The link between gut microbes and the brain was suspected over 20 years ago. At the time, it was dismissed as little more than a curiosity. Times have changed.
“95% of the body’s serotonin is manufactured and stored in the gut.” Dr Felice Jacka
We’re only 10 per cent human. Over 100 trillion microbes live on and in us, and 99.5 per cent of our genetic material is microbial. That makes our bacterial backpackers (our microbiome) almost an organ of sorts.
Today, it seems almost every area of medicine is looking into the role of the microbiome and its relation to health. This is largely because science now has the tools to identify the microbiota and understand what they do.
The intriguing notion that gut bugs can influence mood and behaviour is one of the most exciting areas of research.
What shapes your gut bacteria?
The makeup of your gut bacteria, or microbiota, is influenced by a number of factors. Your age, where you live, how stressed you are, what drugs you take, but above all, by your diet.
Associate Professor Dr Felice Jacka is a Principal Research Fellow within the Deakin University School of Medicine based at Barwon Health in Geelong. She is one of the world’s leading researchers into the connection between diet and mental health.
“The main pathway by which diet exerts its impact on mental health is via the microbiota,” she says.
Your microbiota is sensitive to the type of food you consume, explains Dr Jacka. Fibre, probiotics (think yoghurt) and prebiotics (certain carbohydrates that fuel good bacteria) will all positively impact on the makeup of the bugs in your gut. Excessive fat and animal food products, on the other hand, can have a negative impact.
Where’s the evidence?
Evidence that gut bacteria can influence anxiety and depression initially came from animal studies.
- Strains of two bacteria (lactobacillus and bifidobacterium) were found to reduce anxiety-like behaviour in mice. Humans also carry the same strains of bacteria in their guts.
- Bacteria collected from a strain of mice prone to anxious behaviour were transplanted into other mice inclined to be calm. The result? The calmer mice became more anxious.
- Both bifidobacterium and a widely used antidepressant were equally as effective at reducing levels of stress hormones in mice.
Studies in humans have since followed.
- Probiotics were found to reduce psychological distress and cortisol levels in healthy volunteers.
- Patients with chronic fatigue syndrome experienced improved psychological symptoms when given probiotics.
- Fermented milk products with probiotics had a significant calming effect on the women who consumed them.
- A daily capsule of probiotic bacteria helped healthy male volunteers to better cope with mild anxiety and memory problems.
- Volunteers given certain prebiotics (carbs that bacteria love) showed lower levels of a key stress hormone, cortisol.
5 ways your gut and brain connect
It’s not clear how your gut microbiota affects your brain. Most researchers agree that it’s probably through several channels.
- One possibility is that the bacteria release substances that activate the vagus nerve, which connects the gut to the brain.
- Alternatively, the chemicals they release may end up in the blood supply and reach the brain that way, modifying mood when they get there.
- Scientists have found that gut bacteria produce large quantities of neurotransmitters such as serotonin, GABA and dopamine. These play a key role in mood regulation.
- Gut bacteria may also generate other neuroactive chemicals, including one called butyrate, that have been linked to reduced anxiety and depression.
- The microbiome is intertwined with the immune system, which itself influences mood and behaviour.
“We know that diet is the leading contributor to the composition of the gut microbiota,” says Dr Jacka.
“The gut microbiota affects all the other parameters that are relevant to mental disorder – the stress response, neurotransmitter levels, brain plasticity, the immune system. All of these things are affected by the gut microbiota, probably to a great extent, and influenced by diet.”