Failure of muscle mass maintenance in older adults is attributed to anabolic resistance, or a weakened protein synthetic response to anabolic stimuli. Leucine, an amino acid, is a key food-related anabolic stimulus. There is a scarcity of data on muscle protein synthesis (MPS) following the administration of a single bolus of leucine in aged population. Additionally, long-term single leucine supplementation does not seem to increase muscle mass.

A study by Dijk et al evaluated the MPS response to the administration of a single bolus of leucine or leucine combined with whey protein, in aged experimental models.

Overnight fasted models were administered an oral gavage with leucine or whey-protein enriched with leucine or 0.5 mL water (fasted control). Investigators assessed amino acid concentrations in plasma and right muscle tibialis anterior (TA). MPS and phosphorylation rate of Akt, 4E-BP1 and p70S6k were assessed in left TA.

In aged models, leucine administration did not increase MPS, despite a 6-fold increase in plasma leucine and elevated muscle free leucine levels. However, leucine-enriched whey protein significantly stimulated MPS. Additionally, muscle free EAA, NEAA and the phosphorylation rate of Akt, 4E-BP1 and p70S6k were found to be significantly enhanced only after administration of leucine-enriched whey protein.

The study thus revealed that leucine-enriched whey protein stimulates MPS in aged models but leucine administration does not do so.

Reference

1. Dijk FJ, van Dijk M, Walrand S, et al. Differential effects of leucine and leucine-enriched whey protein on skeletal muscle protein synthesis in aged mice. ClinNutr ESPEN. 2018 Apr;24:127-133.