The menstrual cycle
Normal menstrual cycles are 26-35 days in length and comprise a follicular and luteal phase separated by ovulation. During the follicular phase a single dominant follicle matures and secretes estrogen. In the late follicular phase higher estrogen levels trigger a surge in luteinising hormone (LH) to induce ovulation. During the luteal phase, the corpus luteum develops and secretes progesterone in preparation for fertilisation, implantation and pregnancy. If fertilisation does not occur, the corpus luteum degenerates and the cycle starts again.
Energy availability and the menstrual cycle
As illustrated in the Female Athlete Triad section, if female athletes and exercising women do not have sufficient energy available for adequate hormonal production it can affect their menstrual cycle. Researchers suggest that a specific threshold of energy availability (≥30kcal.kgFFM.day) appears to be necessary to maintain optimal reproductive function. Low energy availability limits secretion of LH from the pituitary gland and estrogen and progesterone from the ovaries. Low estrogen levels can prolong the follicular phase (i.e. slower maturation of the dominant follicle) and further inhibit the LH surge that occurs prior to ovulation. Dependent on the duration and magnitude of low energy availability, menstrual disturbances develop that range in severity from luteal phase defects and anovulation to amenorrhea.
Subtle and asymptomatic energy related menstrual disturbances include luteal phase defects (LPD) and anovulation. LPD cycles are characterised by a prolonged follicular phase resulting from low estrogen levels and a reduction in the rate and extent of follicular development. Although ovulation still occurs, the luteal phase is subsequently shorter.
The most extreme of energy-related menstrual disturbances is amenorrhea which describes the absence of a menstrual period. Amenorrhea is characterised by constant low levels of estrogen and progesterone that completely suppresse follicular development, ovulation and luteal function. Amenorrhea can be classified as primary or secondary. Primary amenorrhea refers to girls whose menstrual cycles have not begun by age 15 years (particularly common among gymnasts and ballet dancers), and secondary amenorrhea refers to previously menstruating women who have an absence of menstrual periods for > 90 days (i.e. at least three consecutive cycles).
The causes of amenorrhea are numerous including many congenital conditions, pituitary gland tumors, anatomic malformations, and other conditions such as anorexia nervosa and bulimia, as well as polycystic ovarian syndrome. The most common cause of amenorrhea is pregnancy, so a pregnancy test is often the first test to be completed. This may be followed up with blood tests to measure various hormones and/or an ultrasound of the vagina, uterus and ovaries to evaluate for anatomic causes. Clearly, diagnosis is a complex process and thus we encourage concerned athletes to seek professional advice (link here to Professional Support page).
Rigorous research conducted by Mary Jane De Souza and colleagues at Pennsylvania State University has highlighted a high prevalence of energy-related menstrual disturbances in reproductively mature, pre-menopausal women who exercise for at least 2hrs per week at an intensity greater than 55% of maximal heart rate. Specifically, the prevalence of subtle menstrual disturbances including LPD and anovulation may be as high as 50%. This finding is sobering because subtle menstrual disturbances are asymptomatic and thus go unnoticed by female athletes and exercising women. Severe menstrual disturbances (i.e. amenorrhea) may also occur in as many as 33% of these recreationally exercising women. Other researchers have identified subtle menstrual disturbances in 79% of recreational runners and amenorrhea in 65% of well-trained runners.