THE NATURE OF THE WEIGHT GAIN INDUCED BY PROGESTERONE IN MICE.

In free-fed mice with progesterone tablet implants and in pseudopregnant mice, an increase in food consumption usually accompanies the body weight gain characteristic of progesterone stimulation; it is not, however, an essential feature since mice on restricted food intakes show similar weight changes. Increases in water consumption usually, but not invariably, accompany the weight gain. The loss of weight after progesterone withdrawal or at the end of pseudopregnancy is accompanied by a reduction in food and water consumed as after parturition or pseudo-parturition. Analyses of the weight gain in progesterone-treated mice with increased food intakes revealed retention of water, fat and protein, together with an increased total energy expenditure of about 10 per cent. Retention of water and protein and increased energy expenditure still occurred with the food intake restricted to sub maintenance levels, i. e. even in the presence of continuous losses of body fat. It is concluded that progesterone fundamentally affects body weight by promoting water and nitrogen retention and, at the same time, increasing energy expenditure. The increase in food intake usually observed is, at least in part, a secondary response to the demands created by the latter two effects and fat deposition occurs also if this response is overadjusted.