Energy Metabolism and Substrate Oxidation as Possible Factors for Net Protein Accretion in Growing Rats

Abstract

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. Oxidation of proteins was quantitatively the same in animals on 350 kcal/kg/day and freely fed reference animals, but it was elevated in rats on 270 kcal/kg/day. The energy cost for net protein accretion was higher in rats receiving mainly fat (60 energy%) as nonprotein calories at 270 kcal/kg/day.

The results show that growth rate is determined by factors that are not directly related to the oxidation rate of carbohydrates and fat above energy balance. We suggest that normal growth is achieved only at a certain intake of amino acids. Below the level net protein synthesis starts to fall irrespective of the nonprotein intake. (Journal of Parenteral and Enteral Nutrition 10:463-469, 1986)