Various Macronutrient Intakes Additively Stimulate Protein Synthesis in Liver and Muscle of Food-Deprived Chicks

We investigated the influence of refeeding food-deprived chicks with either protein, carbohydrate, fat or combinations thereof on the rates of liver and muscle protein synthesis. After 2 d of food-deprivation, chicks were given individual or mixed protein, carbohydrate and fat. At 30 min after refeeding, the protein fractional synthesis rate (Ks) was measured by a large dose injection of l-[2,6-3H]phenylalanine. When chicks were food-deprived for 2 d, liver Ks was 67% lower and muscle fractional synthesis rate was half that of well-fed controls. Upon refeeding starved chicks a complete diet, Ks in the liver and muscle returned to the level of fed controls within 30 min. When food-deprived chicks were refed protein alone or two of the three macronutrients, liver and muscle Ks were significantly higher than those in the starved group. There was no effect of refeeding with carbohydrate or fat alone. Plasma glucose concentration was significantly greater than in fed or starved groups in chicks refed the complete diet, carbohydrate or carbohydrate mixed with either protein or fat. Refeeding chicks with the various macronutrients did not affect the plasma insulin or insulin-like growth factor-I concentrations. These results suggest that intakes of individual macronutrients additively increase liver and muscle protein synthesis and that the acute increase in muscle protein synthesis after refeeding chicks diets containing the three macronutrients was mainly regulated by the change in ribosomal efficiency.