Gut health and mental illness: The hidden link

For years, it was believed that gut disorders—such as irritable bowel syndrome (IBS)—were a byproduct of depression and anxiety. However, current research challenges the notion that the relationship between the brain and gut is monodirectional.^[]

While depression and other mental illnesses are primarily psychological, researchers are discovering that poor gut health may play a major role in the development of mental illnesses.

The brain-gut-microbiota axis

Communication between the gut and the brain is possible due to the gut’s microbiota. According to an article published by Frontiers, gut microbiota shapes brain development and influences brain function, while the brain responds through the nervous system and neuroendocrine pathways.^[] The conversation between the gut and brain is often referred to as the “brain-gut-microbiota axis.”

Because this communication is bidirectional, gut-related physiological processes like motility, secretion, and immune function, are partially governed by brain signals. In turn, the gut sends messages that determine mood and reflex regulation by affecting brain function. 

When gut microbiota are altered by chronic stress, the hypothalamic-pituitary-adrenal (HPA) axis causes cortisol secretion to produce a pro-inflammatory response. The blood-brain barrier and intestinal tract become more permeable in response to the cortisol, which initiates communication between the central nervous system (CNS) and the microbiota.

It’s no surprise, then, that neuropsychiatric disorders, such as mood disorders, schizophrenia, autism spectrum disorder (ASD), Parkinson disease, and other chronic illnesses are linked to microbiota and gut health.

Microbiota and mental illness

Depression, anxiety, and other mental disorders affect up to 20% of people at some point in their lives. Of those with depression who pursue psychological or pharmacological treatments, however, 30% to 40% of patients respond inadequately. 

An article published by the American Society for Nutrition looked at the toll depression and anxiety takes on patients and the healthcare system.^[] The impact warrants a closer look at the connections between diet and mood, as well as the brain-gut-microbiota axis’ effect on neurobiology and behavior. Treatments and prevention plans are a global priority.

According to Frontiers, most studies involving patients with major depressive disorder (MDD) found increased levels of Faecalibacterium in the patients’ gut microbiota. Others found that patients with MDD had decreased levels of Bifidobacterium and Lactobacillus.

In patients with bipolar disorder (BD), several studies saw a decrease in Faecalibacterium and one member of the Ruminococcaceae family. Healthier patients with BD had an abundance of the Clostridiaceae family as well as Roseburia, while clinically depressed patients showed higher levels of Enterobacteriaceae. Meanwhile, higher microbial diversity was discovered in patients who experienced mania when compared with healthy controls.

High microbial diversity was also noted in patients who experienced psychosis. Patients with schizophrenia, on the other hand, showed dysbiosis of gut bacteria and an increase in Proteobacteria.

Dietary interventions to consider

In addition to unearthing the nature of the brain-gut-microbiota axis, researchers are investigating the relationship between diet and mood.

In the meantime, the ASN notes the importance of eating an abundance of fruits, vegetables, and whole grains. These foods contain fermentable carbohydrates, polyols, and phytochemicals, all of which are prebiotic compounds. Prebiotics are known for promoting beneficial gut microbes, which may support better health outcomes for patients with MDD.

Sticking to the Mediterranean diet may also help mitigate brain-gut-microbiome illnesses. It has proven to reduce inflammatory and pathogenic bacteria, while promoting beneficial gut bacteria in mice. Gut inflammation is also reduced by plant-based diets.

A small study published in Cell investigated the role of fermented foods and high-fiber foods in gut microbiome diversity.^[] Thirty-six healthy adults were randomly asked to participate in a 10-week diet. Fermented foods composed the majority of one diet, while the other consisted of high-fiber foods.

The group that ate mostly high-fiber foods did not see a reduction in inflammatory proteins or higher microbiome diversity after the trial. 

However, the group that consumed mostly fermented foods showed an increase in microbiome diversity, a decrease in 19 inflammatory proteins, and a decrease in activation of four types of immune cells. 

Some of the featured fermented foods include kombucha tea, kefir, yogurt, fermented cottage cheese, and fermented vegetables.