BOSTON, FEB. 14 -- Regular intensive exercise, which helps protect the body from a host of serious diseases, also appears to impair fertility in women, researchers reported today.

In a major study of more than 5,000 women, researchers at Harvard University found that active athletes often had irregular menstrual cycles and that many were infertile while training.

Fertility normally returned when the women cut back on their exercise, but the Harvard study -- and others -- raise fundamental questions about the health benefits of intensive exercise.

"You just can't have everything," said Rose Frisch, who teaches at the Harvard School of Public Health. "If you are a marathoner or an Olympian and you want a baby, you are going to have to stop. Even lower levels of exercise interfere with the reproductive system of women."

Researchers have for some time been trying to understand exactly how much exercise is needed to benefit a person's health. These results suggest that going beyond sustained regular workouts to more vigorous activity adds little while damaging reproductive ability.

In Frisch's study, which she presented today at the annual meeting of the American Association for the Advancement of Science, she and colleagues examined the health of 5,398 college graduates ranging in age from 21 to 80.

Confirming evidence from previous research, they found that 2,622 former college athletes were far less likely than the 2,776 nonathletes to have developed breast cancer, diabetes or cancers of the reproductive system. The college athletes were more likely to have exercised earlier in life, as well as to have continued after college.

In the study, athletes were defined as women who had trained regularly (at least twice a week) in an energy-intensive sport, such as hockey, swimming, tennis or gymnastics. Runners were included if they ran at least two miles a day, five days a week.

The Harvard study, and one from the Universty of Alberta, suggested that intense physical activity interferes with the production of estrogen, the hormone that controls reproduction in women.

The Canadian study indicated that production of the male reproductive hormone, testosterone, also was impaired by exercise.

Most of those who work out intensively experience changes in the way the brain controls the reproductive system. Sex hormones enter the bloodstream in smaller quantities. For some men and women, that causes infertility. When the exercise level changes, the brain adjusts the sex hormones it delivers to the body.

One study indicated that most women exercising strenuously have fertility abnormalities, even if they continue to menstruate regularly.

"The message to take away is that there probably is a limit to the advantages of strenuous exercise," said Prof. David C. Cumming of the University of Alberta. "As people increase their load of physical activity, we see a clear dampening down of the reproductive system."

But all the researchers speaking at the session on the effects of intensive exercise on reproduction said the benefits of moderate workouts far outweighed the dangers.

In addition to reducing the risk for cancer, men and women who exercise moderately each week have healthier hearts and are less likely to be obese.

The Harvard group said the most likely explanation of the lower cancer risk for the women athletes is related to differences in the amount and type of estrogen in the two groups studied.

Most of the athletes were leaner than their sedentary classmates. Fat makes estrogen, and the fatter a woman is, the more estrogen she is likely to have. Over decades, the lower estrogen level in leaner women may have reduced the growth of cells that can start tumors.

Recent research also suggests that women who have fewer ovarian cycles are at reduced risk for breast cancer. So regular participation in sports that reduce the frequency of menstruation could help prevent cancer.

"Instead of worrying about whether your daughter's period started so late, you should be happy," Frisch said. "If very young girls exercised early on, all their risks of cancer would be vastly reduced."

Frisch said that the average girl in the United States starts menstruating at 12 1/2 years old. But for girls who exercise, it is 15 1/2 years old, the national average 100 years ago.

Girls start menstruating earlier today because they have a higher percentage of fat -- and estrogen -- in their bodies than they did in the 19th century.

Irregular menstruation seems to be more common in women when exercise is accompanied by marked weight loss, a severely restricted diet and stress.

"We are learning now that many of the benefits we used to associate with intense activity also apply to more moderate exercise," said Dr. Tenley Albright, who participated in the Harvard study. "So moderation is the thing we need more of."