ABSTRACT
Many athletes use dietary supplements with the aim of enhancing adaptations to training and/or competitive performance. One such set of supplements that are widely marketed and available are nitric oxide (NO) precursors. NO is a ubiquitous free-radical gas that is involved in a wide range of signalling and regulatory processes and functions in the human body. The NO synthase family of enzymes comprise the constitutively expressed endothelial (eNOS) and neuronal (nNOS) isoforms, as well as an inducible NOS isoform. eNOS and nNOS are ubiquitously expressed in many tissues, including skeletal muscle fibres and the microvasculature, and catalyze the five-electron oxidation of L-arginine to yield NO and L-citrulline. Oral L-arginine supplementation can increase systemic, but the effects on NO biomarkers and exercise performance in healthy adults is equivocal. Although L-citrulline is co-produced with NO as an end product of NOS activity, it is recognized that L-citrulline can be recycled into L-arginine for subsequent NO generation via the citrulline-NO cycle.
