Many young women with apparently normal menstrual periods do not ovulate every month and as a result may lose significant amounts of bone mass annually, according to a study published today.
Until now, doctors have believed that as long as a woman was menstruating she was probably producing enough hormones, chiefly estrogen, to keep her bones strong. Only when a woman exercised to the point of losing periods or reached the age of menopause, doctors thought, did osteoporosis, also known as bone-thinning, set in.
But in a study of the menstrual cycles of 66 healthy women over one year, researchers found that in 29 percent of the cycles, the women either failed to ovulate or had other hormonal abnormalities that caused them to lose bone mass at a surprising rate.
Among the women who failed to ovulate, the rate of spinal bone loss was 4 percent a year, said the researchers. More research is needed to determine whether this rate of bone loss continues year after year.
Osteoporosis is frequently treated with estrogen supplements and is a serious condition because it leads to easily broken bones.
In the study, published in the New England Journal of Medicine, researchers discovered that ovulatory disturbances that resulted in cycles in which no egg was produced or cycles with hormonal defects in the second phase of the menstrual cycle had a stunning effect on spinal bone loss.
The researchers also found that bone loss appears to be linked not merely to overly low estrogen levels, which has been known for years, but also to decreases in another hormone, progesterone, which had not been thought to play an important part in bone preservation.
In the new study, researchers found that exercise, even training for marathons, had no effect on spinal bone loss. Running, the researchers noted, is still considered good for strengthening the long bones of the legs but does nothing to prevent the more insidious loss of spinal bone mass.
Though more studies are needed on the intricate relationships among exercise, abnormal periods and bone loss, the finding "that rapid bone loss occurs in some women with regular cycles is important," commented Dr. C. Conrad Johnston and Dr. Christopher Longcope in an editorial accompanying the research.
It means that "women may be more at risk of bone loss than we used to think," added Dr. Deborah Riester, an endocrine fellow at the New England Medical Center who is conducting an independent study on bone loss in female athletes.
After a year in which all the women in the study underwent periodic blood tests for hormone levels and took their temperatures daily -- a reliable method of determining when and whether ovulation takes place -- the researchers were surprised by their findings.
In 29 percent of the menstrual cycles, the women did not ovulate, which could indicate low estrogen production, or had defects in the second phase of the cycle, a sign that progesterone levels were too low.
If the bone loss associated with these abnormal cycles persisted from age 20 to age 50, women would be at extremely high risk for bone fractures by the time menopause begins, doctors said. So far, however, women entering menopause do not show as much bone loss as the study would predict, said Dr. Longcope, professor of obstetrics and gynecology at the University of Massachusetts Medical School, in a telephone interview.
In addition to the implications for osteoporosis, the study may shed light on another problem facing modern women: infertility. The unexpectedly high prevalence of menstruation without ovulation and other hormonal abnormalities might explain why more women today seem to have trouble getting and staying pregnant, said the lead researcher, Dr. Jerilynn C. Prior, in a telephone interview.
The new research has several practical implications, according to Dr. Prior and others.
"First, if women are ovulating, they don't need to fear that exercise will do something bad to them, even if they run marathons," said Dr. Prior. "Second, women need to pay attention to their cycles to see if they are ovulating."
A woman can do this by taking her temperature every morning before she gets up and by being alert to premenstrual symptoms such as swollen, tender breasts.
Third, women who are not ovulating may consider taking synthetic progesterone during the latter half of their menstrual cycles if X-rays confirm they are losing bone mass, Dr. Prior said.