Imagine two sisters. Twins. Same genes, same childhood, both now past sixty. For three months, each drinks a powdered drink every morning. Neither knows what’s in it.

One contains fibre that feeds gut bacteria. The other is a placebo.

After twelve weeks, both take the same memory test. And the results start to differ.

This is an illustration. But it’s exactly how the study in this article was set up.

The London twin experiment

The PROMOTe study was published in February 2024 in the journal Nature Communications. Researchers at King’s College London recruited 72 people over sixty – 36 pairs of twins from the TwinsUK cohort. Everyone followed the same programme: muscle-strengthening exercises and a daily dose of amino acids. The only difference: in each pair, one twin also got 7.5 g of prebiotic fibre a day, a blend of inulin and fructo-oligosaccharides (FOS). The other got a placebo.

Choosing twins was clever. They share their genes and much of their lives, so when results differ, it’s very likely down to what each of them was taking.

Results nobody expected

The main aim of the study was muscle strength. After twelve weeks, nothing had changed there.

Memory was a different story. The fibre group did better on tests of memory and learning, especially in a task where you have to remember which picture appeared where on the screen.

And in the gut? In the fibre group, the researchers measured 40 changes in bacterial composition; in the placebo group, just one. The biggest shift was a rise in Bifidobacterium, one of the best-studied markers of a healthy gut microbiome.

Twelve weeks. No medication.

How the gut talks to the brain

The gut and the brain are connected through three routes: nerves, the immune system and hormones. Scientists call it the gut–brain axis.

Prebiotics aren’t bacteria. They’re food for bacteria. Fibre the body can’t digest reaches the large intestine, where beneficial bacteria grow on it. Those bacteria then produce substances that travel through the blood – including to the brain.

Exactly what that does in the brain is still being worked out. But it looks more and more as if the gut isn’t only about digestion.

What it means for your plate

The inulin and FOS in the study aren’t exotic. They occur naturally in chicory, onions, garlic, leeks, asparagus and bananas. You just have to eat them.

The general rule is simple: the more varied your plant foods, the more varied the menu for your gut bacteria. Vegetables, pulses, wholegrain bread, nuts, seeds, fruit. Adults are advised to eat 25–30 g of fibre a day, and most of us don’t come close.

You’ll also find fibre in gfBar. All four flavours are high in fibre, with 10–12 g per box, from wild blueberries, dates and chicory syrup. But the bar isn’t the supplement from the study, and we don’t attribute the study’s results to it.

Back to the sisters

The two sisters at the start are made up. The study isn’t. It shows that what we eat every day is in quiet conversation with the bacteria in our gut – and that the conversation may reach further than we thought.

Tomorrow morning you’ll decide again what to eat. Most foods have nothing to add to that conversation. Fibre does.

Choose your gfBar

Source: Ni Lochlainn M. et al., Effect of gut microbiome modulation on muscle function and cognition: the PROMOTe randomised controlled trial. Nature Communications, 2024. Link to the study