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Nutrient Physiology, Metabolism, and Nutrient-Nutrient Interactions

Whey Protein Ingestion Enhances Postprandial Anabolism during Short-Term Bed Rest in Young Men^1–3,

⁴ Department of Clinical, Technological and Morphological Sciences, Division of Internal Medicine, University of Trieste, I-34149 Trieste, Italy and ⁵ German Space Agency-Institute of Aerospace Medicine, D-51147 Cologne, Germany

^* To whom correspondence should be addressed. E-mail: biolo{at}units.it.

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[2-²H₂]tyrosine and L[ring-²H₅]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 µmol·kg^–1·6 h^–1) than after casein ingestion (34 ± 7 µmol·kg^–1·6 h^–1). During bed rest, net postprandial protein synthesis was greater (P < 0.05) after whey (96 ± 8 µmol phenylalanine·kg^–1·6 h^–1) than after casein ingestion (82 ± 7 µmol 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.