Today I am going to speak to you on the Female Reproductive System and the menstrual cycle. As you know, following puberty women begin to have regular menstrual cycles generally lasting between 24 — 35 days. Women with normal menstrual cycles (eumenorrhea) will have 12 — 13 periods per year, however circumstances exist when irregular (oligomenorrhea) or absent (amenorrhea) menstrual cycles may be present. Oligomenorrhea is defined as a menstrual cycle lasting between 36 — 90 days, whereas amenorrhea is defined as having no menstrual period for at least three months and having less than three periods in the previous 12 months. These menstrual disturbances are more common in exercising women compared to the general population and are associated with low levels of estrogen (the primary female sex hormone). Suppressed estrogen in physically active women is related to bone loss, and more recently has been shown to increase the risk of cardiovascular disease in young, otherwise healthy women. Thus, maintaining regular menstrual periods and more importantly normal levels of estrogen is essential for a woman's health.
Menstrual disturbances in exercising women was originally believed to be the result of low levels of body fat, where a minimum of 22% was necessary to maintain normal reproductive function. However, some women with body fat levels below this threshold still get normal monthly periods, suggesting that other factors play a role in regulating menstrual cycles. So researchers began to look for other causes of these menstrual disturbances. It soon became evident that women who were in an energy deficit were much more likely to experience irregular or absent menstrual cycles (10). What was also discovered is that there are a host of signals from the body that get sent to a specific region of the brain (the hypothalamus) which communicate the body's energy status. The hypothalamus is responsible for interpreting these signals and then provides instructions for how the body should respond. In addition, this region of the brain is also responsible for regulating the menstrual cycle.
Normal menstrual cycles depend on regular pulses every 60 — 90 minutes of a certain hormone called gonadotropin-releasing hormone (GnRH) from the hypothalamus; without this the menstrual cycle would become irregular or absent (6). This hormone then signals another part of the brain to release two other hormones called lutenizing hormone (LH) and follicle stimulating hormone (FSH). Lutenizing hormone also has a pulse which occurs approximately every 60 — 90 minutes. A decrease in the number of LH pulses is used by researchers to identify disturbances in the hypothalamus (because GnRH very easily).
Researchers have shown that the change in LH pulses was the result of an imbalance between energy intake and the energy expenditure from exercise. They showed that when individuals were able to consume the same amount of calories that they expended during exercise, no disruptions of LH pulses were evident, indicating that disruptions in the brain were not caused by the exercise per se, but rather the energy deficit. The disruptions noted are simply to conserve energy in the female. The bottom line, females need to consume enough calories so as not to promote a deficit which in turn, disrupts the LH pulse, and throws off the menstural cycle.