Estrus- and ovariectomy-induced body weight changes: evidence for two estrogenic mechanisms

Estrogenic modulation of body weight in female rats is usually thought to result indirectly from estrogenic modulation of food intake. The data presented here suggest, however, that estrogens influence body weight by at least two mechanisms, one of which is independent of changes in food intake. When female rats were ovariectomized (Ovx) and food intake was limited to preoperative levels, the Ovx animals nonetheless gained large amounts of body weight. Although Ovx rats gained more weight than controls on the same amount of food, during 33 hr of food deprivation Ovx and control animals lost body weight at the same rate, which indicates that the prefasting metabolic rates of the two groups were generally similar. Examination of recovery from food deprivation suggested that the rate of weight gain after Ovx was very gradual. During the first 40 days after surgery, the ano-nasal lengths of Ovx rats increased twice as fast as the ano-nasal lengths of the intact rats, which suggests a mechanism for the gradual increase in body weight induced by Ovx. The body weights of intact rats followed a regular 4-day cycle during ad lib feeding but when the estrus-associated decrease in food intake was prevented the cyclic body weight changes were dramatically altered. Thus estrogens appear to regulate body weight by at least two mechanisms: modulation of food intake (cycling females) and modulation of ano-nasal growth or other metabolic processes (Ovx females).