Moderate physical activity can lessen the possibility of diabetes in individuals at risk. Physical activity could lower the possibility of heart disease and breast and colon cancers. Though the evidence is mixed, physical activity may also provide benefits for a person with osteoporosis.

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

Exercise has forever been touted as the panacea for everything that ails you. For better health, just walk for 20 or thirty minutes a day, boosters say, and you don’t even have to do it all at one time either. 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 lessen your risk of osteoporosis.

If only it were just that simple. Though Exercise has definite benefits, many, if not almost all, of its powers have been oversold. Sure, it can be entertaining. It can give you added energy. And it may increase your mood. But before you turn to a fitness program as the miracle to your particular health or weight concern, think about what science has found.

A good measure of physical activity, such as walking, can decrease the possibility of diabetes in obese and sedentary people whose blood sugar is starting to increase. That outcome was shown in a large federal government research in which individuals 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 less of them went on to develop diabetes.

Physical activity also may decrease the possibility 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 individuals who go from being sedentary to being moderately active, usually by walking regularly. More intense physical activity has been shown to provide only slightly greater benefits. Yet the data from several large researches have not always been clear, because those who exercise seem to be very different from those who don't.

Physically 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 physical activity prevents heart disease or whether individuals who are less likely to get heart disease are also more likely to be physically active.

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

Exercise is normally said to stave off osteoporosis. But 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 usual routine, the exercisers were less likely to fall, possibly because they got stronger or developed better balance. Since falls can lead to fractures in people with osteoporosis, physical activity may prevent broken bones but only indirectly.

And what about losing weight? 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 raise the metabolic rate by only 24 calories a day.

Exercise alone, in the absence of weight loss, has not been found to decrease blood pressure. Nor does it make much difference in cholesterol levels. A decrease in weight 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 problem, 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 associates, An old football coach used to say, ‘I have all my assistants running five miles a day, but they eat 10 miles a day.’