Tryptophan

Tryptophan is an essential aromatic amino acid serving as the sole precursor for serotonin, melatonin, and kynurenine pathway metabolites, making it a critical regulator of mood, sleep, immune function, and central fatigue signalling. During intense or prolonged exercise, plasma tryptophan depletion combined with elevated free fatty acids increases brain tryptophan uptake via reduced competition at the large neutral amino acid transporter, increasing serotonin synthesis and contributing to central fatigue. Cervenka et al. (2017) and Schrocksnadel et al. (2006) established tryptophan and its metabolite ratios as key markers of immune activation and neuroendocrine stress.