Technical and Theoretic Limitations of the Experimental Evidence Supporting a Gut Bacterial Etiology in Mental Illness.

The impact of gut bacteria on the brain and behavior has become the subject of intense research. The brain is sensitive to biochemical and physiologic changes in the body, for example, changes in blood oxygenation or nutritional status. The collection of microorganisms residing within the digestive tract (the gut microbiome) is increasingly considered a major contributor to human physiology. These 2 considerations have led to the hypothesis that human psychology, including complex constructs like emotion and mental illness, could be influenced by the composition or function of gut bacteria. Five lines of evidence have been used to support the concept, including human correlational research, probiotic supplementation, antibiotic use, germ-free animal research, and fecal transplantation. Results from these experiments do not provide substantial support for the theory that complex human psychology is under the influence of gut bacteria. Placebo-controlled interventional research in humans, in particular fecal microbiota transplantation, will be required before a stronger conclusion can be reached.