How Your Gut Controls Your Brain

Mental health issues are notoriously complicated, with a huge number of variables, from genetic predispositions to environmental factors, impacting conditions like depression and anxiety. But a growing body of evidence suggests that one factor may have been long overlooked—the connection between the health of the brain and the condition of the bacteria living in the gut.

It's only in recent years that researchers have turned their sights to the importance of understanding the microbiome, the population of bacteria and microorganisms that live alongside humans and within our bodies. It turns out these creatures, which live in our intestines, on our skin, in our noses, and everywhere else on us, do the dirty work for our bodies. To protect their home—that's you—animals in the microbiome do everything from defending their turf by producing anti-inflammatory chemicals that help fend off foreign microbes to making enzymes that help us digest food.

In 2012, the Human Microbiome Project did the first full census of the bacteria, viruses, archaea, and other microorganisms that call us home, turning up more than 10,000 species of tiny creatures—magnitudes of order more than were anticipated. Microbes are so plentiful that we're actually outnumbered in our own bodies. For every one human cell you have, there are about 10 microorganisms present in your body. Because the creatures are so small, though, they only make up a fraction of our mass, three pounds or so—about the same weight as a human brain.

More and more, researchers are finding that the health of this microbe population could be deeply connected to conditions like depression, anxiety, and even autism. Like most work in its early stages, the research is beginning in mice—in this case, what are known as "germ-free" mice.

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Born into sterile conditions, these mice aren't colonized naturally by the organisms that ordinarily form a microbiome. By studying the fate of mice who go through life without certain microbes, researchers can learn more about the roles those missing microbes play. In an increasing number of cases, that role seems to be in regulating anxiety and depression.

"These mice show many deficits that are relevant to mental health," says Dr. Felice Jacka, an associate professor at Deakin University who studies the link between psychiatry and nutrition.

Among other issues, mice denied a proper microbiome have a hard time relaxing. Their stress responses are through the roof and they produce elevated levels of stress hormones. Their ability to produce mood-affecting chemicals like dopamine and seratonin is also impacted.

Not all mouse microbiome studies use germ-free subjects, though. Some have indicated that changes in diet can affect the mood and brain function of animals with normal microbe populations. In 2011, Dr. John Cryan, a professor at University College of Cork and colleagues published a paper in the Proceedings of the National Academy of Sciences showing that mice fed a strain of Lactobacillus bacteria less anxious than those fed a germ free broth.

When put through a maze, bacteria-fed mice were more likely to venture into open areas, and when forced to swim, they took longer to give up than broth-fed mice. They also showed an increase in the concentration of molecular receptors for GABA—the same receptor, Cryan points out, that is targeted by Valium. The work suggests a potential new role for "psychobiotics"—treatments for mental health issues using microbes that produce effects usually associated with drugs. Another study has even shown that swapping the microbiome populations of genetically distinct mice can switch their anxiety levels, making stressed out mice more relaxed.

A 2010 study showed that the same techniques could hold true in humans, suggesting probiotics could reduce stress levels in a small group of test subjects. And stress levels could be among the least of the health factors impacted by the makeup of our microbiomes.

"If all the research done so far in animal models applies to humans," says Rob Knight, a professor of microbiology at University of California San Diego who runs microbiome identification project American Gut, "microbes could affect our choice of mates or of diet, how much we want to exercise, how much our athletic performance can improve when we do exercise, how much pain we feel, how happy or depressed we are, how we respond to different drugs, and even our cognitive function. The prospects for not only restoring microbial deficits but altering our microbiomes to enhance performance, including cognitive performance, are very exciting."

The wheels of research move slowly, though, and it will be some time before the link between microbiome makeup and mental health is fully understood. In the meantime, though, it could be another reason to make sure you're getting enough fiber, which is thought to be conducive to healthy populations of gut fauna. The same goes for food that has been fermented, suggests Jacka.

"Fermented foods, like sauerkraut, kimchi, kombucha, and tempeh are probably very beneficial as they provide many bacteria that seem to be associated with good health," she says. "On the other hand, high fat and sugar diets are particularly noxious to gut health."