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An alternative to coddling one’s body with products that mimic the effects of exercise is to try non-physically active forms of suffering. This kind of “no pain, no gain” philosophy has inspired a dizzying array of self-inflicted hardships thought to ward off aging (an added benefit is their aura of virtue). Hoping to live longer, people take cold showers, restrict their caloric intake, endure long periods without eating, shun carbohydrates, burn their digestive tracts with spicy food, and more.53 Some of these strategies are downright questionable, and, with the exception of intermittent fasting, none is yet supported by solid evidence as a way to extend human longevity.54 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 allocate enough but not too much energy to producing antioxidants, ramping up the immune system, enlarging and repairing muscles, mending bones, and so on. The challenge is to maintain and repair any damage from physical activity just enough and in the right place and the right time.
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Daniel E. Lieberman (Exercised: Why Something We Never Evolved to Do Is Healthy and Rewarding)