NEW YORK — The oodles of microbes living in the gut may affect brain function, recent studies suggest.
The human body is home to about 100 trillion bacteria — that means there are about 10 times as many bacterial cells as human cells in your body. Increasing evidence shows these microbes — collectively known as the microbiome — play a role in health, including mental health. Studies in mice suggest that microbes living in the digestive tract are linked to depression and anxiety.
"There's a strong relationship between gastroenterology and psychiatric conditions," said gastroenterologist Dr. Stephen Collins of McMaster University in Canada, at a symposium here at the New York Academy of Sciences. [5 Ways Gut Bacteria Affect Your Health]
Many people with inflammatory bowel syndrome (IBS) have depression or anxiety, Collins said. His research team has found several lines of evidence that intestinal microbes influence the brain.
Collins and his colleagues carried out an experiment in which they kept mice in a dark box with access to well-lit outside areas. Some of the mice were "germ-free," because they were raised in sanitized conditions. The mice were allowed to explore at will. The researchers measured the amount of time all the mice spent outside the box: The more time they spent out exploring, the less anxious they were considered to be.
Compared with normal mice, the germ-free mice spent more time exploring outside the box, and standing on high ledges, a sign of risk-taking, Collins said.
The researchers then gave antibiotics to the mice with normal gut bacteria. The rodents became less cautious or anxious, venturing outside the box more than usual. At the same time, their levels of brain-derived neurotrophic factor (BDNF), a molecule linked to lower depression and anxiety, increased. When the mice stopped receiving antibiotics, their less-adventurous behavior and brain chemicals returned to normal levels.
In another experiment, Collins and his colleagues colonized germ-free mice known to have passive behavior with bacteria taken from mice that exhibit daring behavior. The treated germ-free mice became more active and less cautious, they found. Likewise, when they colonized mice that were normally active with bacteria from passive mice, the animals became more passive.
The findings suggest that intestinal bacteria may somehow affect behavior, making mice more or less anxious. But does that mean gut microbes could affect the human psyche too?
Human bugs and brains
Researchers at UCLA led by gastroenterologist Dr. Emeran Mayer did an experiment to find out. They gave healthy women fermented milk, with either a probiotic supplement, or no probiotic, and scanned their brains while showing them photos of people with emotional facial expressions. The women who were given the probiotic showed a reduced brain response to the faces, compared with the women not given the probiotic, the study found.
Others have speculated that late-onset autism and other brain disorders may also be linked with abnormal gut fauna.
Children with autism have a lot of intestinal problems, said Rosa Krajmalnik-Brown, who studies how microbial communities may benefit human health at Arizona State University, in Phoenix. Krajmalnik-Brown led a study that found that children with autism had fewer types of gut bacteria and lower numbers of a few key microbes, compared with typical children.
It remains unclear exactly how gut bacteria may influence mental health. Researchers have noted that the vagus nerve, controls the rhythmic motions of the digestive tract and sends sensory information back to the brain, could be involved.
More research is needed, however, to solve the puzzle of how stomach bugs influence behavior.