“Our gastrointestinal tracts teem with tens of thousands of species of bacteria. These germs are already known to help regulate the digestive process and to play a role in weight and food cravings. Some scientists are finding, though, that these same microbes can also alter our brain chemistry. Just how this happens is still being figured out, but one pathway might be the vagus nerve, which runs from the brain to the stomach. The bacteria stimulate the vagus nerve, and that, in turn, stimulates the production of various neurotransmitters—the brain chemicals that partly determine what we think and how we feel.”
Source: When Yogurt Affects the Brain — The Atlantic by OLGA KHAZAN.