Originally appeared in the article, "How the Bacteria in Your Gut Microbiome Can Impact Mental Health and Cognitive Functioning," by Alondra Segoviano in Well + Good on May 16, 2022.

If the past few years have taught us anything, it’s the importance of prioritizing our mental health in any way we can, whether it be through self-care practices or taking the occasional mental health day. Surprisingly, another great way to support mental health is by implementing habits that support gut health and the production of good gut bacteria.

Supporting our gut health is important for a handful of reasons. For one, our gut contains the enteric nervous system that’s commonly referred to as “the second brain.” Our gut is also responsible for producing 90 percent of serotonin, which is a mood-stabilizing hormone commonly referred to as the “happiness hormone.” It doesn’t stop there—Uma Naidoo, MD, a Harvard-trained nutritional psychiatrist, professional chef, nutritional biologist, and author of the national bestseller, This is Your Brain on Food shares that the bacteria in our gut can support vitamin production, hormone production, sleep and circadian rhythm, infection control, our mental health, and more.

But how exactly can the bacteria in the gut manage to reach the brain and impact cognitive functioning? We can thank the gut-brain axis (or gut-brain connection)—a bidirectional communication network that connects the gut and the brain—for that. “The gut and brain originate from the same cells in the human embryo to later divide up to form two separate organs that are connected by the vagus nerve,” says Dr. Naidoo.

The vagus nerve represents the main component of the parasympathetic nervous system, which is what helps us manage our moods, immune response, and digestion. “The gut-brain axis is an evolutionary phenomenon that connects the microbiome and the brain, and it shows up in the nervous pathway, endocrine pathway, and immune pathway,” says Ali Rezaie, MD, a California-based gastroenterologist and author of The Microbiome Connection. In simple terms, the gut-brain connection can be thought of as a two-way highway sending text messages back and forth between the gut and brain 24/7, 365 days out of the year.

The gut-brain connection has, of course, left many experts wondering what gut health, specifically gut bacteria, can teach us about mental health. We sat down with Dr. Naidoo and Dr. Rezaie—two of the world's top experts on the matter—to better understand what the current research has to say about the relationship between gut health and mental health.

How gut bacteria and mental health impact one another

“There have been studies that associate depression, anxiety, Alzheimer’s, Parkinson's, [and] all these central nervous system and psychiatry disorders with the gut microbiome,” says Dr. Rezaie. For instance, research suggests that those with low diversity in gut bacteria or an altered microbiome composition are more likely to experience depression and anxiety.

Research has also shined a light on the complex ways that eating certain foods impacts the composition of our gut microbiome, which can in turn affect our mental health. “When healthy, nutrient-dense foods get broken down during the digestive process and their byproducts interact with your gut bacteria, you get short-chain fatty acids, which are great for supporting gut health,” Dr. Naidoo shares. Certain foods, like probiotic and prebiotic foods, can also help in the production of postbiotics, which are key for supporting gut health.

But both Dr. Rezaie and Dr. Naidoo underscore the fact that not all foods will have a positive impact on your gut. In fact, some may leave us feeling mentally worse over time. “Certain fast foods, ultra-processed foods, foods high in added sugars and artificial sugars, and/or processed oils can cause damage to the lining of the gut, [which can] cause more chronic and serious inflammation in the gut that has been associated with depression, anxiety, cognitive disorders, and more,” shares Dr. Naidoo. Same goes with alcohol.

Dr. Naidoo adds that the byproducts that result from the breakdown of these foods can be detrimental to the gut, which can damage the gut lining and cause inflammation in the cells. “Rather than functioning at its peak, bacteria in the gut are interacting with nutrient-void foods in the gut environment, which feeds the bad bacteria more than the good. This can lead to dysbiosis,” she says. Dysbiosis is an imbalance of good and bad bacteria in the gut that’s been shown to contribute to various health issues. “I always say gut inflammation is brain inflammation [due to the gut-brain connection], so when the gut is inflamed over time, that’s going to circle back to the connection to the brain. Unfortunately, neuro-inflammation can be correlated to cognitive disorders,” Dr. Naidoo adds.

While it’s quite clear that what we put in our bodies can impact our gut microbiome (and brain functioning), diet is only part of the puzzle that is understanding how the bacteria in our gut impacts mental health, says Dr. Rezaie. Both experts agree that more research needs to be done to understand exactly how the gut plays a role in mental health, but the future is promising.