L-CITRULLINE SUPPLEMENTATION, EXERCISE CAPACITY, AND RELATED PSYCHOPHYSIOLOGICAL RESPONSES DURING EXERCISE IN HEALTHY ADULTS

L-citrulline is a naturally occurring non-essential amino acid that is both endogenously synthesised and obtained from dietary sources. Although it plays a critical role in the urea cycle, L-citrulline is primarily recognised for its contribution to nitric oxide (NO) synthesis, a gaseous signalling molecule generated as a by-product of the conversion of L-arginine to NO. Dietary supplementation with NO precursor L-arginine has shown improved exercise economy and tolerance in healthy adults. However, findings have been inconsistent, likely due to the limited bioavailability of the L-arginine resulting from extensive pre-systemic and systemic metabolism. In contrast, dietary supplementation with L-citrulline can bypass these metabolic barriers, thereby enhancing plasma L-arginine availability and stimulating NO production effectively. As a result, L-citrulline has emerged as a promising nutritional strategy to improve exercise “performance” by increasing NO bioavailability. The scientific literature on the ergogenicity of L-citrulline, however, remains equivocal. Inconsistencies among studies may arise, in part, from differences in exercise protocols and/or variations in supplementation protocols, including dosage. These factors, however, have not been systematically examined, and the occurrence of a dose-response relationship remains still uncertain. To address this knowledge gap, the present thesis investigated the effects of different doses of L-citrulline on exercise tolerance and associated physiological responses during ramp-incremental (Study 1) and constant work-rate cycling tests (Study 2). A group of moderately trained adults visited the laboratory on separate occasions and completed a ramp-incremental cycling test and/or moderate- and severe-intensity step transitions following supplementation of either L-citrulline or energy-matched placebo. Plasma [Orn], [Lys], [Arg], and AAI, blood pressure, and blood lactate were assessed before and after exercise. Pulmonary gas exchange and ventilation and NIRS-derived muscle oxygenation were measured throughout the exercise test. Plasma [Orn], [Arg], and AAI were significantly elevated after L-citrulline supplementation, while blood pressure remained unaltered. Similarly, pulmonary gas exchange and ventilation and muscle oxygenation responses to both ramp-incremental and constant work rate cycling did not differ between conditions. Collectively, the current findings do not support the ergogenicity of L-citrulline under the studied conditions. This result provides valuable evidence concerning the safety and practical applicability of L-citrulline supplementation in healthy, trained adults.