"This is one of the most important questions that exercise scientists can address," says epidemiologist Ralph Paffenbarger, Jr., M.D., Dr.P.H., at Stanford University School of Medicine.

Since many recreational athletes enjoy competition and train hard, we took a careful look for evidence that vigorous physical activity can increase health risks.

We found no consistent and significant evidence that vigorous exercise is linked with disease, unless you have symptoms such as high blood pressure, or have hidden heart disease.

Epidemiology

Moderate activity reduced risk of heart attacks, but when physical activities increased from 2,000 calories a week up to 4,000 calories a week, there seemed to be an increase in the rate of heart attacks, said the first report on the lifestyles and health of Harvard Alumni in 1978.

But "the numbers were not statistically significant and the trend did not appear again in more recent data in this study," says Paffenbarger, who headed the study.

There seemed to be a small increase in death rate in women in the highest fitness group, compared to a moderately fit group, in a 1989 study by Steven Blair, P.E.D., and colleagues at the Cooper Institute for Aerobics Research in Dallas. But, Blair says, "This was not statistically significant, and we haven't seen the trend again in additional data."

Moderate physical activity reduced risk of heart disease in men, but "More vigorous activity does not seem to confer any further protection, and frequent sporting (vigorous) activity may be associated with an increased risk of heart attack," according to statistician Goya Wannamethee, M.Sc., and epidemiologist R.G. Shaper, F.R.C.P., in a 1991 paper from Royal Free Hospital School of Medicine in London, England.

This sounded like a warning, but Wannamethee and Shaper continued working on their data, and in 1994 reported that all the deaths in the highly active men were in a sub-group with high blood pressure.

The risk of heart attack for men with high blood pressure was twice as high for vigorously active men compared to moderately active men. But there was no increase in risk for the most active men compared to moderately active men in the normal blood pressure sub-group.

Summing up, there is no reliable and consistent evidence that the health benefits of moderate physical activities begin to head downhill if you step up to vigorous levels, as long as your blood pressure is normal.

"I do not believe there is evidence that there is a threshold for exercise above which health risks increase," Blair says.

In a new study, in which Blair is collaborating with other laboratories, a comparison between runners consistently logging more than 40 miles a week, and those completing 20 to 40 miles a week, has so far found no increase in cancer and heart disease in the highest-mileage group.

Sudden death

Some people die during vigorous activity. But heart attacks triggered by heavy exertion are more likely in people who are normally sedentary, according to studies led by Murray Mittleman, M.D.C.M., M.P.H., at Harvard Medical School, and by Stefan Willich, M.D., M.P.H., at Free University of Berlin, Germany.

How about those who exercise regularly? The incidence of sudden death is very small, one death per 7,620 joggers, says a review from Paul Thompson, M.D., and colleagues at the Miriam Hospital in Providence, R.I. But if you are going to have a heart attack, you are more likely to have it during or shortly after working out.

However, this doesn't change the fact that moderate and vigorous exercise reduces risk of heart disease in healthy folks.

Sudden deaths in athletes are due either to advanced coronary artery disease, hypertrophic cardiomyopathy (thickening of the heart walls until it just can't pump any more), abnormal or missing arteries, or other rare diseases.

"There is no evidence that exercise hurts the healthy heart," says cardiologist Steven Van Camp, M.D., in San Diego.

Oxygen troubles: free radicals

A small part of the oxygen you breathe in is converted to very active compounds called free radicals. Some of these are so active they damage tissues and genetic material and may lead to numerous diseases, including heart attack and cancer.

Since oxygen uptake increases when you exercise, can these oxidative reactions increase with exercise and lead to increased damage?

The answer is not clear. Some studies report an increase in free radicals during and shortly after exercise. These increases don't seem to be permanent, and resting heart-rate levels can be lower in folks who work out regularly. How could this be? Exercise can also cause an increase in enzymes that defend against free radical reactions.

Consequently, even researchers in the field are tentative in assessing whether exercise really presents oxidative stress problems. Li Li Ji, Ph.D., at the University of Wisconsin in Madison, says, "I don't think that the relationship between free radical production and exercise is firmly established."

"Where is the epidemiological evidence that athletes are harmed by chronic strenuous exercise?" asks Priscilla Clarkson, Ph.D., at the University of Massachusetts in Amherst. "If free radical generation were a problem, one might expect to see a greater incidence of disease related to free radicals in athletes.

"On the contrary, studies have shown that masters athletes are healthier than sedentary controls, and life expectancy has been demonstrated to be greater in various groups of elite athletes than in reference cohorts," Clarkson responds.

She adds: "The more trained an individual is, the more likely they are to be able to counteract an increase in free radicals generated by exercise. The 'weekend warrior,' who may exercise strenuously only on occasion, may be at most risk for oxidative damage to cells."

Although there is uncertainty, some researchers remain concerned that excessive exercise may lead to health risks. Kenneth Cooper, M.D. founder of the Cooper Institute for Aerobics Research and author of the landmark book Aerobics in 1968, discusses possible harm in his new book Antioxidant Revolution.

Cooper says: "I agree that the data is primarily anecdotal and not statistically valid. But we are pursuing studies on whether there is excessive free radical activity in the 'excessive exerciser,' and if there is an effect on antioxidant defense systems."

Overtraining

There is one angle on exercise that everyone agrees is harmful: injury, overtraining and burnout. Regular training improves endurance, strength, and flexibility. The better your training, the more your performance improves. But everyone reaches a point at which further increase without adequate rest becomes stressful.

Overuse of skeletal tissue produces injuries. And even if you avoid injury, sooner or later your performance will go down the tubes; you'll become overtrained, and the only cure is rest.

Although moderate exercise improves your immunity, continuous heavy training, such as that needed to prepare for marathons and triathlons, weakens your immune system. This increases problems such as upper-respiratory infections.

But does this also put you at increased risk of more serious diseases?

Again, we don't know. Overtraining is temporary and seems to be reversed completely by rest, and athletes recover and may even go on to even better performance. Perhaps overtraining is a warning shot, a signal to back off that keep you out of trouble so that rest will let your immune system return to normal, and you'll recover with no long-term ill effects.

Exercise is good

The health benefits of moderate exercise continue as you progress to vigorous or intense exercise. The rare trends in data that suggest otherwise look like artifacts. The trends have not been confirmed, and in most studies did not appear at all.

However, if you have symptoms such as high blood pressure, or a family history of heart disease, then you would be wise to avoid high-intensity efforts at first, and have your doctor monitor your health and exercise program as you progress.

If you are in good health and do not have any risk factors, there does not seem to be any compelling reason why you should not exert yourself as much as you enjoy. But we are all mortal, and there is no such thing as guaranteed protection from disease.

Sudden deaths do occur even in people in good shape. Usually postmortems in these cases reveal hidden disease; very rarely does a heart attack or sudden death remain a puzzle.

For most of us, the psychological benefits and self-esteem earned by an occasional maximal effort far outweigh the tiny risk of serious harm. If you are not happy with this thought, then perhaps you should choose to stay in the moderate zone

Good Luck!