Intuitively we know that what is happening in our minds can influence how our guts are feeling. Before sitting an exam we may experience butterflies in our stomach or need to dash to the bathroom to open our bowels, a phenomenon I have recently heard described as a ‘nervous poo’.
The connection is so deeply established it has seeped into everyday language. We talk about having ‘a gut feeling’ or advise a friend to ‘trust their gut’ when faced with a difficult decision.
Over the past decade scientists have made fascinating discoveries suggesting this connection goes deeper than previously thought, and that information can also travel between brain and gut in the opposite direction.
Could what is happening in our bowels influence our mood?
The story begins with our gut microbiota, or gut flora, a huge and complex ecosystem of microorganisms that live deep within the 7 meters of our intestines.
The microbiota help us digest food and form a vital part of our immune system. In recent years, research has shown that it is possible to optimise this legion of tiny creatures by ingesting probiotics, sometimes known as ‘good bacteria.’
These can be naturally occurring, in fermented foods such as yoghurt, or taken as a supplement.
While it had been established that probiotics can benefit our gut health, a team of neuroscientists from University College Cork in Ireland wanted to find out if these bacteria could have any effect on our mental health.
What can we learn from mice?
Their experiment consisted of two groups of mice, one fed a normal diet and the other a diet heavy in probiotics.
The mice underwent a series of tests designed to evaluate their responses to stressful situations. One such test, the forced swim test, is a behavioural study in which mice are placed in a container of water and closely observed.
The test is used in rodents to evaluate antidepressant drugs, with depressed mice typically stopping swimming earlier due to a phenomenon called ‘behavioural despair.’
This occurs when mice lose hope that they will ever escape the stressful situation. In other words, the depressed mice appear to give up earlier.
Interestingly, researchers found mice that had consumed the probiotics continued to swim for significantly longer than mice fed a normal diet, demonstrating less anxiety and more motivation to escape the water.
Not only did these mice behave differently, tests also showed differences within their bodies. Compared to mice fed a normal diet, bacteria-fed mice were found to have just half the blood levels of the stress hormone corticosterone after the swimming test.
These mice were also found to have clear structural changes within their brains.
The presence of the bacteria seemed to cause a redistribution of brain receptors for GABA, one of the brain’s key inhibitory messenger molecules and the site of action for anti-anxiety medications such as Valium.
As one of the study’s authors put it: “These mice were more chilled out."
What does this mean for humans?
In 2016 another team of researchers from University College Cork compared the faeces of a group of people with severe depression to a group of healthy people.
They found the army of microgramisms living within the faeces of healthy people was far more rich and diverse than their depressed counterparts.
However, the truly astonishing part was observed when faeces from both the depressed and healthy patients were transplanted into rats.
The unfortunate rats who received a faecal transplant from the depressed group began to display depression and anxiety-like behaviours, suggesting that in humans too the creatures in our gut may play a key role in the development of depression.
This is exciting news.
Rates of depression are rising and, as a psychiatric doctor, I have seen that for many people anti-depressants are not the answer.
Studies show that for up to 20% of people these medications simply do not work.
Others experience disabiling side effects or are reluctant to take medication due to the stigma surrounding it. Could treating our bowels, rather than our brains, offer an alternative?
A recent research paper analysed ten studies in which humans were given probiotics for a duration ranging from three weeks to six months.
The studies looked at their impact on either mood, anxiety levels or cognition (for example memory tasks). The evidence in favour of probiotics was described as ‘compelling’, with most of the ten studies showing significant improvements in depressive symptoms.
Should we all rush to the yoghurt aisle?
There has been an explosion of interest in this area, with recent studies also suggesting changes to our gut microbiota may also be linked to the development of autism and schizophrenia. Although the evidence for probiotics in depression is promising, there is not currently enough proof for doctors to be able to recommend probiotics.
Numerous unanswered questions remain, not least how exactly these tiny microoganisms are communicating with our brains.
More practical questions include: What type of bacteria are best? How much should we be taking? And how long for? Given that the makeup of microogamisms in our bowels is as unique as our fingerprint, it may be that what works for one person does nothing for the next.
Research suggests that our bowels are complex and mysterious entities that are doing far more than simply digesting yesterday’s dinner.
It seems our gut should be celebrated, rather than hidden away due to it’s propensity to make troublesome noises and smells.
As we begin to unravel the intricate connection between our brain, our bowels and the bugs that live within them it is becoming clearer that to keep us healthy they need to be healthy.
Time will tell if yoghurt truly can treat depression, but in the meantime eating a healthly, varied diet rich in fruit and vegetables will optimise the living conditions for the trillions of microoganisms that call us home.
Dr Lydia Jones