Preliminary research shows that the gut can play a role in boosting serotonin levels. In a study published in the journal Psychoneuroendocrinology of people with major depression, selective serotonin reuptake inhibitor (SSRI) drugs were augmented with supplements of the probiotic strain known as Lactobacillus plantarum, with impressive results.
Those in the probiotic group had improvements in both cognitive symptoms as well as biochemical markers for the disease.