Combined effect of citrulline and lactoserum on amino acid availability in aged rats

Objective Age-associated sarcopenia is due to anabolic resistance to feeding. Muscle protein synthesis is improved by fast proteins (e.g., lactoserum), which increase peripheral amino acid (AA) bioavailability more rapidly than slow proteins (e.g., casein), and by citrulline. Citrulline, which limits splanchnic sequestration of AA, may more effectively increase peripheral AA bioavailability when combined with lactoserum than with casein when administered as an oral nutritional protein supplement. Methods In this study, 25 fasted aged rats received a single gavage administration of lactoserum or casein 0.4 g/kg, alone or with citrulline 0.4 g/kg, and AA pharmacokinetics, glucose, insulin, triglycerides, and insulin-like growth factor 1 (IGF1) were monitored for 4 h. At 4 h, muscle protein and AA contents and protein synthesis activation were measured. Results While lactoserum was associated with higher AA availability, citrulline exerts only limited effects on the plasma profile of AAs from the two proteins. Maximum plasma citrulline was reached earlier with casein (T90 min) than with lactoserum (T120 min). A protein x citrulline interaction was observed for some plasma and muscle AA levels with a significant activation of mechanistic target of rapamycin complex 1 (mTORC1) signaling suggesting higher anabolism with the combination of citrulline and lactoserum. Lower plasma and muscle AA levels with citrulline and lactoserum compared to lactoserum alone suggest a greater AA utilization in a context of muscle anabolic signaling activation. Conclusion Provision of a citrulline-lactoserum combination as a nutritional supplement could therefore be beneficial in terms of muscle protein balance and prevention of sarcopenia. Further studies are warranted to evaluate the efficacy of this combination.