Responses of skeletal muscle protein synthesis and breakdown to levels of dietary protein and fat during growth in weaning rats.

Skeletal muscle growth, muscle nucleic acids, and muscle protein synthesis and breakdown were measured to evaluate the protein requirement of weanling rats. Male Sprague-Dawley rats were fed purified diets that differed in protein and energy contents. Diets ranged from 5 to 30% casein, 5 to 15% corn oil, and all were methionine fortified. All diets were provided for 14 days beginning at 24 days of age. At 38 days of age, animals were killed and the soleus, plantaris, gastrocnemius, extensor digitorum longus (EDL), and biceps brachii muscles and the inguinal, epididymal, and perirenal fat pads were removed and weighed. The soleus and EDL were used to determine protein synthesis and breakdown and nucleic acid contents. Body weight, muscle weight, protein synthesis, DNA and RNA/DNA increased as the casein content of the diet increased to 15%. Casein levels up to 30% failed to cause any additional increases in any of these parameters. These findings indicate that for growing rats maximum protein synthesis defines the minimum about of dietary protein necessary to achieve maximum growth.