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By E. Dylan Mayer

As stated in an article from Harvard Health, your brain functions like a high-performance vehicle – it works best when it gets premium fuel. The logic behind this view is that eating high-quality foods packed with vitamins, minerals, polyphenols, fiber and healthy fats will help protect your brain from oxidative stress, allowing it function at optimum capacity.

Even though our brain is incredibly resilient and can fully function even with restricted diets or during times of starvation, it is becoming abundantly clear that nutrition plays a significant role in brain health. Regularly consuming ultra-processed foods and refined sugars is damaging to the brain. Studies have found a connection between these “low-grade fuel” substances and impaired brain function, including worsening symptoms of mood disorders, such as depression.¹

“How Can This Be?”

Surprisingly, much of this connection between what we eat, and our brain health comes down to the microbes in our gut. What we eat has a direct effect on the abundance and diversity of microbes and the molecules they generate, some of which in turn affect how our brain functions. Converging evidence has demonstrated that the brain and the gut microbiota are in bidirectional communication.² Rodent studies suggest a causal role of an altered microbiome in the regulation of brain function and behavior, and a possible role of alterations in these brain gut interactions in common brain and brain-gut disorders including food addiction, anxiety, depression, Parkinson’s disease and Autism Spectrum Disorder (ASD).³ In humans, associations between gut microbial composition and function and several brain disorders have been reported, and fecal microbial transplants from patient populations into germ-free mice have resulted in the reproduction of homologous features in the recipient mice.

A statistic that has been thrown around frequently with the explosion of interest in gut health, is that 95% of the body’s serotonin is produced in your gastrointestinal tract. Serotonin is the key hormone that stabilizes our mood, feelings of well-being and happiness. It would only make sense that our gut plays a key role in our emotions as the vast majority of our “feel-good” hormone is produced and stored there.

“Studying the Connection Between Diet and Health”

Being a reader of the MGC Blog, chances are you’re fully aware of the health benefits of the Mediterranean diet. If you aren’t a fan of this tasty cuisine already, here is just one more reason to adopt it. In the last ten years, there has been an increasing amount of research on the connection between diet and health, creating the new discipline of Nutritional Psychiatry.

It has been known for some time that a diet with a wide variety of fruits, vegetables, nuts, seeds and minimal red meat is healthy for you, and that a diet with high levels of saturated fat, refined carbohydrates and ultra-processed foods can contribute to many chronic health problems, such as metabolic syndrome and heart disease. However, it is quite amazing (at least for many patients and psychiatrists) that research has now demonstrated in both animal and human studies a link between diet and mental health as well. But here is a word of caution when extrapolating from animal studies to patients with mental disorders: The causal relations between diet and mental health are confounded by the clear potential for a phenomenon referred to as “reverse causality”. For instance, alterations in food choices/preferences in response to our mental states (such as “comfort foods” in times of low mood, or changes in appetite from stress) are relatively common human experiences.⁴

A study done by one of the pioneers in the field, Felice N. Jacka (see our MGC interview with Dr. Jacka here), found that dietary improvement may provide an efficacious and accessible treatment strategy for the complementary management of [depression], the benefits of which could extend to the management of common co-morbidities.⁵

“The Science”

A recent meta-analysis confirmed that adherence to a ‘healthful’ dietary pattern, comprising higher intakes of fruit and vegetables, fish and whole grains, was associated with a reduced likelihood of depression in adults. Similarly, another meta-analysis reported that higher adherence to a Mediterranean diet was associated with a 30% reduced risk for depression.⁶ On the other hand, a systematic review confirmed relationships between unhealthful dietary patterns, characterized by higher intakes of foods with saturated fat and refined carbohydrates, and processed food products, with worse mental health in children and adolescents.⁶ Even though there are multiple mechanisms by which an unhealthy diet could play a role in depression, the concept of “inflammatory diets” in the form of high-calorie, high-saturated and large amounts of sugar presents a potential pathway through which the standard American diet may also increase the risk of depression in vulnerable people. Observational studies have already shown that a subgroup of patients with depression score significantly higher on measures of ‘dietary inflammation’, characterized by a greater consumption of ‘pro-inflammatory’ foods (e.g., trans-fats and refined carbohydrates) alongside lower intakes of foods with high nutritional value (thought to have anti-inflammatory properties).⁷

In addition to studies showing a link between diet and depression, there is also growing evidence linking diet and cognitive decline. For example, in a longitudinal investigation, it was discovered that a lower intake of nutrient-dense foods and higher intake of unhealthy foods are each independently associated with smaller left hippocampal volume.⁸ The hippocampus is a brain structure which plays a crucial role in both learning and memory, and reductions in volume have been identified in patients with Alzheimer’s disease.⁸

Another study looking at the gut microbiota of 612 non-frail or pre-frail subjects in European countries before and after the administration of a 12-month long Mediterranean diet intervention, also found promising results. Their results showed that the microbial taxa that were enriched by adherence to the diet, were positively associated with several markers of lower frailty (see a previous blog post on frailty here) and improved cognitive function, as well as negatively associated with inflammatory markers.⁹

“Nutritional Psychiatry Is Here to Stay.”

Based on the growing scientific evidence demonstrating a role of diet in mental health, we strongly believe that the new discipline of Nutritional Psychiatry is here to stay. When combined with other healthy lifestyle measures such as exercise, good sleep and a positive mind state, there is a good potential that it may replace pharmaceutical treatments in many patients or may provide important additional benefits in those patients that do require psychoactive medications.

References

1. www.healthline.com/health/depression/sugar-and-depression#quitting-sugar
2. pubmed.ncbi.nlm.nih.gov/28346923
3. https://pubmed.ncbi.nlm.nih.gov/31853944/
4. J. Firth, Mayer et al. BMJ 2020
5. https://bmcmedicine.biomedcentral.com/articles/10.1186/s12916-017-0791-y
6. https://bmcmedicine.biomedcentral.com/track/pdf/10.1186/s12916-017-0791-y.pdf
7. Firth et al. BMC 2020
8. https://bmcmedicine.biomedcentral.com/track/pdf/10.1186/s12916-015-0461-x.pdf
9. gut.bmj.com/content/69/7/1218

E. Dylan Mayer is a graduate from the University of Colorado at Boulder, with a major in Neuroscience and minor in Business. He is currently completing his master’s degree in Human Nutrition from Columbia University. Dylan is fascinated by the close interactions between nutrition, exercise and human health, especially with regard to the brain-gut-microbiome system – and regularly posts his content on his Instagram (@mayerwellness).