LINDA W. GEIGER ANDNANCY L. CANOLTY Department of Nutrition, University of California, Davis, California 95616 ABSTRACT The efficiencies with which a line of mice selected for rapid postweaning weight gain utilized the metabolizable energy from nine diets were compared in this study. Protein supplied 14%, 29% or 43% of the metabolizable energy with fat contributing 17%, 38%, or 51%. The percentages of metabolizable energy supplied as carbohydrate ranged from 5% to 68%. Individually housed mice were fed 158, 193, 220, 264, or 308 kcal/kg0-75 daily from 21 to 42 days of age. Energy deposition coefficients, the slopes of linear regression lines relating carcass energy change and metabolizable energy intake, were determined for total carcass, carcass fat and carcass lean. As dietary fat was increased at the expense of carbo hydrate from low to intermediate levels, all three energy deposition co efficients increased when diets contained 14% or 29% protein but did not change when diets contained 43% protein. When fat was increased at the expense of carbohydrate from intermediate to high levels, energy deposi tion coefficients for total carcass and carcass fat remained unchanged when diets contained 14% or 29% protein but both coefficients decreased when diets contained 43% protein. Energy deposition coefficients for carcass lean decreased as fat was increased from intermediate to high levels when diets contained 14% protein but remained unchanged at dietary protein con centrations of 29% or 43%. This study demonstrates in mice selected for rapid postweaning weight gain that the influence upon energy utilization of changing dietary concentrations of fat and carbohydrate depends upon the level of dietary protein. J. Nutr. 108: 1540-1545, 1978. INDEXING KEY WORDS energy utilization •dietary protein • dietary fat •fat energy gain •lean energy gain •fat energy deposi tion coefficient •lean energy deposition coefficient •selection for rapid postweaning weight gain Energetic efficiency is influenced by dietary fat concentration, with high rather than low fat diets being utilized more efficiently (1-5). Forbes et al. (1) showed that tissue energy gains in rats increased

with increasing fat in the diet. When dietary fat was Substituted isocalorically for (rliirncp ti