In Brief:

While exercise can boost mood, its health benefits have been oversold.

Moderate exercise can reduce the risk of diabetes in people at risk. Exercise may reduce the risk of heart disease and breast and colon cancers.

Though the evidence is mixed, exercise may also provide benefits for people with osteoporosis.

Physical activity alone will not lead to sustained weight loss or reduce blood pressure or cholesterol.

Exercise has long been touted as the panacea for everything that ails you. For better health, simply walk for 20 or 30 minutes a day, boosters say — and you don’t even have to do it all at once. Count a few minutes here and a few there, and just add them up. Or wear a pedometer and keep track of your steps. However you manage it, you will lose weight, get your blood pressure under control and reduce your risk of osteoporosis.

If only it were so simple. While exercise has undeniable benefits, many, if not most, of its powers have been oversold. Sure, it can be fun. It can make you feel energized. And it may lift your mood. But before you turn to a fitness program as the solution to your particular health or weight concern, consider what science has found.

Moderate exercise, such as walking, can reduce the risk of diabetes in obese and sedentary people whose blood sugar is starting to rise. That outcome was shown in a large federal study in which participants were randomly assigned either to an exercise and diet program, to take a diabetes drug or to serve as controls. Despite trying hard, those who dieted and worked out lost very little weight. But they did manage to maintain a regular walking program, and fewer of them went on to develop diabetes.

Exercise also may reduce the risk of heart disease, though the evidence is surprisingly mixed. There seems to be a threshold effect: Most of the heart protection appears to be realized by people who go from being sedentary to being moderately active, usually by walking regularly. More intense exercise has been shown to provide only slightly greater benefits. Yet the data from several large studies have not always been clear, because those who exercise tend to be very different from those who do not.

Active people are much less likely to smoke; they’re thinner and they eat differently than their sedentary peers. They also tend to be more educated, and education is one of the strongest predictors of good health in general and a longer life. As a result, it is impossible to know with confidence whether exercise prevents heart disease or whether people who are less likely to get heart disease are also more likely to be exercising.

Scientists have much the same problem evaluating exercise and cancer. The same sort of studies that were done for heart disease find that people who exercised had lower rates of colon and breast cancer. But whether that result is cause or effect is not well established.

Exercise is often said to stave off osteoporosis. Yet even weight-bearing activities like walking, running or lifting weights has not been shown to have that effect. Still, in rigorous studies in which elderly people were randomly assigned either to exercise or maintain their normal routine, the exercisers were less likely to fall, perhaps because they got stronger or developed better balance. Since falls can lead to fractures in people with osteoporosis, exercise may prevent broken bones — but only indirectly.

And what about weight loss? Lifting weights builds muscles but will not make you burn more calories. The muscle you gain is minuscule compared with the total amount of skeletal muscle in the body. And muscle has a very low metabolic rate when it’s at rest. (You can’t flex your biceps all the time.)

Jack Wilmore, an exercise physiologist at Texas A & M University, calculated that the average amount of muscle that men gained after a serious 12-week weight-lifting program was 2 kilograms, or 4.4 pounds. That added muscle would increase the metabolic rate by only 24 calories a day.

Exercise alone, in the absence of weight loss, has not been shown to reduce blood pressure. Nor does it make much difference in cholesterol levels. Weight loss can lower blood pressure and cholesterol levels, but if you want to lose weight, you have to diet as well as exercise. Exercise alone has not been shown to bring sustained weight loss.Just ask Steven Blair, an exercise researcher at the University of South Carolina. He runs every day and even runs marathons. But, he adds, “I was short, fat and bald when I started running, and I’m still short, fat and bald. Weight control is difficult for me. I fight the losing battle.”

The difficulty, Dr. Blair says, is that it’s much easier to eat 1,000 calories than to burn off 1,000 calories with exercise. As he relates, “An old football coach used to say, ‘I have all my assistants running five miles a day, but they eat 10 miles a day.’”

Publish date: 1/8/08