Prof John Cryan: We humans outsource some of our jobs to microbes

In an exclusive conversation with HT, Prof John Cryan said that he recommends fibre and fermented foods to people. “Increase your fibre intake to as much as you can tolerate. Try and get as much fermented foods into your diet,” said Cyran who also featured in the Netflix documentary ‘Hack Your Health: Secrets of the Gut’, which was released last year

MUMBAI: Growing up, Prof John Cryan didn’t quite understand a phrase his grandmother often used: “You are what you eat, so eat well.” Over the last 15 years however, Cryan, professor and chair, department of anatomy and neuroscience, University College Cork, Ireland, has been working towards understanding just that. His research group is studying the interaction between the brain, gut and microbiome—or microbiota-gut-brain axis—which plays a critical role in neurodevelopmental, age-related and neuropsychiatric disorders through various communication pathways like the immune system and vagus nerve. Gut microbiota comprises microorganisms that include bacteria, virus and fungi that colonise the gastrointestinal system. They can play an important role and also be harmful to health. When it started out, the scientific community then, just like the young Cryan, showed scepticism.

Mumbai, India - Feb. 20, 2025:Professor John Cryan Irish neuroscentist at TIFR, Colaba, in Mumbai, India, on Thursday, February 20, 2025. (Photo by Anshuman Poyrekar/ Hindustan Times) (Hindustan Times)

“The concept that the gut talks to the brain is not very new. That’s how we understand how we feel full during food intake. But the concept that the microbes in the gut affect the brain is relatively new,” said Cryan, who delivered a public lecture ‘Gut Feelings’ on the microbiota-gut-brain axis at the Tata Institute of Fundamental Research on Thursday. “In 2008, when we started publishing studies on the microbiota-gut-brain axis based on experiments on animal models, people were very sceptical. And some still remain so, which is good.”

Since then, several studies have shown that early-life factors, including the birth mode, antibiotics, nutrition, infection, stress and genetics influence the microbiota and shape neurodevelopment and stress responses.

In the case of nutrition, for instance, the hunter-gatherer tribes in Tanzania are shown to have rich diverse microbes owing to their eating practices. “But with the introduction of agricultural practices, the diversity of the microbes in agrarian communities, for instance in Venezuela and Malawi, started diminishing. In westernised society, the introduction of processed and ultra-processed foods, sweeteners and emulsifiers in the 1950s and ’60s has caused absolute extinction of some microbes,” explained Cryan, who has co-authored the bestselling book ‘Psychobiotic Revolution: Mood, Food and the New Science of the Gut-Brain Connection’.

The microbiota-gut-brain axis has emerged as a critical factor during early life. For example, human breast milk contains the highest complexity of sugars of any mammalian species. “It is about 20 times more complex than cow’s milk,” said Cryan. “We have no capacity in our systems to break down these sugars. It’s the microbes that break them down in the child’s gut into key chemicals that support brain development.”

Giving another example of how newborns receive microbes as a kind of “birthday present” from mothers, studies have also shown that a child born by C-section has a different and decreased microbiome than those born through the vaginal pathway. “That’s another factor we’re interested in—to study the enduring effects of birth mode,” he said. “And is there then a public health message in this?”

Cryan says that globally, more data on the microbiota-gut-brain axis is emerging. “The accumulation of real data lays the framework of how microbes in the gut regulate and influence brain and behaviour,” he said. “This in turn is slowly translating into clinical (human) studies, leading to more awareness among people. People are beginning to understand that certain things in our environment like the impact of ultra-processed food, antibiotics, drug and alcohol abuse, and stress across lifespans are bad for the microbiome and affect long-term physical and mental health.”

Stating that good health and disease are often looked at in relation to the impact of genetics, Cryan said that apart from giving credit or blaming parents and grandparents for inheriting genes, there’s isn’t much that can done “But when we understand the relative impact of the microbiome, it gives people agency over their own healthcare in a way that perhaps they never reflected on before.”

With awareness on the importance of lifestyle factors such as exercise and sleep in maintaining mental health during and after the pandemic, Cryan added that nutrition is a key element. “A lot of plant-based food is really good for your microbiome. Fibre and fermented foods are what I recommend to people. Increase your fibre intake to as much as you can tolerate. Try and get as much fermented foods into your diet,” said Cyran who also featured in the Netflix documentary ‘Hack Your Health: Secrets of the Gut’, which was released last year.