The gut, your so-called ‘second brain’, is being recruited in the fight against mental illness.
It has been established for many years that the brain influences gut function; this is why mood disorders such as depression and anxiety are often associated with bowel disorders. However, evidence has emerged substantiating what companies selling ‘probiotic’ foods have long claimed; bacteria in the gut, the so-called ‘second brain’, can influence mental health.
Although the direction of causality is not ascertained, gastrointestinal abnormalities have been recognised for some time to be prevalent in an array of mental health conditions including Autism, child sufferers of which tend to have less diverse gut microbiomes. The microbiome of the gastrointestinal tract, i.e. the 1×1013 microorganisms that inhabit it, impacts cognitive function and fundamental behaviours such as social interaction and stress management. The exact mechanisms by which this occurs are still unclear but are likely associated with signalling to the brain via cells called neurons, chemical signals in the blood called hormones and the immune system which is responsible for recognising and destroying ‘foreign’ bodies such as bacteria. These collective mechanisms influence the brain’s signalling pathways, development, communication with the rest of your body and, thus, behaviour.
Animal models have been used extensively to investigate the gut-brain link. It appears that the microbiome is particularly crucial during early development, impacting upon our behaviour and stress responses. Mice born and maintained in bacteria-free environments were found to exhibit higher levels of risk-taking, elevated levels of the stress hormone cortisol as well as altered levels of the brain chemical BDNF which is involved in human anxiety and depression.
Hsiao and colleagues demonstrated that mice with autistic-type features had lower levels of the gut bacterium Bacteroides fragilis than normal mice, supplementing the levels of which reversed their behavioural abnormalities. This probiotic treatment has also proven to be efficacious in animal depression models.
this field is still in its infancy … many scientists at the forefront of mental health research remain dubious
The evidence of probiotic supplementation’s influence on mental wellbeing is thought to extend to humans. Some bacteria that commonly colonise the human intestines including Bifidobacterium found in some “probiotic” yoghurts produce γ-aminobutyric acid (GABA), which, whilst a waste product for them, is a major signalling molecule in humans, involved in regulating many organs and bodily processes, dysfunctions in its signalling being implicated in many mental health disorders including ADHD, anxiety and depression. GABA is the major inhibitor of signalling cells in the brain called ‘neurons’, thus altering brain activity which promotes relaxation and diminishes anxiety in human subjects. Furthermore, an orally adminstered Lactobacillus helveticus and Bifidobacterium longum combination probiotic has shown significant improvements in depression, anger and anxiety levels as well as reduced levels of the stress hormone cortisol.
However, this field is still in its infancy so don’t start stocking up on yoghurt thinking it’s the key to mental health; many scientists at the forefront of mental health research remain dubious in the face of minimal evidence substantiating the efficacy of oral probiotics in influencing human behaviour. Regardless, these findings should be considered in future experimental design, having found much intra-strain variation in gut bacteria of laboratory mice. New-found understanding of the gut-brain interplay may provide novel therapeutics in treatment and prevention of mental illness based on modulation of gut microbiota.