Younger women with subtle menstrual cycle abnormalities may experience bone loss in their 20s and 30s that makes them more likely to develop osteoporosis after menopause, researchers say.

"This could be an important clue about why some women have abnormal loss of bone and get fractures after menopause and some women don't," Dr. Jerilynn Prior said Wednesday about results of a study of ovulation and bone loss in 66 premenopausal women between the ages of 21 to 42.The study, which involved two groups of women runners and one group of normally active women, also found that strenuous exercise alone apparently did not cause menstrual problems, said Prior, of the University of British Columbia in Vancouver, Canada.

Some previous research found an apparent association between long-distance running in young women and loss of menstrual cycles. But Prior said she and colleagues whose report appeared in The New England Journal of Medicine believe this was due to undernourishment or pre-existing abnormalities and not the effects of exercise.

In that respect, "this is good news for women athletes," she said, noting the researchers found the marathon runners in the study no more likely than others to have bone loss linked to menstrual cycle changes.

In addition, Prior said the research indicates the hormone progesterone may play a far more important role than previously thought in preventing bone loss in women.

The excessive bone loss that characterizes osteoporosis affects about one-fourth of all women who are beyond menopause, and thus whose ovaries no longer produce estrogen or progesterone. It is estimated the disorder causes 1.3 million fractures annually.

The new study involved 21 marathon runners who ran an average of about 85 miles per month, 22 women who ran less intensively and 23 women who had normal levels of activity. The women, who had an average age of 34, were matched in height, weight and caloric intake. None took birth control pills or smoked.

All of the women had two apparently normal menstrual cycles in a row prior to starting the yearlong research. But during the study, Prior said only 13 had entirely normal cycles. Twenty-nine percent of all cycles involved lack of ovulation or unusually short periods between ovulation and the onset of menstruation, during which the ovaries produce progestrone.