Further Experiments on the Relation of Fat to Economy of Food Utilization: II. By the Mature Albino Rat

Abstract

Respiration experiments were conducted by the open-circuit Haldane respiratory quotient procedure, with thirty-six mature albino rats as subjects, to investigate the energy expense of utilization (heat increment) of three complete diets differing in fat content and supplying ten vitamins in much larger quantities than in earlier experiments. The heat increments were measured as the difference in heat production from maintenance and supermaintenance diets containing 2, 10 and 30% of fat, respectively, so compounded and fed as to supply equal quantities of gross energy, protein, and vitamins. The fat content of the diets had little effect on nitrogen utilization, but with the supermaintenance food intake there was slight increase in urinary nitrogen and decrease in nitrogen retention in the order of the increasing fat content of the diets. There were slight decreases in metabolizable energy and larger decreases in heat production in the order of the increasing fat contents of the diets, the fat component, therefore, conferring economy of utilization of food energy. The heat increments of the dietary supplements containing 2, 10 and 30% of fat, respectively, representing the use of food above maintenance, with voluntary activity excluded, were equivalent to 31, 26 and 16%, respectively, of their gross energy. The decreasing energy expense of utilization of the isocaloric intake of the diets in the order of their increasing fat contents was due mainly to decreasing heat from the catabolism of carbohydrates. No fat was catabolized from the 2%-fat diet, while much heat was produced from fat in the 30%-fat diet. Heat production from fat synthesis occurred in the three periods in which the carbohydrate intake was highest.