Exercise: For your health, grin and bear it

Bears hibernate. "But," according to Bernd Heinrich's book, Winter World: The Ingenuity of Animal Survival, "they were for a long time not considered to hibernate, simply because their body temperature showed only modest drops and hibernation was defined in terms of low body temperature." [p. 258]

One of the first issues of hibernating bears to be studied was how, despite maintaining a high metabolic rate (high body temperature), the bear still does not need to drink or urinate all winter....[p. 259]

    The answer is fascinating, but I'm not going to summarize it; you're better off picking up this wonderful book yourself, anyway.
    But bears also

do without...exercise [during their winter hibernation] and suffer no ill effects....
    Hibernating bears accomplish metabolic feats that, if we knew their secrets, would likely lead to cures for many human ills. They have the secrets of how to survive lack of exercise, and then, after five months of resting, of how to get up and walk up a mountain. In all of those months of what amounts to bed rest, they suffer no bed sores. They have marginal loss of muscle mass and no change in muscle fiber type. Despite their non-weight-bearing position for months at a time, they do not suffer from bone loss or osteoporosis. After burning fat for fuel for months during which their cholesterol levels become double those of humans and those they have in the summer, yet they still don't suffer from hardening of the arteries or gallstones, conditions resulting from high cholesterol levels in us. Most of the enigmas that have been revealed in hibernating bears have not been solved [as of 2003, when the book was published], maybe because bears just can't be studied as conveniently as lab rats. We can be reasonably certain, however, that once we understand how bears hibernate through the winter, we will also have a larger window into ourselves. We inadvertently simulate a hibernation-like state of inactivity in our modern environment, a new state of nature to which we are not well-adapted. [pp. 260, 262]

We are not well-adapted to physical inactivity, and we need to avoid it. That's where the book's exercise health advice comes in:

We require mechanical stress of exercise on our skeleton to maintain bone structure and function...Most of the American population subjects itself to the physical stress of inactivity [emphasis mine]...Every hour of vigorous exercise as an adult was repaid [in a study of "the effects of lifestyle on 17,000 Harvard students for twenty-five years"] with two hours of additional life span [how did they concluded that?]. 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. [pp. 260-261]

    Bernd Heinrich has been a marathoner and has also written a book about running: Why We Run: A Natural History.