According to a new study, moderate exercise may extend the time a woman has between her menstrual periods. The research supports one theory on why regular physical activity has been linked to a lower breast cancer risk. Some evidence suggests that hormonal changes brought on by exercise may help lower the chances of breast cancer developing.
However, the authors of the new study point out, this evidence comes largely from studies of highly athletic women who get few periods or have stopped menstruating altogether. Whether more-moderate exercise affects the menstrual cycle significantly is still unclear.
Dr. Barbara Sternfeld of Kaiser Permanente of Oakland, California and her colleagues looked at two groups of women who had participated in separate US studies. The nearly 550 women in both studies had provided information on their exercise habits and menstrual cycles.
The researchers found that exercise, particularly vigorous activity, was associated with a somewhat longer menstrual cycle or time between periods. In one of the studies, both total physical activity and vigorous activities were tied to women's menstrual cycle length. But the effect was not seen among significantly overweight women.
In the other study, only daily vigorous exercise was tied to menstrual cycle length, and the add-on time was modest. The researchers estimate that, during a given menstrual cycle, an increase of 20 minutes per day of vigorous exercise would lengthen that cycle by about two-tenths of a day. "These findings lend modest support to the hypothesis that moderate levels of physical activity can lengthen the menstrual cycle," Sternfeld and her colleagues conclude.
Relatively moderate activity, they add, may have hormonal effects that may lengthen the menstrual cycle, resulting, over a lifetime, in lower levels or less cyclic fluctuations of estrogen and progesterone. This could be a mechanism by which regular exercise may lower breast cancer risk, according to the researchers. They stress, however, that they do not know whether the possible hormonal effects seen in exercisers in this study may translate into any "meaningful" health effects.