Gut feelings: Links between digestion and depression

By Christine Jasoni 15/07/2013


THIS IS A REPOST from the “SOUTHERN GENES” blog that I wrote last week, but figured it should go here instead.  A condensed version appeared in the 2 July 2013 iThink section of the Southland Times.

Can you recall a time when you were nervous about something? The first day of school, a new job, an important speech for work, or maybe you did something you shouldn’t have. Most of us have probably experienced something like this, so you know about ‘butterflies in the stomach’. This happens because when we are stressed, or experience strong emotions, our brain is stressed too. And for some unknown reason our stressed brain sends signals to our digestive system, giving us that queasy feeling.

Even more interesting are peoples’ reports of anxiety or depression that seem to appear at the same time as digestive problems. Or chronic cases of people whose lives are plagued by the duo of mental illness and digestive disorders. The brain and digestive system in partnership may seem a curious relationship, but if we could figure out the details, we may be able to choose foods that can calm both our gut and our brain.

So, how are the digestive system and the brain linked? Studies in animals have shown the importance of microorganisms – creatures, such as bacteria, which are too small to be seen with the naked eye. Within the first few days of life, microorganisms begin to live in our gut. These ‘good bugs’ help our gut function normally, and they act as a defence to keep out the bad bugs that make us sick. But if the good bugs get overwhelmed by bad bugs, we get digestive problems, and sometimes anxiety or depression too1. Mice that are raised in a germ-free environment have no digestive problems, and they are super-resistant to anxiety. What’s more, if normal gut bacteria are introduced to these mice, they show normal anxiety responses, and corresponding changes in brain chemistry3. These observations beg the question: Might we be able to reduce mental illness by ensuring normal gut bacteria? Neuroscience research reported last week is the first to show that this may be possible2.

Healthy women (aged 18-55) were asked to view photos of people with angry or frightened faces. The neural activity of their brains was also recorded using functional magnetic resonance imaging (fMRI) both at rest and during the task. This established the normal brain activity for each person at rest and in response to the emotion-evoking experience. Half of participants were then asked to consume a fermented milk product with probiotic* (think: fancy yoghurt) twice daily for 4 weeks, while the other half consumed a similar yoghurt-like product without the probiotic. The emotionality task and brain monitoring were then repeated and the responses among the groups were compared.

The first rather surprising result was that brain activity was different in women who ate the probiotic when compared with their own brain activity before consuming the probiotic. This suggests that even within an individual, altering diet for as little as 4 weeks can alter brain function. Strikingly, when women who ate the probiotic viewed the emotion-laden photos they showed reduced activity in the emotion and sensation areas of their brains when compared with women who at the normal yoghurt. The authors suggest that the probiotic helped the subjects better to cope with situations that evoke anxiety or negative emotions.

So how is the gut talking to the brain? We already know an awful lot about how the brain controls the gut. This happens through the peripheral nervous system, and also via a direct connection between the brainstem and the gut called the vagus nerve. The vagus nerve is damn important. It controls our diaphragm, our heart and breathing rates, and our pharynx that complicated bit of muscle in our throats that goes through all sorts of contortions when we swallow (and it gets annoyingly filled with snot when we’re sick). Anyway, nerve impulses travel down the vagus (from the brain) and peripheral nerves (from the spinal cord) out into our bodies to control gut motility, so the food passes through our digestive system properly. Turns out, the vagus, and maybe the peripheral nerves too, seems to be able to transmit information back towards the brain. Further details remain to be well worked-out.

So back to the study… Although pretty nifty and one that will undoubtedly have an impact on the field, a couple of key questions remain on both the brain and gut sides of this story. Firstly, there was not a subjective behavioural test of emotionality. So, although the brain scans showed altered neural activity, it was not clear whether that translated into whether the women actually felt better – not as overcome by anxiety or negative emotions – when viewing the emotionally charged photos. Secondly, in situations where people react too strongly to anxiety-evoking situations, then overall dampening of their responses is probably a good thing. But if the subjects were already neurologically normal, then they presumably had emotional responses that were appropriate for the scenario. So might the probiotic actually have a dulling influence? Finally, it will be important to understand the actual biochemical reasons for why this probiotic was able to affect brain function so significantly. Understanding at this basic level will power further research and innovation aimed at developing modern strategies for combatting mental illness.

From both innovation and public good perspectives, this and similar studies provide strong and compelling, not to mention exciting, evidence for the prospect of modulating mental and digestive health through value-added nutritional products and supplements, a cornerstone of one of our new National Science Challenges. It also provides excellent proof of the principle that the brain can be influenced by our diet, and brings an entirely new meaning to the old adage, ‘you are what you eat.’

* Probiotic composition: Bifido- bacterium animalis subsp Lactis, Streptococcus thermophiles, Lactobacillus bulgaricus, and Lactococcus lactis subsp Lactis

References:

1. Foster J-A (2013) “Gut-brain axis: how the microbiome influences anxiety and depression.” Trends in Neuroscience 36: p305.

2. Tillisch K et al. (2013) “Consumption of fermented milk product with probiotic modulates brain activity.” Gastroenterology 144: p1394.

3. Bravo JA (2011) “Ingestion of Lactobacillus strain regulates emotional behaviour and central GABA receptor expression in a mouse via the vagus nerve.” PNAS 108: p16050

@JasoniCL


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