Nutritional Psychiatry and the Microbiota-Gut-Brain Axis
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By MariaLisa Itzoe, DO, MPH
The proverbial saying “you are what you eat” first appeared in English in 1930, attributed to the American nutritionist Victor Lindlahr, though its origins are traced back to 19th century French and German variants, and to the ancient Greek physician Hippocrates’ famous writings. Regardless of when and where exactly the saying emerged, its centuries old presence exhibits a long-standing notion that food plays a critical role in our mental and physical wellbeing. This concept has gained popularity over the last decade alongside the development of nutritional psychiatry: a burgeoning field centered around the science behind how food influences cognitive and psychological function. Dr. Uma Naidoo, a Harvard trained psychiatrist and professional chef describes six pillars of nutritional psychiatry which include the 80/20 rule (80% of diet should be whole, real foods while the remaining 20% allows for flexibility) and the importance of incorporating all different colored plant foods to obtain a variety of nutrients.
“One proposed mechanism by which nutrition impacts mind-body health is through the gut microbiome via the microbiota-gut-brain axis.”
In 2013, The International Society for Nutritional Psychiatry Research (ISNPR) was formed to promote high quality evidence-based research in this area, to help guide healthcare providers with therapeutic options, and to facilitate knowledge sharing with patients. In their consensus statement, ISNPR recognized that whole food dietary modification as well as nutrient-based prescriptions, or nutraceuticals, such as omega-3s, B vitamins, vitamin D, and amino acids, may be used alongside mainstream approaches to treat a variety of psychiatric illnesses from depression and anxiety to schizophrenia. One proposed mechanism by which nutrition impacts mind-body health is through the gut microbiome via microbiota-gut-brain axis.
Diet plays a major role in shaping the gut microbiome, impacting its diversity, abundance, and metabolism. The term psychobiotics has recently been used to describe probiotics (live bacteria that confer health benefits on the host) and prebiotics (which support growth of probiotics) that have been shown in preclinical studies to influence bi-directional communication between the brain and microbiome. Based on these animal studies, it has been suggested that psychobiotics may positively mediate mental health through their effects on the enteric nervous system – which governs gastrointestinal tract function – and central nervous system composed of brain and spinal cord.
“…the combination of [the species of bacteria in someone’s gut] is as unique to them as their fingerprint.”
Over 1000 species of bacteria have been identified in the human gut, though each person harbors on average about 160 species, the combination of which is as unique to them as their fingerprint. Actinobacteria, Bacteroidetes and Firmicutes phyla constitute most of the known beneficial genera including Lactobacillus and Bifidobacterium frequently used in probiotics. Disease associated bacteria include strains of E.Coli, Enterobacteriacae, and an increased Firmicutes:Bacteroidetes ratio, as seen in obesity. The positive and negative effects of various dietary factors on the microbiome has been clearly summarized and potentially modulate gut-brain communication through microbial derived metabolites (i.e. short chain fatty acids or SCFA), neurotransmitters, hormones and the immune system.
One 17-week randomized, prospective study of two microbiome-targeted dietary interventions found that a high fiber diet increased microbiota function (though not diversity), whereas the high fermented food diet increased microbiota diversity and decreased inflammatory markers. Health benefits of fermented foods and high fiber foods including reduction of the systemic inflammatory and immune responses has been discussed in a former MGC post. Increasing fibrous and fermented foods in an obese population was shown to decrease in subjective depression score as well as to increase diversity and abundance of healthy gut bacteria (B. bifidum and S. thermophilius).
“… high adherence to a Mediterranean diet was associated with decreased risk for depression independent of patient age, as well lower risk for cognitive impairment and stroke.”
The traditional Western diet (the Standard America Diet or SAD) high in animal protein, saturated fats, ultra-processed, and high-sugar foods is associated with reduced microbial diversity, shifting the microbiome composition towards a disease associated type. There is also evidence that the SAD is associated with reduced SCFA production which, in animal models, stimulate neuron growth factors, reduce depressive-like behavior, stress responsiveness and intestinal permeability associated with psychosocial stress. Foods with added or refined sugars, industrial seed oils (soy, corn, grapeseed) and nitrate containing processed foods (i.e. deli meats) have inflammatory and anxiety inducing effects. In contrast, a Mediterranean diet characterized by a large proportion of plant-based foods, vegetables, legumes, and fruits, nuts, and olive oil and limited consumption of red meat, saturated fats, and dairy, seems to increase microbial diversity and the abundance of health promoting bacteria (i.e. Bifidobacterium strains). There has also been an association of this diet with increased SCFA production.
One meta-analysis that reviewed 28 studies showed high adherence to a Mediterranean diet was associated with decreased risk for depression independent of patient age, as well lower risk for cognitive impairment and stroke. Adherence to a psychobiotic diet predominantly composed of prebiotic and fermented foods showed a dose-dependent reduction in perceived stress with increased effect linked to stability of microbiota composition over time.
“Dietary modification and nutraceuticals may also positively impact physical health, combating chronic illness such as obesity, diabetes, hypertension, and heart disease.”
In summary, the field of nutritional psychiatry offers promising options to address mental health. Benefits of this therapeutic approach when combined with behavioral treatments like CBT and pharmacologic interventions are that it is non-invasive, easily accessible, and has limited to no side effects unlike many mainstream psychiatric medications. Dietary modification and nutraceuticals may also positively impact physical health, combating chronic illness such as obesity, diabetes, hypertension, and heart disease. One mechanism by which nutrition may exert its effects is through the microbiota-gut-brain axis. More research, specifically on humans, is necessary to further our understanding and application of this fascinating area to clinical practice.
MariaLisa Itzoe, DO, MPH is an Internal Medicine resident at Pennsylvania Hospital of the University of Pennsylvania, with a passion for helping patients who experience disorders of brain-gut interaction. She has a special interest in neuro-gastroenterology and health communication. MariaLisa is also a certified yoga instructor who loves cooking and sharing nutritious meals with family and friends.