If you have a bad feeling in the pit of your stomach, it may be more than just intuition. Scientists have found that at least some of the more than 100 trillion bacteria—there are at least 1,000 different species with more than 3 million genes—that inhabit the human digestive tract have some connection to our emotions. In fact, gut microbiota secrete an incredible number of chemicals, including serotonin, dopamine, and gamma-aminobutyric acid (GABA), which are the same substances neurons use to communicate and regulate mood.
Indeed, 80 to 90 percent of all the body’s serotonin, one of the key neurotransmitters responsible for the feeling of wellbeing, exists in the gut.
Stephen Collins, MBBS, FRCP, found in experiments that relaxed mice became more anxious when injected with bacterial strains present in the intestines of their more apprehensive brethren. Similarly, another groundbreaking study published in Neuroscience in 2010 showed that mice became calmer when administered a particular gut bacterium.
“The nervous system to the digestive tract is called the enteric nervous system and is poorly understood,” says Jonathan Finegold, MD, a gastroenterologist at WESTMED Medical Group. “It is postulated, but not completely proven, that the communication of the enteric nervous system with the central nervous system involves neurotransmitters such as serotonin.” Finegold notes that gut bacteria can be helpful “in terms of completing digestion for nutrients and molecules that are unused, but they may also play a role in neuropsychiatric wellbeing and possibly immunity as well.”
In a well-known study that was published in the June, 2013, issue of Gastroenterology, Emeran Mayer, MD, PhD, of UCLA found that brain scans revealed women who ingested a probiotic-containing yogurt twice daily were shown to be dramatically calmer than a control group that did not consume probiotics.
But Dr. Feingold warns that the connection between gut microbes and mood are only beginning to be understood. “Unfortunately, clinical trials have yet to really explain how these probiotics work exactly and what, if anything, they really do in a positive way as opposed to just placebo effect.”
Despite the fact that this area of medicine is still in its earliest stages, Feingold still sees enormous promise. “I think this is the potential dawn of a new era where we will finally get an understanding of the role of bacteria in the bowel and the chemical linkage between the nervous system and the immune system.”