What's good for your aging gut may also be good for your aging brain. The first study of its kind in twins has found that taking daily protein and prebiotic supplements can improve scores on memory tests in people over the age of 60.

The findings are food for thought, especially as the same visual memory and learning test is used to detect early signs of Alzheimer's disease.

The double-blinded trial involved two cheap plant fiber prebiotics that are available over the counter in numerous nations around the world. Prebiotics are non-digestible consumables that help stimulate our gut microbes.

One is called inulin, and it is a dietary fiber in the class fructan. Another is called fructooligosaccharides (FOS), and it is a plant carbohydrate often used as a natural low calorie sweetener.

To test the effect of these supplements on the aging brain, researchers at King's College London enrolled 36 pairs of twins over the age of 60. Each duo was randomly split so that one twin was assigned a daily prebiotic in a protein powder and the other was assigned a daily placebo in a protein powder.

The twin who unknowingly took inulin or FOS generally scored higher on a cognitive test three months later.

What's more, the daily fiber supplements were linked to slight changes in the gut microbiome between twins. The beneficial Bifidobacterium, for instance, were more plentiful in twins taking inulin or FOS.

Studies on mice suggest Bifidobacterium reduces cognitive deficits by regulating gut-brain connections.

"We are excited to see these changes in just 12 weeks. This holds huge promise for enhancing brain health and memory in our aging population," said Mary Ni Lochlainn, a geriatric medicine researcher at King's College London, when the findings were published in March.

"Unlocking the secrets of the gut-brain axis could offer new approaches for living more healthily for longer."

King's College is home to the United Kingdom's largest adult twin registry, and twin studies are highly valuable when it comes to differentiating between the effect of genetics and the environment on human health.

Past studies on rodents suggest that high-fiber supplements, like inulin and FOS, can 'feed' the colon's microbiome, allowing 'good' bacteria to thrive.

Some of these bacterial players are also linked to improved cognitive function in both mice and humans.

Evidence for the close relationship between the gut and the brain is growing year after year. Some experts are now so convinced by the results, they refer to the gut as the body's 'second brain'.

But the way these two nervous systems work together remains a mystery.

The recent twin study at KCL suggests that consuming certain 'brain foods' may be a promising way to treat cognitive decline. But while prebiotics might improve some aspects of cognitive function in an aging brain, like memory and processing times, there don't appear to be significant physical benefits.

Muscle loss didn't improve among aging twins taking high-fiber supplements, despite the fact that inulin and FOS are important factors in musculoskeletal maintenance.

"These plant fibers, which are cheap and available over the counter, could benefit a wide group of people in these cash-strapped times. They are safe and acceptable too," said geriatrician Claire Steves at KCL.

"Our next task is to see whether these effects are sustained over longer periods and in larger groups of people."

The twins that participated in the current trial were mostly female, and even though the researchers adjusted for sex differences in their findings, they acknowledge that there may be some selection bias amongst KCL's twin cohort.

That said, females are more susceptible to Alzheimer's disease, and studies like the current one support the emerging idea that cognitive decline is not always a disease of the brain, but may involve external factors, too.

The gut has its fingers in many bodily 'pies', including the immune system and the central nervous system. Feeding its microbiome certain prebiotics and probiotics could open the door to treating a plethora of illnesses and diseases.

The study was published in Nature Communications.

An earlier version of this article was published in March 2024.