Longitudinal changes in reproductive hormones and menstrual cyclicity in cynomolgus monkeys during strenuous exercise training: Abrupt transition to exercise-induced amenorrhea

Cross-sectional studies of exercise-induced reproductive dysfunction have d ocumented a high proportion of menstrual cycle disturbances in women involv ed in strenuous exercise training. However, longitudinal studies have been needed to examine individual susceptibility to exercise-induced reproductiv e dysfunction and to elucidate the progression of changes in reproductive f unction that occur with strenuous exercise training. Using the female cynom olgus monkey (Macaca fascicularis), we documented changes in menstrual cycl icity and patterns of LH, FSH, estradiol, and progesterone secretion as the animals developed exercise-induced amenorrhea. As monkeys gradually increa sed running to 12.3 +/- 0.9 km/day, body weight did not change significantl y although food intake remained constant. The time spent training until ame norrhea developed varied widely among animals (7-24 months; mean = 14.3 +/- 2.2 months) and was not correlated with initial body weight, training dist ance, or food intake. Consistent changes in function of the reproductive ax is occurred abruptly, one to two menstrual cycles before the development of amenorrhea. These included significant declines in plasma reproductive hor mone concentrations, an increase in follicular phase length, and a decrease in luteal phase progesterone secretion. These data document a high level o f interindividual variability in the development of exercise-induced reprod uctive dysfunction, delineate the progression of changes in reproductive ho rmone secretion that occur with exercise training, and illustrate an abrupt transition from normal cyclicity to an amenorrheic state in exercising ind ividuals, that is not necessarily associated with weight loss.