Exercised: The Science of Physical Activity, Rest and Health

by Daniel E. Lieberman

There is no disagreement, however, that moderate levels of exercise are usually beneficial rather than detrimental, but once one is fighting any serious infection high levels of exercise are seriously inadvisable because the immune system needs all the energy it can get.

Today, the lifetime risk of osteoporosis is 40 to 50 percent among women and 13 to 22 percent among men, contributing to more than ten million fractures per year in developed nations.

Sarcopenia is the most obvious beneficiary of exercise. Because muscles are costly (right now, you are spending about one out of five calories simply maintaining your muscles73), they are the classic example of the “Use It or Lose It” principle of energy allocation.

We spend the first twenty to thirty years of life building up the bones in our skeleton, and thereafter gradually lose bone mass and density at a slow rate—as much as 1 percent per year.

One way to avoid osteoporosis is to develop as youngsters strong bones better able to withstand later losses.

Although good nutrition, including lots of calcium and vitamin D, helps youngsters develop strong skeletons and prevents the elderly from resorbing their bones, the forces bones experience from physical activity are equally vital.

In particular, weight-bearing activities that load the skeleton cause bone-growing cells to add more bone when we are young, and they prevent bone-resorbing cells from removing bone as we age.78 Consequently, lifelong weight-bearing exercise helps prevent the disease.

Physical activities like running that load joints repeatedly and heavily do not cause higher rates of osteoarthritis and may sometimes be protective.

Indeed, if physical activity were a problem, we’d expect the disease to have become less, not more, common in today’s more sedentary world. Instead, the disease occurs more commonly today as we age because of inflammation within joints that eats away cartilage.

Muscles benefit from all physical activities, but they respond most strongly to weight-bearing activities that require them to contract forcefully without changing length (isometric contractions) or as they lengthen (eccentric contractions).

Thus activities like jumping, running, and weight lifting that place demanding loads on bones help develop and maintain a strong skeleton much more than lower-impact activities like swimming or using an elliptical.

Cartilage degeneration is probably countered by physical activity, but it is unknown how and to what extent different kinds of exercise help prevent osteoarthritis.

Probably, the biggest benefit of physical activity is to prevent and reduce obesity, thus limiting inflammation as well as abnormally high pressures.

increase the quantity and quality of cartilage in joints.84 Finally, exercise, especially weights, strengthens the muscles around joints, reducing the likelihood they will be damaged from aberrant loads (like twisting a knee).

Prevention includes not only exercising but also recognizing that the disease is a form of evolution gone dreadfully wrong.

Every once in a while, however, cells develop mutations that disrupt their function, and a tiny fraction of those mutations trigger cells to compete with each other. When such mutations occur, the cells become malignant. At this point, they divide uncontrollably, migrate throughout the body, and voraciously consume calories. If your immune system fails to kill these cancerous cells quickly enough, they overtake organs, disrupt their function, and starve other cells.

The most common cancers occur in reproductive organs, intestines, skin, lungs, and marrow because cells in these tissues divide frequently and are exposed to external influences like radiation, toxins, and hormones that affect their likelihood of dividing or mutating.

However, some cancers may be partly mismatches. Without the sophisticated technology available in modern hospitals, cancer is difficult to diagnose, but limited evidence suggests that cancer rates are lower in hunter-gatherer and nonindustrial populations.

2040, a 62 percent increase from 2018.91 Because cancer isn’t going away, we must figure out better ways not just to fight it but also to prevent or tame it. Fortunately, for several kinds of cancers, that includes physical activity.

Please don’t react that way for cancer, however, because the cancer-fighting potential of exercise is underappreciated and insufficiently explored.

Breast and colon cancers are most strongly affected by exercise.

According to one estimate, three to four hours of moderate exercise a week is likely to reduce a woman’s risk of breast cancer by 30 to 40 percent, and both men’s and women’s risk of colon cancer by 40 to 50 percent.

How and why physical activity helps ward off cancer is only partly understood, but as an evolutionary perspective predicts, the mechanisms appear to be linked to energy.

High levels of physical activity divert energy from cancerous cells in at least four possible ways.

Women who exercise moderately produce more than enough hormones to reproduce, but the bodies of sedentary women naturally shunt more energy toward reproduction, leading to 25 percent higher levels of estrogen.95 Because reproductive hormones like estrogen induce cell division in breast tissue, inactivity increases the risk of breast cancer, while exercise has the opposite effect.

Some cancer cells have a sweet tooth. In fact, many cancer cells tend to get their energy directly from sugars, which they burn anaerobically without oxygen. High levels of blood sugar from metabolic syndrome are thus associated with increased rates of cancer.

Inflammation, which goes hand in hand with chronic positive energy balance and obesity, is a risk factor for many cancers.

Physical activity stimulates the body to spend energy on repair and maintenance systems to mop up the damage that exercise might cause in the first place.

An especially promising discovery is that vigorous exercise potently enhances the effectiveness of natural killer (NK) cells, the immune system’s primary weapon that recognizes and destroys cancerous cells.

It was as if the disease had stolen her mind, leaving behind just her body.

While Alzheimer’s symptoms and progression are well known, its causes are not.

Mounting novel evidence suggests Alzheimer’s is a kind of inflammatory autoimmune disease that initially affects cells in the brain known as astrocytes.

Astrocytes, which number in the billions, normally regulate and protect neurons and their connections. When needed, astrocytes also produce toxin-like chemicals to defend the brain from infection. According to this theory, Alzheimer’s occurs when astrocytes produce these toxins in the absence of infections, thus attacking other cells in the brain.

According to this idea, ApoE4, which can be expressed by cells in the brain, might have evolved long ago to help protect the brain when infectious diseases were ubiquitous. Those of us today who live in bizarrely sterile environments without many germs and worms, face an increased chance that these formerly protective immune mechanisms turn against us.

(The hygiene hypothesis also helps explain increased rates of allergies and many other autoimmune diseases.

No effective drugs have yet been developed to treat Alzheimer’s, and there is inconclusive evidence that keeping your mind sharp with mental games staves off dementia.

Exercise is by far the most effective known form of prevention and treatment. Further, the effects are impressive.

Physical activity also slows the rate of cognitive and physical deterioration in Alzheimer’s patients.

The most well supported is that physical activity—especially of longer duration but also more vigorous activities—causes the brain to produce a powerful molecule known as BDNF (brain-derived neurotrophic factor).

BDNF is a sort of growth tonic for the brain that nourishes and induces new brain cells, especially in regions involved in memory. But because we never evolved to be persistently sedentary, we never evolved a mechanism other than physical activity to produce high levels of BDNF.

Physical activity may also lower the risk of Alzheimer’s by increasing blood flow to the brain, by suppressing inflammation, and by lowering damaging levels of oxidative stress.

Abundant evidence shows that physical activity is probably the single best way to lower the risk of Alzheimer’s disease, but how much and what type are most effective is poorly known.

While the ups and downs of daily moods are a normal part of being alive, they should not be confused with depressive and anxiety disorders, which are very different, serious clinical syndromes that affect one in five of us at some point.

While some anxiety disorders are directed toward specific dreads (like speaking in public or acts of violence), generalized anxiety disorders involve chronic obsessive worries about nonspecific threats that are potential rather than actual.

It is obviously adaptive for fear to lead us to avoid threats like poisonous snakes or being attacked by strangers. In anxiety disorder, however, these normal anxieties become irrational and uncontrolled.

Likewise, it may sometimes be adaptive to be discouraged and unmotivated, hence disinclined to engage in behaviors unlikely to be successful like fighting someone who might kill us or wooing a lover who spurns us. In depression disorder, however, these low moods become directed persistently at ourselves, not the outside world.

With these caveats in mind, some but not all studies suggest a trend toward higher rates of depression and anxiety disorders in societies undergoing modernization.

Dozens of more focused, high-quality analyses—many of them prospective randomized control studies—confirm that exercise helps prevent and treat depression disorders and to a lesser extent generalized anxiety disorder.

How exercise alleviates depression and anxiety is less clear, and we should remember that some of the benefits of physical activity may arise from our physiology being poorly adapted to excess sedentariness.

First, physical activity has many direct effects on the brain. One is to flood the brain with mood-altering chemicals.