The purpose of this experiment is to investigate whether the dependence of luteinizing hormone pulsatility on energy availability declines during adolescence.

The incidence of menstrual disorders declines during adolescence. This has long been attributed to the gradual "maturation" of the hypothalamic-pituitary-ovarian axis, but the mechanism of this "maturation" is not known. Ovarian function critically depends on the pulsatile secretion of gonadotropin-releasing hormone (GnRH) by the hypothalamus and on the consequent and more readily assessed pulsatility of luteinizing hormone (LH) secretion by the pituitary. LH pulsatility has been shown to depend on energy availability, operationally defined as dietary energy intake minus exercise energy expenditure. The effects of energy availability on LH pulsatility are thought to be mediated by certain metabolic substrates and hormones.

Comparison: By manipulating diet and exercise regimens, contrasting energy availability treatments of 10 and 45 kilocalories per kilogram of fat-free mass per day are being administered to adolescents with 5-8 years of gynecological age and to adults with 14-18 years of gynecological age for five days in the early follicular phase of separate menstrual cycles. Effects of low energy availability on LH pulsatility and on selected metabolic substrates and hormones are being measured.

Inclusion Criteria:

• 5-8 or 14-18 years of gynecological age
• menstrual cycles of 26 to 32 days for the previous 3 months
• luteal phase length >11 days
• 18-32% body fat

Exclusion Criteria:

• smoking
• oral contraceptives
• hematocrit <35%
• diet with <35 or >55 kcal/kgFFM/day of energy
• habitually performing more than 60 minutes/week of aerobic exercise
• histories or evidence of heart, liver, or kidney disease, diabetes, menstrual or thyroid disorders, pregnancy, lactation, and congenital or acquired orthopedic abnormalities