Rapid maturation of the reproductive axis during perimenarche independent of body composition

The development of the reproductive axis is thought to be a gradual process , but our understanding of the complex endocrine changes that accompany the transition from premenarche to reproductive life in women has been hampere d by the paucity of longitudinal studies. We studied 112 premenarchal Cauca sian females at B-month intervals over 4yr and obtained a detailed reproduc tive and dietary history. We quantified reproductive hormones in 24-h urine collections as a measure of daily output and measured body composition bio metrically and with the use of dual energy x-ray absortiometry scans. The p ercent body fat did not change appreciably in the study period (range, 21-2 4%) and was unrelated to menarche. Sex steroid and gonadotropin levels chan ged exponentially in the year approaching menarche. FSH levels peaked at me narche and then progressively declined thereafter. Estradiol output increas ed rapidly in the year approaching menarche and then plateaued thereafter. The frequency of menstrual bleeding increased rapidly and plateaued at 1 yr postmenarche. At 1 yr, 65% of these adolescent women had established a pat tern of 10 or more menstrual episodes/yr, and by 3 yr postmenarche this fig ure exceeded 90%. There were no significant changes in dietary intake of pr otein, carbohydrate, or fat in the same period. Menarche occurs as a result of rapid maturation of the reproductive axis and heralds the reestablishme nt of a negative sex steroid feedback loop that parallels the adult thresho ld. These events appear to develop independent of changes in body compositi on and diet, but may reflect the improved nutrition and socioeconomic statu s of the late 20th century.