Gut feelings. Microbiome and mental health

For decades, researchers have known of the connection between the brain and the gut. It is well known that anxiety or depression can change appetite or be linked with digestive upset, constipation or diarrhea. It is also known that the gut can produce enough neuro-hormones to be considered a “second brain”.

Yet, until a few years ago, scientists believed the communications between these two organs was one way only (from the brain down to the gut), but new research on the human gut microbiome – the intestinal microbial population – reveals this communication process is like a two ways superfast connection that is similar to many other neural patways. We also know that by changing bacterial populations in the gut it’s possible to modify human behaviour and this is changing the way we understand and treat mental disorders and eating disorders as well.

We know that the exposures of newborns and toddlers in their earliest years is critical to the development of a robust microbiome and the acquisition of intestinal microbiome in the immediate postnatal period has a defining impact on the development and functions of the gastrointestinal, immune, neuroendocrine and metabolic systems. For example, the presence of gut microbiota regulates the set point for hypothalamic-pituitary-adrenal (HPA) axis activity.

Also, animal studies demonstrate that administration of oral antimicrobials to SPF (specific-pathogen-free) mice transiently alter the composition of the microbiome and increase exploratory behavior and hippocampal expression of the neurotrophic factor BDNF which has been implicated in development of anxiety and depression.

If we consider how many antimicrobials are routinely prescribed to our children and new experimental evidence that the baseline stress characteristics of germ-free mice can only be changed until nine weeks of age (and after that, no variety of bacterial additions to the mice’s guts could properly regulate stress and anxiety levels), we should become worried about future numbers of mental illnesses incidence in the population.

Fortunately, there is also evidence that tweaking these bacteria later in life can yield profound behavioral and psychological changes. In a  recent study, anxious mice dosed with the probiotic bacterium Lactobacillus rhamnosus showed lower levels of anxiety, decreased stress hormones, and even an increase in brain receptors for a neurotransmitter that’s vital in curbing worry, anxiety, and fear.

Although plenty of questions remain, the benefits of using probiotics to treat human behavior are becoming increasingly obvious. In 2013 researchers at UCLA showed that healthy women who consumed a drink with four added probiotic strains twice daily for four weeks showed significantly altered brain functioning on an fMRI brain scan; those who had consumed the probiotic drink showed significantly lower brain activity in the neural networks that help drive responses to sensory and emotional behavior.

Until that study was published, the idea that probiotic bacteria administered to the intestine could influence the brain seemed almost surreal–like science-fiction. Well, science yes, but maybe not fiction.  Might people suffering from certain forms of mental health problems benefit from a fecal transplant from someone with more happy-go-lucky bacteria? I don’t know, but this apparent ability of probiotics to affect brain processes is one of the most exciting recent developments in probiotic research.