Evidence-Based Reviews

Gut microbiota and its implications for psychiatry: A review of 3 studies

Dr. Pastis is Clinical Assistant Professor, Department of Psychiatry, East Carolina University Brody School of Medicine, Greenville, North Carolina. Dr. Saeed is Professor and Chair, Department of Psychiatry and Behavioral Medicine, East Carolina University Brody School of Medicine, Greenville, North Carolina. Dr. Muthukanagaraj is Assistant Professor, Department of Internal Medicine and Psychiatry, East Carolina University Brody School of Medicine, Greenville, North Carolina.

The authors report no financial relationships with any companies whose products are mentioned in this article, or with manufacturers of competing products.




  • Patients who were depressed had lower IgG levels against dairy products compared to controls when there was high dairy consumption. However, there was no overall difference between patients and controls in mean IgG concentration against food products.
  • Patients who were depressed had higher levels of cortisol. Levels of cortisol had a positive correlation with HAM-D-17 score. Patients with depression had lower levels of TNF-alpha.


  • Patients with depression had lower levels of IgG against dairy protein. Patients with depression had high cortisol levels but decreased levels of TNF-alpha, which could explain an immune suppression of IgG in these patients. There were no differences in IL-6 or IL-1beta levels.

Hypercortisolemia is present in approximately 60% of patients with depression. Elevated cortisol levels have a negative effect on lymphocyte function. B-lymphocytes (CD 10+ and CD 19+) are sensitive to glucocorticoids. Studies in mice have demonstrated that elevated glucocorticoid levels are associated with a 50% decrease in serum B-lymphocytes, and this can be explained by downregulation of c-myc protein, which plays a role in cell proliferation and cell survival. Glucocorticoids also decrease levels of protein kinases that are vital for the cell cycle to continue, and they upregulate p27 and p21, which are cell cycle inhibitors. Therefore, if high cortisol suppresses B-lymphocyte production, this can explain how patients with depression have low IgG levels, since B-lymphocytes differentiate into plasma cells that will produce antibodies.6

Depression can trigger an inflammatory response by increasing levels of inflammatory cytokines, acute phase reactants, and oxidative molecules. The inflammatory response can lead to intestinal wall disruption, and therefore bacteria can migrate across the GI barrier, along with food antigens, which could then lead to food antigen hypersensitivity.6

The significance of diet

Many studies have looked into specific types of diets, such as the Mediterranean diet, the ketogenic diet, and the addition of supplements such as probiotics, omega-3 fatty acids, zinc, and multivitamins.7 The Mediterranean diet is high in fiber, nuts, legumes, and fish.7 The ketogenic diet includes a controlled amount of fat, but is low in protein and carbohydrates.7 The main point is that a balanced diet can have a positive effect on mental health.7 The Mediterranean diet has shown to decrease the incidence of cardiovascular disease and lower the risk of depression.7 In animal studies, the ketogenic diet has improved anxiety, depression, and autism.7 Diet clearly affects gut microbiota and, as a consequence, the body’s level of inflammation.7

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