The Gut-Brain Axis: Literature Overview and Psychiatric Applications
Importance: Literature exploring the relationship between the intestinal microbiome and its effects on general health and well-being has grown significantly in recent years, and our knowledge of this subject continues to grow. Mounting evidence indicates that the intestinal microbiome is a potential target for therapeutic intervention in psychiatric illness and in neurodegenerative disorders such as Alzheimer disease. It is reasonable to consider modulating not just a patient’s neurochemistry, behavior, or cognitive habits, but also their intestinal microbiome in an effort to improve psychiatric symptoms.
Observations: In this review paper, we show that intestinal microbiota possess the ability to directly influence both physical and mental well-being; therefore, should be included in future discussions regarding psychiatric treatment.
Conclusions: Clinicians are encouraged to consider patients’ gut health when evaluating and treating psychiatric conditions, such as anxiety and depression. Optimization and diversification of gut flora through the use of psychobiotics — probiotics that confer mental health benefits — may soon become standard practice in conjunction with traditional psychiatric treatment modalities such as pharmacotherapy and psychotherapy.
The Role of Diet
Although there has been tremendous focus on new and improved therapeutics to address MH conditions, such as depression and anxiety, there also has been renewed interest in the fundamental importance and benefit of a wholesome diet. Recent literature has shown how diet may play a pivotal role in the development and severity of mental illness and holds promise as another potential target for treatment. A 2010 cross-sectional population study of more than 1000 adult women aged 20 to 93 years demonstrated that women with a largely Western dietary pattern (ie, largely composed of processed meats, pizza, chips, hamburgers, white bread, sugar, flavored milk drinks, and beer) were more likely to have dysthymic disorder or major depression, whereas women in this same cohort with a more traditional dietary pattern (ie, composed mainly of vegetables, fruit, lamb, beef, fish, and whole grains) were found to have significantly reduced odds for depression or dysthymic disorder as well as anxiety disorders.45
Several other large-scale population studies such as the SUN cohort study, Hordaland Health study, Whitehall II cohort study, and RHEA mother and baby cohort study have demonstrated similar findings: that a more wholesome diet composed mainly of lean meats, vegetables, fruits, and whole grains was associated with significantly reduced risk of depression compared with a largely processed, high fat, and high sugar diet. This trend also has been observed in children and adolescents and is of particular importance when considering that many psychological and psychiatric problems tend to arise in the formative and often turbulent years prior to adulthood.46
The causal relationship between diet and MH may be better understood by taking a closer look at a crucial intermediate factor: the gut microbiome. The interplay between diet and intestinal microbiome was well elucidated in a landmark 2010 study by De Filippo and colleagues.47 In this study, the microbiota of 14 healthy children from a small village in Burkina Faso (BF) were compared with those of 15 healthy children from an urban area of Florence, Italy (EU). The BF children were reported to consume a traditional rural African diet that is primarily vegetarian, rich in fiber, and low in animal protein and fat, whereas the EU children were noted as consuming a typical Western diet low in fiber but rich in animal protein, fat, sugar, and starch. Comparison revealed that EU children had a higher F/B ratio than their BF counterparts, a metric that has been associated with obesity.47 Furthermore, increased exposure to environmental microbes associated with a fiber-rich diet has been postulated to increase the richness of intestinal flora and serve as a protective factor against noninfectious and inflammatory colonic diseases, which are found to be more prevalent in Western nations whose diets lack fiber. BF children were found to have increased microbial diversity and increased abundance of bacteria capable of producing SCFA relative to their EU counterparts, both of which have a positive influence on the gut, systemic inflammation, and MH.47
Conclusions
Diet has a powerful impact on the intestinal microbiome, which in turn directly impacts our physical and MH in myriad ways. The well-known benefits of a wholesome, nutritious, and well-varied diet include reduced cardiovascular risk, improved glycemic control, GI regularity, and decreased depression. Along with a balanced diet, patients may achieve further benefit with the addition of probiotics.
With regard to psychiatry in particular, increased awareness of the intimate relationship between the gut and the brain is expected to have profound implications for the field. Given this mounting data, immunology, microbiology, and GI pathophysiology should be included in future discussions regarding MH. Their application will likely improve both somatic and mental well-being. We anticipate that newly discovered probiotics and other psychobiotic formulations will be routinely included in a psychiatrist’s pharmacopeia in the near future. Unfortunately, as is clear from our review of the current literature, we do not yet have specific interventions targeting the intestinal microbiome to recommend for the management of specific psychiatric conditions. However, this should not deter further exploring diet modification and psychobiotic supplementation as a means of impacting the intestinal microbiome in the pursuit of psychiatric symptom relief.
Dietary modification is already a standard component of sound primary care medicine, designed to mitigate risk for cardiovascular disease. This exploration can occur as part of otherwise standard psychiatric care and be used as a form of behavioral activation for the patient. Furthermore, explaining the interconnectedness of the mind, brain, and body along with the rationale for experimentation could further help destigmatize the experience of mental illness.
