‘The way to a human's brain and immune system is through the gut'
It seems our guts do indeed talk to our brains.
In a recent post on his Substack Ground Truths, Dr Eric Topol describes the complex interplay involving the gut-brain axis. There are four principal interactions by which the gut influences the brain: through the nervous system; by means of hormones; through the immune system; and via the microbiome.
The gut microbiome is the community of all the bacteria and viruses in our intestines.
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'The abundant and diverse bacteria in the gut microbiome [are made up] of tens of trillion cells of more than 3,000 species. These gut bacteria and their metabolites have an outsize impact by producing or stimulating different neurotransmitters and metabolites that communicate with the brain and the immune system,' Topol writes.
In addition, cells in the gut produce hormones such as glucagon-like peptide (GLP-1), gastric inhibitory peptide (GIP), secretin, ghrelin and gastrin. The gut's interaction with the immune system is critical to maintaining the integrity of the gut lining and the blood-brain-barrier, Topol says. The gut microbiome has been described as the thermostat that controls the level of inflammation in our body. Every time we eat we contribute good and bad microbes to the microbiome.
Illustration: Getty Images
One way to think of the process is to compare it to taking oral medication. The microbes in our gut make compounds that enter our bloodstream, in the same way that when you take a pill by mouth, it makes its way through your blood system. Those compounds from our gut microbes are small enough to cross the blood-brain barrier and enter the brain, just like medications do.
Research shows that microbes found in our gastrointestinal tract can have a direct impact on mood. In one Canadian study, researchers found that when 10- and 11-year-olds met recommendations for diet, as well as sleep, physical activity and screen time, they were less likely to need
mental health
interventions as adolescents. Conversely, separate research found that participants who had diets high in ultra-processed foods had the highest risk of depression. And we know that following a Mediterranean diet can prevent and reduce symptoms of depression and anxiety.
There is also evidence of the microbiome's effect on our neurological health through the use of faecal transplants. This involves giving a small sample of stool from the colon of a healthy person to a recipient for therapeutic purposes. It is now a recognised treatment for recurrent bowel infection with the bacteria Clostridium difficile. The goal with the transplant is to have the healthy bacteria outcompete the Clostridium difficile and wipe out the stubborn infection.
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Dr Muiris Houston: Lifestyle changes to reduce your risk of stroke, dementia and late-life depression
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Studies are now under way looking at using faecal transplants for anorexia nervosa in teenagers. Research suggests there is a gut-brain connection, and scientists are finding there is an association between anorexia and imbalances in the gut microbiome, which could influence a person's behaviour. And there is a case report of a child with
autism
experiencing improvements in behaviour while being treated with faecal transplant for Clostridium difficile. The new GLP-1 family of drugs (such as Ozempic), developed for the treatment of diabetes and obesity, are now thought to influence the gut-brain axis. They are recognised for treating a broadening range of illnesses including
heart
failure, kidney disease, liver disease and sleep apnoea.
Topol notes that even before there is weight loss with these drugs, there is evidence from experimental models of reduced body and brain inflammation. While drugs such as
Ozempic
have little direct penetrance to the brain, they exert their effect chiefly through the gut-brain axis.
Topol speculates that, in time, the old saying, 'the way to a man's heart is through his stomach' will be revised to 'the way to a human's brain and immune system is through the gut'.
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