Recovery of body weight from below "set point" in mature female rats.

In this study we determined the time course of changes in body composition and of serum concentrations of some hormones in mature rats returning to "set point" following weight loss. Groups of six female rats were fed ad libitum for 10 days. Carcass composition of one group was determined. A control group continued to eat ad libitum. The others were restricted to 5 g per rat per day for 22 days. Carcass composition of groups of rats was determined after 0, 2, 6, 13 and 20 days of refeeding. Of 60 g of body weight lost during restriction, 21 g were fat and 12 g were protein. The fat was replaced by the sixth day of refeeding. Protein and body weight were recovered by the thirteenth day. Calculated efficiency of energy retention was increased until body fat was replete. This was not due to a decrease in metabolism of brown fat, measured in vitro. Serum thyroxine (T4), L-3,3',5'-triiodothyronine (rT3), insulin, corticosterone and free fatty acids did not change with body composition. T3 (L-3,3',5-triiodothyronine) decreased in restricted rats and returned to control levels when body weight, protein and food intake had been restored. These results do not support the concept that body weight is controlled by regulating body fat content.