Reprinted from Eating Disorders Review
January/February 1999 Volume 10, Number 1
©1999 Gürze Books
Leptin, a recently discovered product of the human OB (obesity) gene, is emerging as a useful measure of reproductive function. Leptin may well be the hormone that signals the nutritional status of a woman to body centers that regulate reproductive function.
According to a National Institutes of Health study, menstrual function in young premenopausal women declined linearly with a decline in body fat and plasma leptin concentrations. The authors concluded that women are at increased risk of impaired reproduction when their body fat is below 15% and/or plasma leptin concentrations fall below 3 nanograms (ng)/ml (Int J Obesity 21:818, 1997).
Exercise training had no effect
Among the 34 premenopausal women recruited for the study, those who did not participate in any regular aerobic exercise were classified as sedentary. The remaining women were trained and exercised at least 5 hours a week. All menstruating women were studied in the luteal phase (7 to 10 days after ovulation). A fasting blood sample was taken to measure plasma leptin.
Abnormal reproductive function was associated with low body fat and low plasma leptin concentrations. But there was no independent association between plasma leptin levels and menstrual function, and no effect of exercise training was found on plasma leptin concentration or menstrual function, independent of percentage body fat. This lack of effect from exercise is also true in cases of extreme nutritional deprivation, such as anorexia nervosa.
The authors point out that it is important not to assume that critical levels of fatness or of plasma leptin concentration are needed for disruption of the hypothalamic-pituitary gonadal axis, for there was a wide variation of percentage body fat and plasma leptin concentrations in their study. In addition, other hormones, such as insulin and cortisol, also respond to energy deficits. Finally, they note that changes in reproductive function, including anovulatory cycles, can also occur at higher body fat levels, such as 20%, and with plasma leptin concentrations near 5 ng/ml.