Whey Protein Ingestion Enhances Postprandial Anabolism during Short-Term Bed Rest in Young Men

We tested the relative ability of rapidly digested whey and slowly digested casein to stimulate net whole-body protein synthesis during prolonged physical inactivity. We studied 8 young male Volunteers after they consumed isonitrogenous casein or whey mixed meals on d 12 or d 14 of experimental bed rest. Rates of phenylalanine hydroxylation were measured by primed-constant oral administration Of L[2-H-2(2)]tyrosine and L[ring-H-2(5)]phenylalanine for 3 h in the postabsorptive state and 6 h after an isonitrogenous bolus meal containing Sucrose (0.27 g/kg) and casein or whey (0.40 g/kg). Net protein synthesis in the fed state was calculated during the first 6 h postmeal as the difference between phenylalanine hydroxylation and phenylalanine content in the ingested casein or whey. In the fed state, the integrated changes in phenylalanine hydroxylation were lower (P < 0.05) after whey (-2 +/- 8 mu mol.kg(-1) 6 h(1)) than after casein ingestion (34 +/- 7 mu mol.kg(-1) . 6 h(-1)). During bed rest, net postprandial protein synthesis was greater (P < 0.05) after whey (96 +/- 8 mu mol phenylalanine kg(-1) 6 h(1)) than after casein ingestion (82 +/- 7 mu mol phenylalanine kg(-1). 6 h(-1)). The rapidly digested whey protein was more efficient than the slowly digested casein in increasing postprandial net protein synthesis during short-term bed rest. J. Nutr. 138: 2212-2216, 2008.