Traditionally, Western science has treated the mind and the body as separate entities, but the flood of research on the gut microbiota, and our understanding of the role it plays in physical health, is also having an impact on how we understand mental and cognitive health. It has also increased interest in learning how nurturing a diverse gut microbiota can help us be both healthier and happier.
Scientists have known for years about the gut-brain axis, the two-way communication between the central nervous system (CNS) and the enteric nervous system (ENS). The CNS has about 100 billion neurons (nerve cells) that communicate with other neurons. The ENS, which covers the entire gastrointestinal tract, has about 500 million neurons, so 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 your gut and your brain, also sending signals in both directions. This two-way connection is clear when we become aware of digestive pain or distress, when we have “a gut feeling,” 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 our gut microbes interact directly not just with our intestinal cells and the ENS, but also with the CNS. The microbiota has even been called the “peacekeeper” between the gut and brain.
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Anxiety and depression
Both human and animal studies have demonstrated that consuming probiotics (beneficial microbes) from supplements or food can 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 that 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, may 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 (IBS) — now considered to be a disorder of gut-brain interaction — also have increased levels of anxiety and depression symptoms, and about 60 percent of IBS sufferers report that their first symptoms coincided with increased stress levels. Many people with IBS experience “visceral hypersensitivity” — in other words, their perception of pain or discomfort in the intestines is more heightened than normal. People with severe IBS symptoms are likely to exhibit alterations to their gut microbiota, whereas people with mild symptoms are not.
Microbial diversity and brain health
Normal development of the gut microbiota is necessary to support normal brain development shortly after birth and may have long-lasting effects on behavior and cognitive function. A sparse microbiota early in life may be associated with increased risk of anxiety, autism and IBS while a sparse microbiota later in life is associated with Alzheimer’s and Parkinson’s disease. There is also growing evidence that the origins of schizophrenia and other psychiatric illnesses may lie in early brain development.
It is also noteworthy 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 also 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 may improve cognitive function in various neurological diseases.
Eating for a healthy gut and brain
Research to date supports the role of gut bacteria on brain development and function, but most of this research has been done in animals. Information from human studies is limited for several 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 needs to be done to understand the mechanisms involved in the microbiota-gut-brain axis so that scientists can develop therapeutic strategies.
Despite the interest in probiotics, it’s not yet clear how specific strains of bacteria or combinations of strains might be used therapeutically to target certain health conditions or neurological issues. Until then, a diet rich in whole plant foods, with the addition of probiotic-rich fermented foods, is a good bet for supporting both physical and mental health. The bottom line is that 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 the award-winning independent newsletter written by nutrition experts dedicated to providing readers up-to-date, accurate information about health and nutrition in clear, concise English. For more information, visit www.environmentalnutrition.com.)