The gut–brain axis is the biochemical signaling that takes place between the gastrointestinal tract and the central nervous system. Changes in the composition of the gut flora due to diet, drugs, or disease correlate with changes in levels of circulating cytokines, some of which can affect brain function. The gut flora also release molecules that can directly activate the vagus nerve. Around 70% of people with autism also have gastrointestinal problems, indicating that there may be a connection between autism and gut flora.