The Muscle Protein Synthetic Response to Whey Protein Ingestion Is Greater in Middle-Aged Women Compared With Men

Rationale: Muscle mass maintenance is largely regulated by the postprandial rise in muscle protein synthesis rates. It remains unclear whether postprandial protein handling differs between women and men.Methods: Healthy men (43 +/- 3 years; body mass index, 23.4 +/- 0.4 kg/m(2); n = 12) and women (46 +/- 2 years; body mass index, 21.3 +/- 0.5 kg/m(2); n = 12) received primed continuous infusions of L-[ring-H-2(5)]-phenylalanine and L-[ring-3,5-H-2(2)]-tyrosine and ingested 25 g intrinsically L-[1-C-13]-phenylalanine-labeled whey protein. Blood samples and muscle biopsies were collected to assess dietary protein digestion and amino acid absorption kinetics as well as basal and postprandial myofibrillar protein synthesis rates.Results: Plasma phenylalanine and leucine concentrations rapidly increased after protein ingestion (both P <0.001), with no differences between middle-aged women and men (Time x Sex, P = 0.307 and 0.529, respectively). The fraction of dietary protein-derived phenylalanine that appeared in the circulation over the 5-hour postprandial period averaged 56 +/- 1% and 53 +/- 1% in women and men, respectively (P = 0.145). Myofibrillar protein synthesis rates increased (Time, P = 0.010) from 0.035 +/- 0.004%/h and 0.030 +/- 0.002%/h in the postabsorptive state (t test, P = 0.319) to 0.045 +/- 0.002%/h and 0.034 +/- 0.002%/h in the 5-hour postprandial phase in middle-aged women and men, respectively, with higher postprandial myofibrillar protein synthesis rates in women compared with men (t test, P = 0.005). Middle-aged women showed a greater increase in myofibrillar protein synthesis rates during the early (0 to 2 hours) postprandial period compared with men (Time x Sex, P = 0.001).Conclusions: There are no differences in postabsorptive myofibrillar protein synthesis rates between middle-aged women and men. The myofibrillar protein synthetic response to the ingestion of 25 g whey protein is greater in women than in men.