Caloric balance, brain to body ratio, and the timing of menarche

Caloric availability regulates female reproductive function. Menses do not normally occur until the amount of stored fat is sufficient to meet the nutritional requirements of pregnancy. Unusual energy drains such as heavy exercise cause a reversible cessation of menses. The brain appears to monitor the balance between the availability of calories and their utilization, and reproduction is suppressed when the balance is unfavorable. We postulate that the juvenile human brain, with its unique metabolic requirements, constitutes an important energy drain in the premenarchal girl, and that caloric utilization by the brain may explain the delay of reproductive competence in man. It follows that the changing energy requirements of the brain relative to the body during normal premenarchal growth may play a key role in the timing of menarche. Pathological examples of altered brain to body ratios support this hypothesis: an increase of the brain to body ratio has been observed to delay menarche, a decrease of the brain to body ratio to advance menarche. We explore several clinical implications of this hypothesis and present suggestions for its experimental evaluation.