The effects of progesterone administration on the weight and composition of the body have been studied in rats. Female rats injected with 5 mg. progesterone/day initially gained weight at an average rate of 2 g./day, compared with 0·4 g./day for controls. When treatment was continued for a month or more their weight stabilized at 40–50 g. above the control level. The bodies of the progesterone-treated rats contained increased amounts of water, fat and solids other than fat. These effects were smoothly related to the dose of progesterone.
In terms of percentage composition, fat increased at the expense of the other two constituents. The composition of the fat-free solids did not change, but the proportion of water in the fat-free body increased. About a tenth of the gain of live weight was accounted for by an increase in the contents of the alimentary tract. The composition of the rest was equivalent, typically, to 43% lean tissue, 26% water additional to that in the lean tissue, and 31% fat.
Male rats treated with progesterone showed no changes other than a small gain of water.
It seems likely that in females progesterone reproduces the changes in body composition which occur in pregnancy. The gain of lean tissue seems to reflect increased growth it, and the accumulation of fat, may both be consequences of the production of a positive energy balance.