Background: Exercise and citrulline (CIT) are both regulators of muscle protein metabolism. However, the combination of both has been under-studied yet may have synergistic effects on muscle metabolism and performance.

Methods: Three-month-old healthy male rats were randomly assigned to be fed ad libitum for 4 weeks with either a citrulline-enriched diet (1 g·kg−1·day−1) (CIT) or an isonitrogenous standard diet (by addition of nonessential amino acid) (Ctrl) and trained (running on treadmill 5 days·week−1) (ex) or not. Maximal endurance activity and body composition were assessed, and muscle protein metabolism (protein synthesis, proteomic approach) and energy metabolism [energy expenditure, mitochondrial metabolism] were explored.

Results: Body composition was affected by exercise but not by CIT supplementation. Endurance training was associated with a higher maximal endurance capacity than sedentary groups (P<0.001), and running time was 14% higher in the CITex group than the Ctrlex group (139±4 min versus 122±6 min, P<0.05). Both endurance training and CIT supplementation alone increased muscle protein synthesis (by +27% and +33%, respectively, versus Ctrl, P<0.05) with an additive effect (+48% versus Ctrl, P<0.05). Mitochondrial metabolism was modulated by exercise but not directly by CIT supplementation. However, the proteomic approach demonstrated that CIT supplementation was able to affect energy metabolism, probably due to activation of pathways generating acetyl-CoA.

Conclusion: CIT supplementation and endurance training in healthy male rats modulates both muscle protein and energy metabolisms, with synergic effects on an array of parameters, including performance and protein synthesis.

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