Mechanisms of postprandial protein accretion in human skeletal muscle. Insight from leucine and phenylalanine forearm kinetics.

The relative role of protein synthesis and degradation in de- termining postprandial net protein deposition in human muscle is not known. To this aim, we studied forearm leu- cine and phenylalanine turnover by combining the arterio- venous catheterization with tracer infusions, before and following a 4 h administration of a mixed meal in normal volunteers. Forearm amino acid kinetics were assessed in both whole blood and plasma. Fasting forearm protein deg- radation exceeded synthesis ( P , 0.01) using either tracer, indicating net muscle protein loss. The net negative forearm protein balance was quantitatively similar in whole blood and in plasma. After the meal, forearm proteolysis was sup- pressed ( P , 0.05- , 0.03), while forearm protein synthesis was stimulated ( P , 0.05- , 0.01). However, stimulation of protein synthesis was greater ( P , 0.05- , 0.01) in whole blood (leucine data: 1 50.4 6 7.8 nmol/min 3 100 ml of fore- arm; phenylalanine data: 1 30.4 6 11.6) than in plasma (leu- cine data: 1 17.8 6 5.6 nmol/min 3 100 ml of forearm; phe- nylalanine data: 1 5.7 6 2.1). Consequently, the increment of net amino acid balance was approximately two to fourfold greater ( P , 0.04- , 0.03) in whole blood than in plasma. In conclusion, meal ingestion stimulates forearm protein depo- sition through both enhanced protein synthesis and inhib- ited proteolysis. Plasma data underestimate net postpran- dial forearm protein synthesis, suggesting a key role of red blood cells and/or of blood mass in mediating meal- enhanced protein accretion. ( J. Clin. Invest. 1996. 98:1361- 1372.) Key words: postprandial stateforearm protein syn- thesisforearm proteolysisleucine oxidation