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
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 out 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
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
However, the truly astonishing part was observed when faeces
from both the depressed and healthy patients were transplanted into
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
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
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
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
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
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