Body weight, body fat, and ovulation.

Women with moderate weight loss (10%-15% of ideal weight) have secondary or primary amenorrhea. Athletic women also may have amenorrhea, or anovulatory cycles, though body weight may be in the normal range, since muscles are heavy (80% water compared with 5%-10% water in adipose tissue). The amenorrhea, which is usually reversible with weight gain, decreased exercise, or both, is due to hypothalamic dysfunction; the pituitary-ovary axis is intact, suggesting that this type of amenorrhea is adaptive, preventing an unsuccessful pregnancy. Evidence is presented that the high percentage of body fat (26%-28%) in mature women is necessary for regular ovulatory cycles and may influence reproductive ability directly: (a) as an extragonadal source of estrogen by aromatization of androgen to estrogen, (b) by influencing the direction of estrogen metabolism to more potent or less potent forms, or (c) by changes in the binding properties of sex-hormone-binding globulin. Indirect signals may be abnormal control of temperature and changes in energy metabolism that accompany excessive leanness.