Why we should exercise

If the benefits of physical activity are legion, so are the reasons for avoiding it. We've got suggestions for adding some to your day.

You already know that exercise is good for you. What you may not know is just how good — or exactly what qualifies as exercise. That's what this issue of the Health Letter is all about. The notion that physical activity helps keep us healthy is very old news indeed. Hippocrates wrote about the dangers of too little activity (and too much food). Tai chi, an exercise system of graceful movements that originated in China, dates from the 12th century B.C. Yoga's roots in India go back much further.

But old ideas aren't necessarily good ones, or have much evidence to back them up. This isn't a problem for exercise — or physical activity, the term many researchers prefer because it's more of a catchall. A deluge of studies have documented its health benefits. Many are observational, which always pose the problem of showing associations (people who exercise happen to be healthy) not proof of cause and effect (it's the exercise that makes those people healthy). But after statistical adjustments, these studies suggest that the connection between exercise and health is more than just an association. Besides, results from randomized clinical trials, which are usually seen as making the case for causality, also point to exercise making people healthier.

What's impressive about this research, aside from the sheer volume, is the number of conditions exercise seems to prevent, ameliorate, or delay.

We're used to hearing about exercise fending off heart attacks. The American Heart Association promulgated the country's first set of exercise guidelines in 1972. And it's not hard to envision why exercise helps the heart. If you're physically active, your heart gets trained to beat slower and stronger, so it needs less oxygen to function well; your arteries get springier, so they push your blood along better; and your levels of "good" HDL cholesterol go up.

It's also not much of a surprise that physical activity helps prevent diabetes. Muscles that are used to working stay more receptive to insulin, the hormone that ushers blood sugar into cells, so in fit individuals blood sugar levels aren't as likely to creep up.

But exercise as a soldier in the war against cancer? It seems to be, and on several fronts: breast, colon, endometrial, perhaps ovarian. The effect of physical activity on breast cancer prevention may be stronger after menopause than before, although some research suggests that it takes quite a lot to make a difference: four to seven hours of moderate to vigorous activity a week. Three studies have found that if you've had colon cancer or breast cancer, physical activity reduces the chances of it coming back.

To top things off, moving the body seems to help the brain. Several studies have found that exercise can reduce the symptoms of depression, and it changes the brain in ways similar to antidepressant medications. In old age, physical activity may delay the slide of cognitive decline into dementia, and even once that process has started, exercise can improve certain aspects of thinking.

Easy to avoid

We have to eat, so following nutritional advice is a matter of making choices. Swap out saturated fats for healthy oils. Eat whole grains instead of refined carbohydrates.

But in this day and age, many (perhaps most) people don't need to be physically active unless they choose to be. And most evidence suggests that the choice of the kind of activity is far less important than whether to be active at all. About half of adult Americans don't meet one of the most oft-cited guidelines, which calls for at least 30 minutes of moderate-intensity activity (a fast walking pace) most days of the week — and you can accumulate that total in bouts of 10 to 15 minutes. About a quarter of American adults say they devote none of their free time to active pursuits.

Clearly some of us are less athletic than others — and some unathletic individuals were simply born that way. Twin studies suggest that about half of the difference in physical activity among people is probably inherited. And researchers are making headway in identifying particular genes that may influence how we respond to physical exertion. For example, they've identified some of the genes responsible for variation in the beta-agonist receptors in the lungs. How your lungs and heart react to strenuous exercise depends, in part, on those receptors.

But genetic explanations for behaviors like exercising only go so far. Many other influences come into play: family, neighborhood, cultural attitudes, historical circumstances. Research has shown, not surprisingly, that active children are more likely to have parents who encouraged them to be that way. Perceptions of how active parents are also seem to matter. The safety and layout of neighborhoods is a factor, particularly for children. In a dangerous place, having children stay home and watch television instead of going to the park to play might be the healthier choice simply because it's safer.