5-HTP

[Keith]

The latest news is spore-forming gut bacteria are responsible for serotonin synthesis. It's said 95% of our serotonin is intestinal.
http://www.sciencedaily.com/releases/2015/04/150409143045.htm

The bacteria per the paper are mainly the butyrate producing clostridium clusters IV and XIVa. These probiotics are clostridium butyricum:
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But it's tryptophan which crosses BBB to raise brain serotonin which inhibits glutamate toxicity. So which microbes are responsible for raising tryptophan? It may be a matter of overgrown microbes such as E. coli producing tryptophanase leading to tryptophan depletion.
http://www.pnas.org/content/106/10/3698.full

Bifidobacteria probiotics may be useful to displace that problem:
http://www.ncbi.nlm.nih.gov/pubmed/12612972

 

[Andrew]

Perhaps the most important factor about tryptophan is how it's converted to niacin

Tryptophan metabolism
Deficiency in another B vitamin, niacin, is easily prevented by adequate dietary intakes. The dietary requirement for niacin and the niacin coenzyme, nicotinamide adenine dinucleotide (NAD), can be also met, though to a fairly limited extent, by the catabolism of the essential amino acid tryptophan in the tryptophan-kynurenine pathway (Figure 2). Key reactions in this pathway are PLP-dependent; in particular, PLP is the cofactor for the enzyme kynureninase, which catalyzes the conversion of 3-hydroxykynurenine to 3-hydroxyanthranilic acid. A reduction in PLP availability appears to primarily affect kynureninase activity, limiting NAD production and leading to higher concentrations of kynurenine, 3-hydroxykynurenine, and xanthurenic acid in blood and urine (Figure 2) (9). Thus, while dietary vitamin B6 restriction impairs NAD synthesis from tryptophan, adequate PLP levels help maintain NAD formation from tryptophan (10). The effect of vitamin B6 inadequacy on immune activation and inflammation may be partly related to the role of PLP in the tryptophan-kynurenine metabolism (see Disease Prevention).

https://lpi.oregonstate.edu/mic/vitamins/vitamin-B6

Aromatic l‐amino acid decarboxylase (AADC; dopa decarboxylase; E.C. 4.1.1.28 ) is a PLP‐dependent enzyme that catalyses the production of dopamine and serotonin from l‐3, 4‐dihydroxyphenylalanine (l‐dopa) and l‐5‐hydroxytryptophan (5‐HTP), respectively.

https://onlinelibrary.wiley.com/doi/full/10.1111/j.1471-4159.2010.06742.x