The enteric nervous system, or neurons in the digestive tract, affects our mental health: See this article that refers to research on this topic, plus how IBS (Irritable Bowel Syndrome) and osteoporosis can arise from too much serotonin in the digestive tract, and also be related to such conditions as M.S. (multiple sclerosis) and autism, and also the strength of the immune system. This rather new field, called neurogastroenterology, fits very nicely in the holistic model of natural medicine, where we have discussed for years that many conditions are tied to the health of the digestive system. This is especially true in the model of Chinese Medicine, where we strengthen the digestion to treat such diverse conditions as menstrual irregularities, menopausal complaints, headaches, mental and emotional problems, headaches, diabetes, etc. along with IBS, MS, and immune system weakness!
Scientific American article Think Twice: How the Gut’s “Second Brain” Influences Mood and Well-Being