Differential metabolic effects of casein and soy protein meals on skeletal muscle in healthy volunteers

Background & aims: Dietary protein intake is known to affect whole body and interorgan protein turnover. We examined if moderate-nitrogen and carbohydrate casein and soy meals have a different effect on skeletal muscle protein and amino acid kinetics in healthy young subjects. Methods: Muscle protein and amino acid kinetics were measured in the postabsorptive state and during 4-h enteral intake of isonitrogenous [0.21 g protein/(kg body weight. 4 h)] protein-based test meals, which contained either casein (CAPM: n = 12) or soy protein (SOPM; n = 10) in 2 separate groups. Stable isotope and muscle biopsy techniques were used to study metabolic effects. Results: The net uptake of glutamate, serine, histidine, and lysine across the leg was larger during CAPM than during SOPM intake. Muscle concentrations of glutamate, serine, histidine, glutamine, isoleucine and BCAA changed differently after CAPM and SOPM (P < 0.05). Muscle net protein breakdown decreased significantly (P < 0.05) to zero during feeding of both CAPM and SOPM, but differences in their (net) breakdown rates were not significant. Muscle protein synthesis was not different between CAPM and SOPM. Conclusion: Moderate-nitrogen casein and soy protein meals differently alter leg amino acid uptake without a significant difference in influencing acute muscle protein metabolism. (C) 2010 Elsevier Ltd and European Society for Clinical Nutrition and Metabolism. All rights reserved.