In recent years, there has been an increase in the literature surrounding research into the communication between the gut and the brain moderated by the microbiota, commonly referred to as the microbiota-gut-brain axis.

This communication is modulated by various mechanisms, including nervous, endocrine, and immune pathways⁽¹⁾. The microbiota interacts with the brain through neurologically and biologically active molecules, and can influence the immune system (and modulate inflammation) through microbial-produced metabolites which can also interfere with the intestinal barrier. The molecules produced by the microbiota are also capable of modulating communication with brain structures⁽²⁾. Due to the importance of this communication, it is vital to study the factors capable of influencing the microbiota, such as medication, lifestyle (physical activity, etc.) and genetics, among others. Diet in particular has been shown to be one of the factors with major impact on the microbiota and, consequently, the molecules it produces.

In a review paper by J. Horn et al.⁽¹⁾, which compiles several scientific studies on the subject, the authors highlight the impact of diet on the composition, diversity, and function of the microbiota and its consequent effect on brain structure and function, thereby influencing mental health. The metabolites produced by the microbiota are one of the main focus of the paper, as they can promote either an anti-inflammatory or pro-inflammatory state. For example, a healthy balanced and diversified dietary pattern, like the Mediterranean Diet, might promote the proliferation of bacterial strains that produce anti-inflammatory metabolites, such as short-chain fatty acids, thus preventing metabolic endotoxemia (associated with elevated systemic immune activation markers). This effect can also be potentially achieved through the administration of prebiotics, that is, foods and/or nutrients (such as polyphenols and oligosaccharides) that promote the growth of beneficial bacterial strains.

The metabolites produced by the microbiota can also stimulate serotonin secretion in nerve endings and promote its systemic circulation. The authors highlight that signals from nerve endings can generate responses in brain areas responsible for very important functions, such as appetite regulation, sleep regulation, and even emotional regulation, thus contributing to our mental health.

Beyond individual nutrients, the authors note that a varied diet rich in fibre (through dietary intake of whole grains, vegetables, legumes/pulses and nuts), polyphenols (found in vegetables,  fruits, nuts and extra-virgin olive oil), micronutrients (such as zinc, folate, B vitamins), n-3 polyunsaturated fatty acids (from fatty fish and nuts), and specific amino acids (from protein food sources) can positively modulate microbial composition and their metabolites. This subsequently improves other factors impacting brain health, such as metabolic endotoxemia and inflammation, thereby contributing to better mental health and wellbeing.

Text by Rita Cassilda Ribeiro (nutritionist intern, 4136NE)

References

1. Horn J, Mayer DE, Chen S, Mayer EA. Role of diet and its effects on the gut microbiome in the pathophysiology of mental disorders. Translational Psychiatry. 2022; 12(1):164.
2. Ribeiro G, Ferri A, Clarke G, Cryan JF. Diet and the microbiota – gut – brain-axis: a primer for clinical nutrition. Curr Opin Clin Nutr Metab Care. 2022; 25(6):443-50.
3. Cryan JF, O’Riordan KJ, Cowan CSM, Sandhu KV, Bastiaanssen TFS, Boehme M, et al. The Microbiota-Gut-Brain Axis. Physiol Rev. 2019; 99(4):1877-2013.