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.