The Effect of Nutrition and Exercise on Menstrual Cycles
Amenorrhea, or absence of menstrual periods, can be due to a lack of appropriate signaling from the brain to the ovaries. The part of the brain that begins the whole reproductive process is the hypothalamus, and it secretes pulses of “releasing factors” that stimulate the nearby pituitary gland to release hormones into the blood which will exert effects on their specific target organs. When the hypothalamus fails to release sufficient quantities of gonadotropin releasing hormone, or GnRH, the pituitary gland fails to secrete enough of the gonadotropin hormones FSH and LH, and the reproductive system essentially turns off. This is akin to a pre-pubertal state, and the woman will not ovulate or menstruate. This so-called hypothalamic amenorrhea (HA) is often associated with excessive physical or mental stress and low body weight and body fat percentage.
A minimal disruption of the normal GnRH secretion can manifest as a luteal phase defect (LPD) in which the amount and duration of progesterone release form the ovary after ovulation is insufficient. With LPD, patients may complain that their luteal phases are short. Sometimes the ovary actually starts to release some progesterone without ovulation, so patients (and doctors!) can be fooled into thinking that there is regular or normal ovulation when this is not the case.
Menstrual irregularity is more common in female athletes than in non-athletic women. While the prevalence of menstrual irregularity in the population is estimated at about 5%, the prevalence in female athletes is close to 80% in some studies. In fact, up to 25% of female athletes experience complete cessation of their periods until such time as they actually stop exercising. Amenorrhea is more prevalent in women participating in exercise activities that result in a low body weight such as ballet, gymnastics and long-distance running.
Body composition is a strong predictor of reproductive health. Women with an increased lean-fat ratio appear to be particularly susceptible to menstrual irregularities. Accomplished runners and ballet dancers have a 15% body fat ratio that is in contrast to the relatively “high” percentage of 20% in swimmers. However, it has been observed that athletic amenorrhea can occur despite stable weight and that women who stop exercising but maintain weight can resume normal menstrual cycles. It’s therefore likely that the metabolic abnormalities resulting from altered body composition contribute to the abnormalities of the GnRH pulse generator.
Recent intriguing studies have demonstrated a clear link between body composition and the reproductive axis via release of a fat tissue secreted hormone called leptin. Leptin exerts a permissive effect on brain production of GnRH, and overly lean women, especially anorectics, have little leptin to facilitate the natural release of GnRH.
It is believed that an energy deficit resulting from low number of fat calories leads to a negative energy balance in athletes, anorectics and women with seemingly unexplained or “functional” HA. This energy imbalance leads to a reduced metabolic rate. Studies of women with FHA compared to controls revealed they have a lower fat body mass, 50% less fat and twice as much fiber consumption. Furthermore, decreased pituitary LH pulse frequency resulted in lower 24-hour LH. Such changes in LH secretion were completely reversed in exercising women when the caloric intake was increased to meet the energy demands despite continued exercise. This suggests that the nutrient restriction and not the increased physical activity per se results in the observed gonadotrophin abnormalities.
It is therefore important to optimize as much as is possible any problematic eating or exercise predilections that may negatively affect reproductive health before more serious interventions are considered.