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January 22 - February 24, 2021
physical activity. In turn, natural selection favored older individuals whose bodies stimulated repair and maintenance mechanisms in response to ...
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however, compare how much Americans and Hadza walk. A study of thousands found that the average twenty-first-century woman in the United States aged eighteen to forty walks 5,756 steps a day (about two to three miles), but this number declines precipitously with age, and by the time they are in their seventies, American women take roughly half as many steps.
23 Thanks to an active lifestyle without retirement, there is no significant age-related decline in walking speed among Hadza women, whose average pace remains a brisk 3.6 feet per second (1.1 meters per second) well into their seventies.
Darwinian perspective, the best strategy is to live long and actively and then die fast when you become inactive. An even better strategy, however, would be to avoid any deterioration with age in the first place.
Aging is inexorable, but senescence, the deterioration of function associated with advancing years, correlates much less strongly with age. Instead, senescence is also influenced strongly by environmental factors like diet, physical activity, or radiation, and thus can be slowed, sometimes prevented, and even partly reversed.
The oxygen we breathe generates energy in cells but leaves behind unstable oxygen molecules with free, unpaired electrons. These reactive oxygen species (charmingly also called free radicals) steal electrons indiscriminately from other molecules, thereby “oxidizing” them. That theft sets off a slow chain reaction by creating other unstable, electron-hungry molecules obliged to steal electrons from yet more molecules.
But there is hope. Aging and senescence are not inextricably linked, because to varying degrees most of these destructive processes can be prevented or slowed and the damage they cause can be mended. Oxidation, for example, is halted by antioxidants, compounds that bind with reactive
oxygen species, thus rendering them harmless. Some antioxidants like vitamins C and E come from food, but our bodies synthesize many other antioxidants in abundance. Similarly, mitochondria can be regenerated, and some products of glycation can be repaired by enzymes that scavenge or break down these compounds.37 Inflammation can be turned off by anti-inflammatory proteins produced by white blood cells and muscles; telomeres can be lengthened; DNA can be repaired; and cells can be induced to restore or repair dozens of functions. Indeed, almost every cause of aging in almost every tissue (with
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In the mid-1960s, a team of physiologists in Dallas decided to compare the effects of sedentariness with those of exercise on health by paying five healthy twenty-year-olds first to spend three weeks in bed and then to undergo an intensive eight-week exercise program. The bed rest was ruinous. When they were finally allowed to arise from their beds, the volunteers’ bodies resembled forty-year-olds’ by many metrics: they were fatter, had higher blood pressure, higher cholesterol levels, less muscle mass, and lower fitness.40 The eight ensuing weeks of exercise, however, not only reversed the
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Three decades of typical American lifestyles had not been kind to the original volunteers: they had each gained about fifty pounds, had higher blood pressure and weaker hearts, and were less fit and healthy in numerous ways. But they agreed to be studied once more as they tried to undo the consequences of thirty sedentary years with a six-month program of walking, cycling, and jogging. Fortunately, this second late-in-life exercise intervention helped the volunteers lose about ten pounds and, most astoundingly, largely reversed their decline in cardiovascular fitness. After six months of
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If exercise is so destructive, why is it healthy? One explanation is that once she stopped exercising, my wife’s body reacted by repairing whatever harm she caused and, crucially, also repairing some of the damage that she had accumulated beforehand when she wasn’t exercising.
Although these maintenance and repair processes are not nearly as costly as her workout, they nonetheless require calories, thus elevating her resting metabolic rate very slightly for some time. Studies show that people’s afterburns can last from two hours to two days depending on the intensity and duration of the physical activity.44
While exercise restores most structures (what biologists term homeostasis), in some cases it may make things even better than before (this is termed allostasis).
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.
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.
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.
All in all, the modest physiological stresses caused by exercise trigger a reparative response yielding a general benefit, a phenomenon sometimes known as hormesis.
Pauling was a brilliant chemist, but his advocacy of vitamin C was quackery. Dozens of studies have found that taking antioxidant pills is no substitute for physical activity to fight senescence.
To add insult to injury, additional studies suggest antioxidants may sometimes do more harm than good when combined with exercise.
Muscle biopsies taken before and after their exercise bouts showed that, as expected, physical activity induced plenty of oxidative stress, but those who took antioxidants incurred more oxidative damage because their bodies produced much lower levels of their own antioxidants.
Why is regular physical activity the best way to delay senescence and extend life? Recall that according to the costly repair hypothesis, organisms with restricted energy supplies (just about everyone until recently) must allocate limited calories toward either reproducing, moving, or taking care of their bodies, but natural selection ultimately cares only about reproduction. Consequently, our bodies evolved to spend as little energy as possible on costly maintenance and repair tasks. So while physical activities trigger cycles of damage and restoration, selection favors individuals who
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The capacity is the ability to maintain, often through repair, a stable internal environment so we can adequately and effectively perform those functions needed for survival and reproduction.
Unfortunately, this marvelous system has one big flaw. Apparently, we never evolved to activate these maintenance and repair responses as effectively in the absence of regular physical activity. As we have seen, almost no one in the Stone Age, least of all grandparents, managed to avoid hours of walking, running, digging, climbing, and other manual labors. Hunter-gatherers of all ages would have stimulated their body’s natural reparative mechanisms nearly every day in response to the demands posed by their way of life. So just as our species never evolved to diet or cope with jet lag, we never
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is a useful way to think about the effects of physical activity on aging. In a nutshell, persistent physical inactivity along with smoking and excess body fat are the biggest three factors that influence the likelihood and duration of the major illnesses that kill most people who live in industrial, westernized contexts.
That doesn’t mean, however, that physical activity is a surefire Fountain of Youth, and remember it doesn’t delay mortality by preventing aging per se. Instead, physical activity triggers a suite of mechanisms that increase the chances of staying healthy with age by retarding senescence and preventing many chronic diseases that contribute over time to mortality.
In my opinion, if we want to promote exercise effectively, we need to grapple with the problem that engaging in voluntary physical activity for the sake of health and fitness is a bizarre, modern, and optional behavior. Like it or not, little voices in our brains help us avoid physical activity when it is neither necessary nor fun. So let’s reconsider both of these qualities from an evolutionary anthropological perspective.
Paradoxically, for the first time in history, wealthier people get more physical activity than the working poor.
These negative reactions are probably ancient adaptations. Like most organisms, we have been selected to enjoy and desire sex, eating, and other behaviors that benefit our reproductive success and to dislike behaviors
If our Stone Age ancestors found unnecessary physical activities like optional five-mile jogs unpleasant, they would have avoided squandering limited energy that could have been allocated toward reproduction.
But there are three shortcomings in this reward system for today’s non-exercisers. First, dopamine levels go up only while we exercise. So they don’t get us off the couch.
Elevated levels of serotonin induce a feeling of well-being (the drug ecstasy exaggerates this feeling by boosting serotonin levels sky-high), and we become better at controlling nonadaptive impulses.
Endocannabinoids. For years, endorphins were thought to cause the infamous runner’s high, but it is now evident that endocannabinoids—the body’s natural version of marijuana’s active ingredient—play a much greater role in this phenomenon.24 Despite causing a truly pleasurable high, this system has little relevance for most exercisers because it usually takes several hours of vigorous physical activity before the brain releases these mood- and sensory-enhancing drugs. Further, not everyone has the genes that make a runner’s high possible.25 I suspect the runner’s high evolved primarily to
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Because we never evolved to be inactive and out of shape, the adaptations that make physical activity feel rewarding and become a habit develop only after the several months of effort it takes to improve fitness. Slowly and gradually, exercise switches from being a negative feedback loop in which discomfort and lack of reward inhibit us from exercising again to being a positive feedback loop in which exercise becomes satisfying.
Going to college is essentially a highly social commitment contract for adults that includes carrots and sticks. Students in my university pay a fortune to have professors like me compel them to read, study, and work under penalty of getting bad grades or failing. My students compete for and agree to these conditions because they know they would not learn as much without the school’s nudges, shoves, and requirements.
Young people need to move. Because our thrifty physiologies evolved to build capacity in response to demand, sufficient physical activity during the first decades of life is indispensable for developing a healthy body. At least an hour a day of moderate to vigorous physical activity reduces children’s risk of obesity and helps them grow healthy muscles, bones, hearts, blood vessels, digestive systems, and even brains. Children who get more physical activity learn more and are smarter, happier, and less prone to depression and other mood disorders.
Some of these concerns are real, but all can be addressed. The least persuasive concern is coercion.
studies (including one contending that regular chocolate consumption adds nearly a year to your life3),
Middle-aged alumni who exercised more than two thousand calories per week had a 21 percent lower risk of dying than their sedentary classmates, and those who were over seventy and exercised the same amount had half the risk of dying in a given year as their inactive classmates.
Yes, half. This study when published was the first unequivocal evidence for a powerful dose-response relationship between exercise and mortality. Exercise is no panacea, but the more you exercise, the longer you are likely to live, and the effects of physical activity on longevity become vastly greater as we age.
Finally, alumni who took up exercise later in life had similarly lower rates of mortality compared to those who had been active all along. It’s never too late to start.
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.
follows a common pattern. In every study, the largest benefit came from just ninety weekly minutes of exercise, yielding an average 20 percent reduction in the risk of dying. After that, the risk of death drops with increasing doses but less steeply. If we assume the studies’ median to be a reasonable guide, to attain another 20 percent reduction in risk beyond the benefits of ninety weekly minutes, we’d have to exercise another five and a half hours for a total of seven hours per week.
Because hunter-gatherers are generally only moderately physically active without being either couch potatoes or ultramarathoners, we are probably adapted for moderate rather than extreme exercise.
As the philosopher George Santayana once quipped, “Skepticism, like chastity, should not be relinquished too readily.”
As we will see in chapter 13, there is evidence that regular, moderate physical activity can help protect against some contagious diseases, but we need more clinical data on the extent to which high doses of exercise suppress the immune system’s ability to ward off infections and under what conditions.
An experiment that gave mice a deadly form of influenza and then forced them to exercise before their symptoms developed found that low levels of moderate exercise (twenty to thirty minutes of daily running) doubled their rate of survival compared to sedentary mice, but extremely high levels of exercise (two and a half hours of running a day) caused them to die at even higher rates.
Because plaques contain calcium, which shows up nicely in a CT scan, doctors routinely score plaques by their calcium content: a coronary artery calcium (CAC) score. CACs above 100 are generally considered cause for concern.
One massive analysis of almost twenty-two thousand middle-aged and elderly men found that the most physically active individuals had the highest CAC scores but the lowest risk of heart disease.27
So can you exercise too much? Perhaps at extreme levels, and most certainly if you are sick with a serious infection or injured and need to recover. You also increase your risk of musculoskeletal injuries if you haven’t adapted your bones, muscles, and other tissues to handle the stresses of repeated high forces of Olympic-level weight lifting, playing five sets of tennis a day, running marathons, or overdoing some other sport that obsesses you. In other respects, the negative effects of too much exercise appear to be ridiculously less than the negative effects of too little.
Because the fundamental challenge of aerobic activity is to deliver more oxygen at a faster rate to muscles and other organs, this demand stimulates the chambers of the heart to grow stronger, more capacious, and more elastic. These adaptations in turn increase the heart’s cardiac output, the product of heart rate and the volume of blood pumped per contraction. In the blood, aerobic exercise augments the red blood cell count but also increases the volume of plasma, reducing viscosity so the heart can pump blood more easily. Sustained increased cardiac output also stimulates the expansion of
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