THE COMPOSITION OF THE GAINS IN WEIGHT AND THE UTILIZATION OF FOOD ENERGY IN GROWING RATS

Abstract

The assumption that gain in weight during growth is a measure of the nutritive value of food consumed is challenged. Rats on the same ration and growing approximately at the same rate put on gains differing greatly in composition. The composition of the growth increment was determined by analyses of empty rat carcasses taken after different periods of growth, compared with analyses of litter mates killed at the beginning of the experiment. The amount of energy accounted for by tissue gains was only 6-14% of the metabolizable energy of the food taken. Total energy accounted for by tissue gains plus basal heat production, as computed from the estimated body surface of the rats and the average heat production of rats, was 60-75% of the metabolizable energy of the food; it was lower for femalcs than for malcs of the same litter, indicating greater activity of the females. Evidence is presented that in rats on a maintenance ration the specific dynamic effect of food is inappreciable.