The English language offers a clue to the deep connection between the gut and the brain, says Irish neurologist John Cryan. Phrases like “gut feelings” and “butterflies in our tummies” reflect an underlying biology that scientists are now decoding.
Cryan, who leads the department of anatomy and neuroscience at University College Cork, has spent decades studying the gut-brain axis. In recent years, his focus has shifted to the microbiome—the trillions of bacteria and viruses living in our intestines. These microbes, he explains, influence not just digestion but also brain function and behaviour.
The Microbiome's Role in Social Behaviour
To test the importance of gut microbes, Cryan’s lab raised mice in a germ-free environment. Unlike normal mice, these animals avoided social interaction. “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 told European Pulse. Similar patterns appear across the animal kingdom, from honeybees to baboons.
In humans, this has implications for disorders such as autism, social anxiety, and schizophrenia. “Social behaviour is at the heart of a variety of disorders,” Cryan says. “All of these are now implicating the microbiome in their overall pathophysiology.”
What Disrupts the Microbiome?
The gut microbiome is sensitive. Stress, antibiotics, and diet can throw it off balance. “Many aspects of our Western diet—processed food, sweeteners, emulsifiers—have been shown to negatively impact the composition of the microbiome,” Cryan notes. Diversity is key: as humans moved away from hunter-gatherer diets, our microbiomes became less diverse, and new inflammatory disorders like irritable bowel syndrome (IBS) and multiple sclerosis (MS) emerged.
Some microbes our ancestors carried have gone extinct. “We’re beginning to grapple with this as a society,” Cryan says. “How do we realign the microbiome that our ancestors had?”
The Psychobiotic Diet
Cryan and colleague Ted Dinan, a psychiatry professor at Cork, coined the term psychobiotics to describe interventions that target the microbiome for mental health benefits. These include probiotics (specific bacterial species), prebiotics (supplements that feed good bacteria), and postbiotics (chemicals produced by bacteria).
In a study, Cryan’s team put participants on a psychobiotic diet rich in fibre and fermented foods. “We found a reduction in their feelings of stress, in their overall mood-related readouts, and their sleep also improved,” he says. The worst offender? Processed food. “Work predominantly from Australia has shown that highly processed food has a negative effect on our mental health.”
Key foods that support mental health include Omega-3 fatty acids (found in fish and flaxseeds), polyphenols (in berries, olives, and soy beans), fibre (lentils, avocados, broccoli), and fermented foods like kimchi, yoghurt, and miso.
The findings are encouraging because they suggest simple, affordable steps. “It’s not about creating an expensive product,” Cryan says. “You can change your fibre intake reasonably cheaply.” As Europe grapples with rising stress and mental health challenges, this research offers a practical path forward.
