To begin to understand the link between the gut and the brain, all you have to do is look at the English language, according to Irish neurologist John Cryan.
“We use phrases like gut feelings, gut instincts, we have butterflies in our tummies when we’re nervous,” Cryan told Euronews Next. “So perhaps there’s an underlying biology to these phrases.”
The most basic connection we can observe every day revolves around eating, Cryan explained. When we’re hungry, our stomach sends a message to our brain to tell us to eat. It also tells us when we’re full and need to stop.
Cryan has been studying this gut/brain axis for decades, most recently at University of Cork in Ireland, where he leads the department of anatomy and neuroscience.
In recent years, he says a new variable has entered the equation: the microbiome. Scientists have discovered that the trillions of benevolent bacteria and viruses living in our intestines can have a huge impact on our brain and behaviour.
And feeding your microbes foods that they like can help reduce stress and even alleviate symptoms of anxiety and depression.
“We're beginning to really understand that these microbes that we have within our gut are really important for most aspects of our physiology,” Cryan said. “But what my lab has been interested in is how they're also playing a role in shaping our brain and our behaviour.”
How important is our microbiome?
The easiest way to know if something is important or not in the body is to take it out and see what happens, Cryan says.
So that’s what his lab at University of Cork did – they conducted a study on mice, raising them in a germ-free environment to see how this affected their behaviour. These germ-free mice were then given the option to spend time with other mice or to spend time alone in a chamber.
“Mice are quite social like humans, so they normally gravitate towards a social environment, but if they didn’t have microbes in their guts, they didn’t,” Cryan said.
Across the animal kingdom, from honeybees to baboons, the same patterns were observed – if you changed the microbiome, social structures and behaviour also changed.
In humans, the growing evidence that microbes can change behaviour has huge implications on a wide variety of mental disorders and conditions.
“Social behaviour is at the heart of a variety of disorders, like autism,” Cryan said. “We’re also studying it in the context of social anxiety disorder. It’s very much important for schizophrenia. And all of these are now implicating the microbiome in their overall pathophysiology.”
What throws our microbiome out of whack?
Our gut microbiome is sensitive, and it can be thrown off by a number of different things, like the environment, stress, antibiotics, and diet, Cryan said.
“Many aspects of our Western diet, the increase in processed food, sweeteners, emulsifiers, etc. have been shown to negatively impact the composition of the microbiome,” he said. “Diversity is really important in all aspects of life, and diversity is really important for our microbiomes.”
As humans evolved and moved away from our hunter-gatherer origins, our diets became less diverse, as did our microbiomes. New disorders of inflammation started showing up in Western populations, like irritable bowel syndrome (IBS) and multiple sclerosis (MS), both of which affect the brain.
Some of the microbes our ancestors had in their guts have gone completely extinct, Cryan said.
“We’re beginning to grapple with this as a society,” Cryan said. “How do we realign the microbiome that our ancestors had and how do we have the diets that are going to be good in fostering an appropriate microbiome?”
Can eating differently improve your mental health?
Cryan and his colleagues in Cork began to wonder - if diet can negatively affect the microbiome and brain, then maybe changing what we eat could enrich our microbiome and have a positive effect on mental health.
He and colleague Ted Dinan, a psychiatry professor at Cork, had come up with the term psychobiotics to describe any interventions that target the microbiome and have positive effects on mental health.
These include specific species of bacteria called probiotics, supplements that support good bacteria called prebiotics and the chemicals produced by the bacteria called postbiotics.
But Cryan and his team set out to prove that a psychobiotic diet could also exist.
“We brought people in and we either gave them some normal dietary advice or we got them to change their diet completely to this psychobiotic diet, really ramping up the fibre and fermented foods,” he said. “What we found was that there was a reduction in their feelings of stress, in their overall mood-related readouts and their sleep also improved.”
What foods can improve your mental health?
When it comes to feeding your microbiome, the worst thing you can eat is processed food, Cryan says.
“Work predominantly from Australia has shown that really highly processed food in particular has a negative effect on our mental health,” he said. “Diets that are really extreme in any way have also been shown to be quite negative on our mental health.”
There are four main things in our diets that have been found to improve mental health – Omega-3 fatty acids, polyphenols, fibre and fermented foods.
Omega-3 fatty acids are found in fish and flaxseeds; while polyphenols give plant-based food colour and are found in berries, olives and soy beans. Fibre-rich foods include lentils, avocados and broccoli. Fermented foods include kimchi, yoghurt and miso.
More studies are still being done to get a better idea of how diet and mental health are related, but Cryan says the findings are encouraging, because they suggest there are simple steps everyone can take to reduce stress.
“It’s not about creating an expensive product that’s only available in health food stores,” he said. “It’s about really telling people that you can change your fibre intake reasonably cheaply, you can change your fermented foods. This can be done without a huge expense and therefore we’re hoping that this could be implemented as a preventative mechanism to help people who have busy, stressful lives deal with the stress.”