We hear again and again why exercise is so good for our health, but for many, it is so difficult to fit into our day. Daniel E. Lieberman tells us in his new book, Exercised: Why Something We Never Evolved to Do is Healthy and Rewarding, why this is, what we can do to fix it and the amazing benefits including decreasing our risk for Alzheimer’s. Read the excerpt below.
When my grandmother’s short-term memory started to fail, we thought it was caused by the stress of taking care of my ailing grandfather. But after he died, her mind continued to decline slowly and relentlessly, memory by memory. At first she couldn’t remember where she had put things, whom she had just spoken to, and what she had eaten for lunch. Then, as her Alzheimer’s progressed, she started having trouble recognizing family members and friends and remembering basic words and key events in her life. Eventually, she lost her sense of both the present and the past. It was as if the disease had stolen her mind, leaving behind just her body.
What Is the Hypothesized Mismatch?
Alzheimer’s is a complicated, poorly understood disease that must be in part an evolutionary mismatch. Studies of dementia in nonindustrial populations are limited, but conservative epidemiological studies that correct for differences in life expectancy indicate that Alzheimer’s disease is about twenty times more common in industrial than non-industrial populations.103 And it’s getting more common: worldwide prevalence of the disease is projected to increase fourfold in the first half of the twenty-first century.104 Genes alone cannot explain this epidemic.
While Alzheimer’s symptoms and progression are well known, its causes are not. The most common theory is that Alzheimer’s results from plaques and tangles that smother nerve cells (neurons) near the surface of the brain, depriving the cells of nutrients, not unlike the way hair clogs a drain.105 Treating these plaques and tangles, however, doesn’t appear to reverse or prevent the disease, and many elderly people with plaques and tangles never develop Alzheimer’s.106 Mounting novel evidence suggests Alzheimer’s is a kind of inflammatory auto-immune 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.107
One evolutionary explanation and preliminary support for this hypothesis comes from studies of Amazonian forager-farmers, the Tsimane (remember, they are the population without evidence of coronary heart disease). Although Westerners who carry the two copies of a gene called ApoE4 (a protein that transports fats in the bloodstream) are three to fifteen times more likely to get Alzheimer’s in old age, elderly Tsimane with the same ApoE4 gene are less likely to show declines in cognitive performance if they suffer from many infections.108 Alzheimer’s may thus be an example of an evolutionary phenomenon called the hygiene hypothesis. 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.109)
How Does Physical Activity Help?
Regardless of what causes Alzheimer’s, if you are worried about the disease, then exercise. 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.110 Exercise is by far the most effective known form of prevention and treatment. Further, the effects are impressive. An analysis of sixteen prospective studies including more than 160,000 individuals found that moderate levels of physical activity lowered the risk of Alzheimer’s by 45 percent.111 More physically intense activities may be associated with reduced risks for the disease.112 Physical activity also slows the rate of cognitive and physical deterioration in Alzheimer’s patients.113
How physical activity helps prevent and treat Alzheimer’s is poorly known, but there is evidence for several evolved mechanisms. 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 first evolved to help mammals get energy during physical activity and at some point took on additional roles in the brain.114 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. In a classic mismatch, absence of exercise deprives us of doses of BDNF that have been shown to improve memory and cognition and to maintain neuronal health that apparently helps prevent Alzheimer’s.115 One prospective study that followed more than two thousand individuals for decades found that women with the highest levels of BDNF have half the risk of developing Alzheimer’s as those with the lowest levels.116 Because BDNF helps astrocytes take care of brain cells and their connections, elevated levels of exercise-induced BDNF may help pre- vent the kind of astrocyte-induced damage hypothesized to underlie Alzheimer’s.117 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.118 Rodents that run on treadmills develop fewer plaques and tangles in their brains and have lower levels of inflammation associated with Alzheimer’s.119
How Much and What Kind of Exercise Are Best?
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. One analysis of nine- teen studies found that aerobic physical activity is most beneficial, but other reviews favor a mix of aerobic exercise, weights, and exercises that improve balance and coordination.120 In addition, limited evidence suggests there may be a dose-response relationship between exercise intensity and risk.121
From EXERCISED by Daniel E. Lieberman, published by Pantheon Books, an imprint of the Knopf Doubleday Publishing Group, a division of Penguin Random House LLC. Copyright (c) 2020 by Daniel E. Lieberman.