What is the gut-brain connection — and why is it important for overall health?

Traditionally, Western science has treated the mind and the body as separate. But a flood of research on the gut microbiota and our growing understanding of the role it plays in physical health is also having an impact on how we understand mental and cognitive health.

That’s increased interest in learning how a nurturing a diverse gut microbiota can help us be healthier and happier.

Scientists have known for years about the gut-brain axis — the communication between the central nervous system, which has about 100 billion nerve cells (neurons) that communicate with other nerve cells, and the enteric nervous system, which covers the gastrointestinal tract and has about 500 million neurons.

The gut-brain axis links your brain’s emotional and cognitive centers with your intestinal functions.

The vagus nerve is one of the biggest nerves connecting gut and brain, which is clear when we become aware of digestive pain or distress or when stress or anger causes our stomach to “be tied up in knots.”

A more recent twist in our knowledge of the gut-brain connection is the concept of a microbiome-gut-brain axis. Research suggests that gut microbes interact not just with our intestinal cells and the enteric nervous system but also with the central nervous system. The microbiota has even been called the “peacekeeper” between the gut and brain.

Human and animal studies have demonstrated that consuming probiotics — beneficial microbes — from food or supplements can help reduce inflammation, anxiety and signs of distress.

A 2013 study randomized 36 healthy women to one of three groups: probiotic yogurt, non-fermented milk product with no probiotics or no yogurt or milk products. After four weeks of twice-daily consumption, brain scans indicated the women who ate the probiotic- rich yogurt had less of a negative emotional response when shown photos of people who were angry, sad or fearful.

Prebiotic fiber, which feeds gut bacteria, might also influence mental health. Research from the Women’s Health Initiative found that a diet high in refined carbohydrates increased the risk of depression in postmenopausal women. That study found that a diet high in fiber from whole grains, vegetables and whole fruit was associated with a lower risk of depression.

Many people with irritable bowel syndrome — now considered to be a disorder of gut-brain interaction — have increased levels of anxiety and depression symptoms, and about 60% of IBS sufferers report that their first symptoms coincided with increased stress.

Many with IBS experience visceral hypersensitivity: Their perception of pain or discomfort in the intestines is more heightened than normal. People with severe IBS are likely to exhibit alterations to their gut microbiota; people with mild symptoms are not.

Normal development of the gut microbiota is necessary to support brain development after birth and might have long-lasting effects on behavior and cognitive function. A sparse microbiota early in life might be associated with an increased risk of anxiety, autism and IBS, while a sparse microbiota later in life is associated with Alzheimer’s and Parkinson’s disease.

It’snoteworthy that an altered gut microbiome likely plays a central role in the onset of celiac disease, which can produce neurological symptoms including loss of coordination, headache and cognitive dysfunction. The microbiota can trigger production of several neurotransmitters, including serotonin, 95% of which is produced in the gut.

Bacteria in the large intestine ferments dietary fiber to produce short-chain fatty acids, which might improve cognitive function in people with neurological diseases.

Research supports the role of gut bacteria on brain development and function, but most of this research has been done on animals. Information from human studies is limited for reasons including the increased complexity of studying the human microbiome, broader variations in the human diet, environmental influences, genetic variation and the difficulty of measuring subtle changes in human emotional and cognitive function. More research is needed to understand the mechanisms involved in the microbiota-gut-brain axis so scientists can develop therapeutic strategies.

Despite the interest in probiotics, it’s not clear how specific strains of bacteria or combinations of bacteria might be used to target certain health conditions or neurological issues.

But a diet rich in whole plant foods and probiotic-rich fermented foods is a good bet for supporting physical and mental health.

Our diets do influence the composition and health of our gut microbiota, and eating a diet rich in different types and sources of fiber will help support healthy microbial diversity.

Environmental Nutrition is an independent newsletter written by experts on health and nutrition.