Exercise training, menstrual irregularities and bone development in children and adolescents.

Weight bearing physical activity plays an important role in bone development. This is particularly important in children and adolescents since bone mineral density reaches about 90% of its peak by the end of the second decade, and because about one quarter of adult bone is accumulated during the two years surrounding the peak bone growth velocity. Recent studies suggested that the exercise-induced increase in bone mineralization is maturity dependent, and that there is a "window of opportunity" and a critical period for bone response to weight bearing exercise during early puberty and premenarchal years. This supports the idea that increase in physical activity during childhood and adolescence can prevent bone disorders (like osteoporosis) later in life. In contrast, strenuous physical activity may affect the female reproductive system and lead to "athletic amenorrhea". The prevalence of "athletic amenorrhea" is 4-20 times higher than the general population. As a consequence, bone demineralization may develop with increased risk of skeletal fragility, fractures, vertebral instability, and curvature. Menstrual abnormalities in the female athlete result from hypothalamic suppression of the spontaneous pulsatile secretion of gonadotropin releasing hormone. Recent studies suggested that reduced energy availability (increased energy expenditure with inadequate caloric intake) is the main cause of the central suppression of the hypothalamic pituitary-gonadal axis. Therefore, effort should be made to optimize the nutritional state of female athletes, and if not successful, to reduce the training load in order to prevent menstrual abnormalities, and deleterious bone effects in particular during the critical period of rapid bone growth.