modest physiological stresses caused by exercise trigger a reparative response yielding a general benefit, a phenomenon sometimes known as hormesis.

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The resulting paradox is that our bodies never evolved to function optimally without lifelong physical activity but our minds never evolved to get us moving unless it is necessary, pleasurable, or otherwise rewarding.

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Because exercise by definition isn’t necessary, we mostly do it for emotional or physical rewards, and on that horrid April day in 2018, the only rewards were emotional—all stemming from the event’s social nature. For the last few million years humans rarely engaged in hours of moderate to vigorous exertion alone. When hunter-gatherer women forage, they usually go in groups, gossiping and otherwise enjoying each other’s company as they walk to find food, dig tubers, pick berries, and more.

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One of the most popular is a twelve-step program, “Be as Strong as a Lioness,” that alternates many repetitions of moderate weights with fewer repetitions of heavier weights. Another clever prince writes “Walk, Run, and Live Forever,” a ten-step plan that begins with long walks and then adds short runs that gradually increase to ten miles. Other crowd favorites are “Seven Minutes or Your Life,” which promises “optimal health” from just seven minutes a day of high-intensity interval training, and “Live Longer than a Caveman,” which replicates a paleo fitness regime with barefoot walking, tree climbing, and rock lifting. Yet more plans advocate stretching, swimming, biking, jogging, dancing, boxing, yoga, and even pogo sticking. Some of the prescriptions consider genetic variation, others have different plans for men and women, many are designed to maximize weight loss, and one is cleverly tailored to integrate with a woman’s monthly cycle. While the judges ponder, journalists, bloggers, celebrities, enthusiasts, and trolls fiercely argue the merits of every entry. With each day it seems there is a new consensus favorite.

what is the best way to exercise

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The exercise guru Jack LaLanne (who lived to ninety-six) liked to say, “People don’t die of old age, they die of inactivity.”2 That’s hyperbole, but ever since the dawn of civilization

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In 1995 and 1996, all three panels published essentially the same advice: to reduce the overall risk of chronic disease, adults should engage in at least 30 minutes of moderate-intensity exercise at least five times a week.6 They also concluded that children should engage in 60 minutes of physical activity a day. Since then, these prescriptions—150 minutes per week for adults and 60 minutes a day for kids—have been revisited, confirmed, and only slightly modified many times.

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In the end, the 2018 HHS panel concluded that some physical activity is better than none, that more physical activity provides additional health benefits, and that for “substantial health benefits” adults should do at least 150 minutes per week of moderate-intensity or 75 minutes per week of vigorous-intensity aerobic physical activity, or an equivalent combination of the two. (Moderate-intensity aerobic activity is defined as between 50 and 70 percent of your maximum heart rate; vigorous-intensity aerobic activity is 70 to 85 percent of your maximum heart rate.) They also reaffirmed the long-standing recommendation that children need an hour of exercise a day. Finally, they recommended everyone also do some weights twice a week.

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However, many other supposedly worrisome risk factors reported in athletes appear to be misinterpretations of evidence by doctors who compare the hearts of athletes with those of “normal” sedentary individuals with no diagnosis of disease. As we have repeatedly seen, being sedentary is by no means normal from an evolutionary perspective, and such individuals are more likely to develop chronic illnesses and die at a younger age than more active people. The medical habit of erroneously considering sedentary individuals “normal” controls has led to some diagnostic blunders such as mistaking normal repair mechanisms for signs of disease. A prime example is coronary calcification.

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HIIT workouts became especially popular among runners and other endurance athletes after the great Finnish middle- and long-distance runner Paavo Nurmi (the “Flying Finn”) trained for and won nine Olympic gold medals in the 1920s by doing short four-hundred-meter runs over and over as fast as he possibly could.

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Some exercises involve using muscles against an opposing, heavy weight that resists their efforts to contract. It bears repeating that when working against substantial loads, muscles can shorten (concentric contractions), but they are more stressed and grow larger and stronger in response to forceful contractions in which they stay the same length (isometric contractions) or stretch (eccentric contractions)

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In the eighteenth century it was fashionable to lift church bells that were silenced (made “dumb”) by having their clappers removed, hence the term “dumbbells.” Today’s gyms are stocked with an assortment of dumbbells, free weights, and contraptions that can be adjusted to place a constant level of resistance on muscles throughout their entire range of motion.

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However you do them, resistance activities are critical for maintaining muscle mass, especially fast-twitch fibers that generate strength and power. Resistance exercise can also help prevent bone loss, augment muscles’ ability to use sugar, enhance some metabolic functions, and improve cholesterol levels. As a result, every major medical health organization recommends we supplement cardio with weights, especially as we age. A consensus suggestion is two sessions per week of muscle-strengthening exercises involving all major muscle groups (legs, hips, back, core, shoulders, and arms).

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if you were a doctor back in the old days, you’d be a pee connoisseur. As a matter of routine, you would collect your patients’ “liquid gold” to examine its taste, color, smell, and consistency. Much of what doctors discerned from urine was nonsense, but an exception was its sweetness. The English physician Thomas Willis (1621–1675) coined the term “diabetes mellitus” (Latin for “honey sweetened”), what we now call diabetes, from urine that was “wonderfully sweet as if it were imbued with honey or sugar.”

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The mechanisms by which physical activity helps prevent and treat type 2 diabetes are well studied. Most basically, exercise (in conjunction with diet) can ameliorate every characteristic of metabolic syndrome including excess organ fat, high blood pressure, and high levels of blood sugar, fat, and cholesterol. In addition, exercise lowers inflammation and counteracts many of the damaging effects of stress. And most remarkably, exercise can reverse insulin resistance by restoring blocked insulin receptors and causing muscle cells to produce more of the transporter molecules that shuttle sugar out of the bloodstream.21 The effect is akin to unclogging a drain and flushing out the pipes. Altogether, by simultaneously improving the delivery, transport, and use of blood sugar, exercise can resuscitate a once resistant muscle cell to suck up as much as fiftyfold more molecules of blood sugar. No drug is so potent.

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As an example, one worldwide best-selling book claimed that wheat and other foods with gluten cause inflammation of the brain. The data, however, indicate that unless you have celiac disease, eating wheat (especially whole wheat) or other grains will not cause your body, including your brain, to become inflamed unless you eat too much and become obese. For credible, peer-reviewed, evidence-based studies, see Lutsey, P. L., et al. (2007), Whole grain intake and its cross-sectional association with obesity, insulin resistance, inflammation, diabetes, and subclinical CVD: The MESA Study, British Journal of Nutrition 98:397–405; Lefevre, M., and Jonnalagadda, S. (2012), Effect of whole grains on markers of subclinical inflammation, Nutrition Review 70:387–96; Vitaglione, P., et al. (2015), Whole-grain wheat consumption reduces inflammation in a randomized controlled trial on overweight and obese subjects with unhealthy dietary and lifestyle behaviors: Role of polyphenols bound to cereal dietary fiber, American Journal of Clinical Nutrition 101:251–61; Ampatzoglou, A., et al. (2015), Increased whole grain consumption does not affect blood biochemistry, body composition, or gut microbiology in healthy, low-habitual whole grain consumers, Journal of Nutrition 145:215–21.

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