In a perfect physiological environment, the menstrual cycle will occur every month and always be the same duration. But that doesn’t always happen, especially with many girls and women who train with high volumes and high intensities and have a low percentage of body fat. They often experience irregular or even absent menstrual cycles, which reduce estrogen levels. Research has shown that girls who start intense training before their first period delay their menstruation nearly a year later than girls who already have menstrual periods when they start training. Once menstrual activity starts, its continued occurrence is also sensitive to training. In response to heavy training, the first change in the menstrual cycle is a shortening of the luteal phase, followed by cycles without ovulation and, finally, cessation of menses, called amenorrhea, which is defined as having three or fewer periods per year, and results in constantly low levels of estrogen and progesterone. A female runner with amenorrhea has about one-third the estrogen concentration and about 10 to 20 percent the progesterone concentration of a normally menstruating woman. Thus, endocrinologically, an amenorrheic runner experiences an estrogen-deficient state similar to that of a postmenopausal woman.
The incidence of menstrual irregularity or amenorrhea varies from woman to woman. Some female runners can train at high volumes and never disrupt or lose their menstrual cycle, while other women notice changes in their cycles with relatively little training. Long-distance runners, in particular, are at an increased risk for menstrual irregularity or amenorrhea, in large part due to the enormous number of calories that are burned from running high volume. Research has shown that consuming fewer calories than what is burned, rather than the stress of exercise itself, is responsible for the loss of the menstrual cycle, and that consuming enough calories to replace the calories burned from running can prevent amenorrhea. Therefore, if you run a lot, you need to increase the number of calories you consume throughout the day to keep up with the large number of calories you burn by running.
One of the biggest consequences of menstrual irregularity or amenorrhea is poor bone health. Estrogen is extremely important in facilitating the absorption of calcium into bones. Any disruption to the menstrual cycle can cause a decrease in bone density, increasing the risk for osteoporosis and stress fractures, which can occur with only minimal impact to the bones. Female runners with irregular or absent menstruation have significantly lower bone density than runners with regular menstruation and even compared to nonathletes, particularly at the lumbar spine.
How do you train when you don’t have a period, or if your menstrual cycle is irregular? An irregular menstrual cycle makes planning the training more complicated than when not having a menstrual cycle at all, because you can’t predict the months that will have a normal cycle and the months that won’t, unless your cycle is regularly irregular and therefore predictable. If your cycle is irregularly irregular, you need to plan the training month-to-month or even week-to-week.
With no menstrual cycle, you can train without consideration to the hormonal environment, since estrogen and progesterone won’t fluctuate throughout the month. It’s perfectly okay to run a lot without a menstrual cycle (plenty of girls and women do!). However, in the face of a lack of bone-protecting estrogen, you need to take extra precaution in regard to your bone health, especially if you run a lot. It’s a good idea to regularly get your bone density evaluated as part of an annual medical check-up to determine whether or not you’re at risk for a running-related stress fracture. Strategies like meticulously-planned training that avoids rapid increases in volume and intensity, calcium and vitamin D supplements, oral contraception to provide estrogen, and intense strength training to increase bone density can all help mitigate the risk for bone injuries.