Energy metabolism and substrate oxidation as possible factors for net protein accretion in growing rats

This study describes an experimental model with growing rats in which we have measured energy expenditure and substrate oxidation as possible factors for regulation of net protein synthesis in intravenously fed growing rats. Orally fed rats were used as a reference group. Rats were given intravenous nutrition for 10 days at a high (350 kcal/kg/day) and a low (270 kcal/kg/day) energy level with four different fat-to-carbohydrate compositions at each energy level: N (no fat), L (low fat, 6% nonprotein calories), M (medium fat, 30% nonprotein calories) and H (high fat, 60% nonprotein calories). Continuous O2-consumption and CO2 production of the animals were measured. Whole body substrate oxidation was calculated from respiratory gas exchange and nitrogen excretion. Body composition was assessed in all animals at the end of infusions. Energy balance with zero growth was reached at 240 kcal/kg/day. Growth rate was normal in animals receiving 350 kcal/kg/day compared with freely eating reference animals irrespective of the glucose-to-fat ratio of the intravenous solutions. Animals on 270 kcal/kg/day showed retarded growth rate, but the differences in growth rate among the groups were mainly explained by carcass fat. Net protein accretion did not differ among most of the groups irrespective of intravenous or oral intake at different levels. The whole body oxidation rate of glucose was directly proportional to the infusion rate of glucose, while the net fat oxidation was inversely correlated to the infusion rate of glucose. The oxidation rate of amino acids and proteins was not directly correlated to the oxidation of glucose or fat.(ABSTRACT TRUNCATED AT 250 WORDS)