Main idea: indole production by the gut microbiota may play a role in the onset of mood disorders in humans, urinary excretion of 3-indoxyl sulfate can be a marker for indole production.
Gut microbiota metabolizes tryptophan(an essential amino acid) into indole, which can influence the brain and behavior. Some oxidized derivatives of indole have neuroactive properties. Indole overproduction by the gut microbiota induces an anxio-depressive phenotype in rodents. The aim of the current study was to investigate in humans whether there was a relationship between recurrent depressive symptoms and indole production by the gut microbiota. A case-control study was conducted on 45-to 65-year-old women, who were participants in the observational prospective NutriNet-Santé Study. Urinary excretion of 3-indoxyl sulfate, the major final metabolite of indole, was used as a biomarker of indole production by the gut microbiota. The study found a positive association between urinary 3-indoxyl sulfate concentrations, grouped in tertiles, and recurrent depressive symptoms.