Winter World_ The Ingenuity of Animal Survival - Bernd Heinrich [105]
Scientists at the NASA Ames Research Center have studied the effects of inactivity in healthy young volunteers who got well paid to lie flat on their backs in bed for a few days to a few weeks at a time (Miller 1995). Since 1971 more than 500 participants at the Ames Center have proven the dramatic implications of sedentary lifestyle to humans. There is not only bone loss and weakened muscles, but also slower absorption from the gut and prediabetic resistance to insulin. The scientists concluded that the physical stresses placed on the body during space flight are virtually identical to those of prolonged bed rest, or those of a hibernating bear. Again the question was: How does the bear’s body stave off bone loss?
Most of the American population subjects itself to the physical stress of inactivity. One in three Americans over fifty is completely sedentary. Therefore, our muscles, deprived of exercise, become resistant to insulin that normally promotes the absorption of glucose; blood sugar reaches dangerously high levels whenever we consume sugar-containing products and so we risk the onset of adult (Type II) diabetes. Our bodies are not adapted to inactivity. In our evolutionary history, in contrast to bears, exercise was a constant, and we’re not made to tolerate being idle for long. We’re adapted as long-distance endurance predators, as I’ve elaborated on in Why We Run: A Natural History. Inactivity adversely affects every organ system in the body, at least so long as we continue to eat. However, I suspect that a caloric surplus could be a relevant variable, too, since that is often the result of inactivity.
Ralf Paffenbarger, a physician at Stanford University, studied the effects of the lifestyles of 17, 000 Harvard students for twenty-five years after they graduated from college and concluded that exercise is a prime variable for health and longevity. That is, the stresses of inactivity mimic the aging response. Every hour of vigorous exercise as an adult was repaid with two hours of additional life span. There is, obviously, a limit to the benefits of human exercise or else more exercise could make us immortal. Instead, too much exercising increases the aging process as well. I suspect the debate of optimum exercise for maximum longevity may relate less to how much exercise we get than to how many calories we take in versus how many we burn off. There is a correlation between eating less and having a longer lifespan. But of course starvation shortens life, and there is thus also a correlation between eating more and having a longer lifespan. The difference is in the range of food intake versus the amount of exercise.
Rest does not as a rule speed up the deteriorating effects of aging, since hibernators that enter torpor have a longer life span than nonhibernators. That is not surprising, because during hibernation physiological functions are put on hold, presumably those that result in degradation as well as those of regeneration. Thus, the long period of low body temperatures characteristic of hibernation are like death to the animal, so that in effect its life span may be extended, even if the number of hours it spends living (as we define eating, defecating, moving, and sleeping) are curtailed.