WHAT DO WE KNOW?

New scientific studies suggest that the microbiota is affecting areas of the body well beyond the gut, as far as the brain.  The relationship between food and mood is an exciting area of research that has gravitated from observing the effect of individual nutrients on depression to looking at overall dietary patterns. Studies from across the world have shown that following a diet rich in fruits, vegetables and whole grains can be protective of mental health, and possibly making us happier (1).

The opposite has also been suggested to be true with less nutritious and diverse dietary patterns, negatively impacting our mental state. The western style dietary pattern for example, has been associated with an increased risk of depression(2).

New research has hinted at links between the gut microbiome and the brain and these findings have led to speculation that altering the gut microbiome may play a pathophysiological role in human brain diseases. An exciting recent landmark study in Australia, the first randomised control trial of its kind, displayed that Mediterranean dietary advice and nutritional counselling from the dietitian could be an effective strategy for treating depression, though gut microbiome analyses were not undertaken (3).

HOW IS THE GUT MICROBIOME INVOLVED?

Interestingly, the brain and the gut microbiome both weigh about the same, between 1-2kg (4). The composition of our gut microbiome and its diversity of different species and families of bacteria has been suggested to play a role in the development of the brain as well as our behaviour.

FEELING HAPPY

We know that the microbiota in depressed individuals tends not have the usual diversity and richness as those without depression (5). This may play a role in producing symptoms of depression. A 2016 study observed rats who had received faecal transplants from depressed individuals, and promptly developed symptoms of depression, as well as decreased metabolism of tryptophan an essential amino acid involved in the production of the happy hormone serotonin (6). Germ-free mice have also displayed higher traits of anxiety and less motor control (7). While we should be careful to extrapolate the results from mice and rat studies to humans, they do provide us with a small preview of a possible effect.

HUMAN STRESS RESPONSE

The gut microbiome may have an impact on our response to stress as well as mood and psychological disorders, by interfering with the control of neurotransmitters and other signalling molecules within the hypothalamic-pituitary-adrenal axis (the hypothalamus, pituitary and adrenal glands). While most of our gut bacteria have little or no impact on mental health, there are a number which have been identified as having a possible health benefit on our mental state. These bacteria have been named psychobiotics (8).

One example of a strain that has been suggested to have psychobiotic activity is Bifidobacterium Longum 1714 (9). Researchers in Ireland observed that study participants felt less anxious, and less stressed on this psychobiotic, with reduced levels of the stress hormone cortisol and some modestly improved cognitive function. While this is a new and exciting area of research, there is still much more progress to be made before targeted psychobiotic therapies can be provided, with further research required in humans before any definitive conclusions can be made for treating brain disorders or diseases.

WHAT DOES THE FUTURE HOLD?

The relationship between the microbiome and the brain is an emerging field of research still in its early stages, though there appears to be potential for the future treatment of some psychological disorders such as depression, anxiety and Alzheimer’s disease. Some neuroscientists have discussed the explosion of research around the gut microbiome as a paradigm shift, potentially transforming our existing understanding of brain function in health and disease which could offer new treatment therapies. While there is still a long way to go until we have more definite answers, the next few years may uncover intriguing connections between our gut microbiota and our brain that may impact our mental health.