What We Know About the Gut-Brain Connection — and What We Still Don’t Understand

Our health can be influenced by thousands of different factors, from our diet to our environment to our relationships. But when it comes to our mood and our mental health, research has shown that one system more than almost any other plays an important role: the microbiome.

The microbiome, according to the Harvard T.H. Chan School of Public Health, is made up of the trillions of microorganisms such as bacteria, fungi, parasites and viruses that live in our small and large intestines or "gut."

While your microbiome starts to develop at birth, the microorganisms that populate your gut change over time depending on your diet and other environmental factors, according to a July 2019 article in Genome Biology.

"In our remedies for gut health, we keep making the mistake of valuing quantity over quality."

For example, antibiotics, though lifesaving, can wipe out beneficial bacteria in the gut (along with the infection-causing bacteria). This imbalance within the gut is linked to diseases like obesity, asthma, depression and others, according to a May 2019 Chinese Medical Journal article.

Both the food we eat and the air we breathe can contribute to either a narrow spectrum of bacteria in our guts or a diverse spectrum — and when it comes to a healthy gut, diversity is better, according to Zach Bush, MD, who specializes in internal medicine and endocrinology.

"In our remedies for gut health, we keep making the mistake of valuing quantity over quality," Dr. Bush says. "Chronic use of probiotics, as well as eating processed foods, creates a monoculture in the gut."

Probiotics, according to the National Center for Complementary and Integrative Health, are live microorganisms that, when ingested, can replenish the gut with certain types of "good bacteria."

But ingesting the same kinds of bacteria for long periods of time means there's not enough bacterial diversity, Dr. Bush says — in other words, that "monoculture." A monoculture has been tied to many diseases, according to a May 2016 Molecular Metabolism report, such as obesity and inflammatory bowel disease (IBD).

Our brains get most of the credit, but our gut lining, influenced by a diverse microbiome, actually produces the majority of chemicals responsible for regulating our mood. For instance, 95 percent of the body's serotonin, a chemical that regulates mood and sleep, is produced in the gut, according to the American Psychological Association.

The microbiome is so essential to our mood that researchers at Johns Hopkins refer to it as our enteric nervous system, or our "second brain." The gut can actually send signals to the brain that result in mood changes or even mental health conditions such as anxiety or depression, per Johns Hopkins.

The bacteria that are in your gut — or, perhaps, ones that are not present — can affect your mental health, often referred to as the gut-brain connection.

Researchers looked at the gut health of more than 1,000 people in Belgium, and found that two microbes, Coprococcus and Dialister, were present in lower levels in people with depression, per the February 2019 study published in Nature Microbiology.

There appears to be some sort of two-way relationship between digestion problems and depression.

People with irritable bowel syndrome (IBS) have higher levels of anxiety and depression than those who do not, per an April 2014 systematic review and meta-analysis in the European Archives of Psychiatry and Clinical Neuroscience.

That's true with IBD, too: Depression rates are higher in people with IBD than in their healthy peers, per a March 2017 article in Gastroenterology and Hepatology. Inflammation may contribute to depression, and depression can also drive inflammation, per the article, which describes the relationship between the two conditions as "bidirectional."

When it comes to gut health, what we eat is crucial, Dr. Bush says.

Processed foods reinforce a narrow spectrum of bacteria, per a March 2018 article in Nutrients. In contrast, getting a wide variety of nutrients from fresh, plant-based and fermented foods means more biodiversity for the gut — and could contribute to better mental health.

"Probably the best thing to do in terms of gut health is to get a variety of good bacteria from fermented foods," Dr. Bush says. "Eating miso, kimchi [and] sauerkraut have historically all been an important way to stimulate gut health and microbiome diversity."

It's likely best to get probiotics in your diet through food rather than supplements, Dr. Bush says, since taking the same strain of probiotics through supplements only promotes a monoculture of bacteria. Probiotic supplements are also not rigorously tested or regulated by the FDA, according to the Harvard T.H. Chan School of Public Health, so it's hard to tell whether a product will provide the same quality of bacteria you could get through food.

A deficiency of certain bacterias — or an abundance of others — may play a role in the development of mood disorders such as bipolar disorder (BD), per a small February 2019 study in Bipolar Disorders, which compared gut microbiota composition for people with BD and those who did not have the condition.

Researchers aren't certain whether there's a cause-effect relationship between gut microbiota and disease. What is clear, however, is that some relationship exists and plays a role in many diseases, per a January 2019 review in the Journal of Experimental Medicine.

While we need more research to understand exactly how good bacteria affect our health, we do know that eating a varied diet of fresh and minimally processed foods will help our microbiome diversity — and that appears to be a good thing.