The concept of the gut-brain axis—a bidirectional line of communication by which the gut and brain jointly maintain health—is not new, but there is now evidence to suggest that the gut microbiota plays a key role in communication between the gut and the brain. The gut-brain axis links the cognitive and emotional areas of the brain with peripheral gut functions, such as intestinal permeability and immune system stimulation (Del Toro-Barbosa et al. 2020). The gut microbiota may influence brain function through neural, endocrine, and immune pathways, and the brain can in turn utilize the same pathways to influence the composition of the gut microbiota ((Butler et al. 2019). These pathways are generally altered in psychiatric disorders (Mörkl et al. 2020).
There are many factors that impact the gut microbiota, including diet, which is the major determinant of gut microbiota composition in healthy adults (Bambury et al. 2018).
Probiotics are live bacteria that have been shown to support a healthy gut microbiome and thus confer a beneficial effect on the health of the host, when administered in adequate amounts. A novel class of probiotics called psychobiotics has attracted the attention of researchers who are exploring their potential to benefit the mental health of the host when consumed in adequate amounts. Some researchers have expanded the definition of psychobiotics to include not only probiotics but also prebiotics (Bambury et al. 2018), which are indigestible food substances, usually carbohydrates, that support the growth of specific gut bacteria. Fructans and oligosaccharides are the prebiotics most often studied for their psychological effects (Sarkar et al. 2016).
Several trials investigating the effects of probiotics on psychiatric disorders have found that supplementation with Lactobacillus and Bifidobacterium species improved depression symptoms (Mörkl et al. 2020). The composition of the gut microbiome is, in fact, significantly different in people with diagnosed depression compared with healthy controls, though the differences seem to vary from one person to the next (Butler et al. 2019).
Research into the effects of probiotic treatments for anxiety disorders is more limited. Germ-free animals (i.e., those born and raised in a sterile environment) and animals treated with antibiotics have demonstrated increased anxious behaviors. A few studies in healthy human adults have reported reduced stress and anxiety with probiotic supplementation (Choi et al. 2020). However, very few trials have looked at the effects of probiotics in adults with clinically diagnosed anxiety disorders, and additional research is needed (Mörkl et al. 2020).
Though researchers have a general understanding of the means by which psychobiotics interact with the brain, the specific mechanisms are not well understood. One proposed mechanism involves the hypothalamic-pituitary-adrenal (HPA) axis, the neuroendocrine system primarily responsible for the body’s response to stress. The HPA axis regulates the production of cortisol, an immunosuppressant hormone overproduced in chronic stress that increases threat sensitivity and negative mood. The gut microbiota is thought to modulate the HPA axis, thus ameliorating the body’s stress response (Del Toro-Barbosa et al. 2020). Another proposed mechanism of action relates to the gut microbiota’s production of neurotransmitters, including serotonin and dopamine, via the metabolism of indigestible fibers and other prebiotic foods (Sarkar et al. 2016). It is thought that the neurotransmitters produced in the gut act directly on neurons and other cells of the gastrointestinal tract, but the exact mechanism is unclear (Choi et al. 2020).
While psychobiotics appear promising as a possible strategy for alleviating mental illness, research in this area is limited. To date, studies have been able only to establish the existence of a correlation between the gut microbiota and mental health. Many challenges, such as specific strains of bacteria, dose, and viability, remain in understanding the role of psychobiotics on mental health. This is further complicated by the lack of understanding of the mechanism(s) by which the gut microbiota and brain communicate. For now, dietary advice for those suffering from mental illness should focus on consuming a well-balanced diet that includes both probiotic- and prebiotic-containing foods.