Serotonin

Serotonin is a neurotransmitter synthesized from the amino acid tryptophan. B vitamins support the conversion, while insulin can favour tryptophan uptake over competing amino acids. Seasonal temperature, food availability, and light influence whether tryptophan feeds serotonin production or alternative pathways such as niacin and NAD.

Serotonin is distributed differently between the gut and brain, so its balance is compartment-specific rather than a matter of simply increasing it. Vitamin D supports brain synthesis through TPH2 while inhibiting gut production through TPH1, and the microbiome and the omega-3 fatty acids DHA and EPA also participate. In darkness, brain serotonin can be converted into melatonin.