Confession: I enjoy exercise. I played competitive sports in school. Before the current fitness craze, I used to jump rope to relieve stress. Now I do aerobics. I love the pounding music, the kicks and lunges, the L-step, the grapevine. When I miss a couple of sessions, I feel lousy in muscle and mind.

So I am definitely pro-exercise.

But I am anti-exercise-hype.

Not a day goes by that some study does not hail the miracle powers of exercise. Keep on stepping, flexing, jogging--and you will be forever young, hip, sexy and smart. The drumbeat comes not just from magazine images of fitness fanatics with rippling muscles but from earnest health officials who have made 30 minutes a day of physical activity an official recommendation.

According to the government report Healthy People 2000, "a physically active lifestyle is essential for a healthy life." Regular exercise has "protective effects for several chronic diseases, including coronary heart disease, hypertension, noninsulin-dependent diabetes mellitus, osteoporosis, colon cancer, and depression and anxiety."

My friend, who doesn't go to the gym, reads this and now she is afraid that she's going to get sick and that it will be her fault.

I reassure her. Getting sick is far more complicated than just not exercising. Especially with cancer, the exercise connection is particularly loose. And people who suffer from major depression such as manic-depressive illness often have a genetic component that leads to the disorder. Going to aerobics is not going to cure depression or prevent cancer. Failing to exercise is not going to cause these diseases.

The studies, I tell her, suggest that exercise can reduce the risk of developing certain diseases. It may help people manage certain medical problems. That's a good benefit. But there is no guarantee that daily physical activity will prevent or control an illness in a particular individual.

The research is strongest on the link between physical activity and heart disease. In a study of more than 8,000 recreational runners, Paul T. Williams of the Lawrence Berkeley National Laboratory found that long-distance runners significantly reduced their risk of heart disease by lowering their overall cholesterol levels and blood pressure. Men who ran 50 miles a week, for example, had more "good cholesterol," showed greater reductions in blood pressure, and took fewer medications for hypertension and high cholesterol than men who ran 10 miles or fewer a week.

Still, runners who have heart problems--no matter how much they run--need to get medical treatment. Exercise "doesn't replace controlling cholesterol and blood pressure, or not smoking," cautions Williams.

There is also some self-selection going on here. Most runners are in pretty good shape to begin with. Indeed, men who had high amounts of the good cholesterol, HDL, at the start of the study were the ones who ran the greatest distances. In fact, having a high level of HDL was a predictor of who would be a long-distance runner. This suggests a genetic component at work, and the people who do a lot of exercise tend to have a lot of good cholesterol to begin with. Even if they were forced to sit on the sidelines, they might still have a lower risk of heart disease.

In any case, the general message from this research is that you have to exercise pretty vigorously, pretty regularly to see real health benefits. Studies on walking draw a similar conclusion. Women who walk briskly--3 miles per hour or faster--every day for 30 minutes can reduce their risk of heart disease, but slow walkers, though they may enjoy themselves, get no such health benefit.

The distinction between vigorous and moderate exercise sometimes gets lost in public statements on the benefits of physical activity. The recommendations from the Centers for Disease Control and Prevention, the American College of Sports Medicine and the President's Council on Physical Fitness and Sports advocate a lifestyle approach that includes "walking upstairs (instead of taking the elevator), gardening, raking leaves, dancing and walking part or all of the way to or from work."

This easy-does-it guideline is aimed at the large majority of Americans who are "exercise-challenged"--the quarter of adults who are totally sedentary and the more than half who are considered "inadequately active" by exercise researchers. The message takes into account that many people don't like going to exercise classes, or they don't have the time or money to join a fitness center.

Earlier this year, an article in the Journal of the American Medical Association reported the lifestyle approach was as effective in reducing risk factors for heart disease as a structured exercise program at a fitness center.

But researchers such as Williams have criticized the study and question health benefits from modest physical activity. "The message is being sold without the science," he says. Certainly some exercise is better than nothing. But it takes more than a stroll in the park to have a significant impact on health--and even vigorous exercise has its limits.

Don't get me wrong. I like physical activity. I always feel better afterwards. But I am not so naive as to think that because I do aerobics, I will live to be 120.