Vitamin B6, particularly in its active form pyridoxal 5'-phosphate (PLP), plays a critical biochemical role in the synthesis of serotonin by serving as a coenzyme for the enzyme aromatic L-amino acid decarboxylase. This enzyme catalyzes the conversion of 5-hydroxytryptophan (5-HTP) to serotonin, a key neurotransmitter involved in mood regulation, sleep, and other physiological functions. Multiple clinical and preclinical studies have demonstrated that B6 deficiency can reduce serotonin levels, and in some cases, supplementation may support serotonin synthesis, particularly in individuals with low B6 status.

Evidence exists from both observational studies and small clinical trials indicating that adequate B6 levels are associated with better mood and may reduce depressive symptoms, likely via serotonin pathways. However, direct evidence that supplementation with B6 alone reliably increases central serotonin or consistently improves mood in healthy individuals is limited, and results are mixed. Most scientific validation supports B6's role as a necessary cofactor in serotonin biosynthesis, but not necessarily as an effective stand-alone intervention for serotonin-related disorders unless deficiency is present. Thus, while the use of B6 to support serotonin metabolism is grounded in clear biochemical rationale and some human data, the strength of clinical evidence for supplementation in non-deficient populations remains moderate.