Women who exercise enough to disrupt their menstrual periods may suffer an irreversible loss of strength in their bones that could lead to serious fractures or a kind of premature aging, new studies suggest.

A 24-year-old woman whose periods are irregular can have the bones of a 50-year-old, said Dr. Charles H. Chesnut of the University of Washington. "Once that loss has occurred, it appears there's very little we can do about it."The bone loss can lead to potentially serious spinal fractures and to less serious stress fractures that will keep women from exercising.

Women can regain regular periods by reducing their level of exercise, but the bones don't seem to fully recover, said Chesnut. Recreational athletes, or women who are just trying to stay fit, don't suffer disruption of periods.

The loss or disruption of menstrual periods, known technically as amenorrhea, occurs most in runners, cyclists, swimmers and ballet dancers, said Anne Loucks, a research endocrinologist at the University of California, San Diego, and an authority on the effect of exercise on women.

Researchers don't know why intense exercise affects menstrual periods. "Most people think it's probably a combination of factors," said Barbara Drinkwater, an exercise and environmental physiologist at the Pacific Medical Center in Seattle who has collaborated with Chesnut.

"We want to be very sure that people don't get the idea that exercise causes low bone mass," Drinkwater said. "Exercise is beneficial to bone. It's the amenorrhea that's responsible for the decrease of bone density."

The mechanism of bone loss in amenorrheic women is not completely clear, but researchers say they have a plausible hypothesis: Women who have lost their periods do not produce as much of the female sex hormone estrogen, which leads to calcium loss. So the researchers assume that exercise-related bone loss "is due to the low estrogen levels in the amenorrheic athletes," Loucks said.

The same thing happens after menopause, leading to the disorder called osteoporosis, characterized by loss of calcium and fragile bones.