Brain bran: The protective effect that fibre has on cognition

Eating fibre can improve health, lengthen lifespans, and even protect our brains. Yet many of us are still coming up short on this "essential nutrient".

A diet high in wholegrains, fruits, pulses, nuts and seeds – all of which are full of fibre – can have huge benefits for both our bodies and our brains.

Emerging research shows fibre supercharges the microbiome and influences the gut-brain axis, which is the communication channel that runs between the gut and the brain; slowing down symptoms of cognitive decline.

Increasing fibre intake is one of the most impactful dietary changes for cognitive health, says Karen Scott, professor of gut microbiology at the Rowett Institute, University of Aberdeen. Alternatively, a fibre deficit has been found to be a leading dietary risk factor for ill health.

Yet many of us aren't eating enough. In the US, about 97% of men and 90% of women do not eat enough fibre. Most eat less than half the recommended daily amount. In the UK, over 90% of UK adults fall short, with many other countries showing similar deficiencies. 

So why exactly is fibre so beneficial, and how can we eat more of it?

Gut feeling: How fibre functions

Fibre is a carbohydrate that can't be easily broken down by the digestive enzymes. Most therefore passes through the gut largely unchanged.

It increases the size of stools. It keeps us feeling fuller for longer and, as we digest it slowly, it leads to a more gradual rise in blood sugar levels. Those who eat more wholegrains per day have been shown to have a lower BMI and less belly fat than those who eat refined grains.

A diet high in fibre can also help lengthen a lifespan and should therefore be considered an essential nutrient, says John Cummings, emeritus professor of experimental gastroenterology at the University of Dundee.

Serenity Strull/ BBC

A review which Cummings co-authored found that those who ate the most fibre showed a 15-30% reduced risk of mortality compared to those who ate the least. Sufficient fibre consumption, which the researchers say is about 30g per day, lowers the risk of coronary heart disease, stroke, type 2 diabetes and colon cancer – and translates as 13 fewer deaths per 1,000 people.

The greatest benefits were seen at 25-29g of fibre per day. To achieve that, you could incorporate fibre into every meal and snack, with portions of fruits or vegetables in each meal. A skin-on jacket of potato with baked beans followed by an apple will provide about 15.7g of fibre for instance. Snacking on nuts and seeds will also increase your intake – a handful of nuts (about 30g) contains 3.8g of fibre.

Key to this impact is fibre's relationship to the gut microbiome. 

As our gut bacteria digest fibre, beneficial byproducts are produced, including the short-chain fatty acids acetate, propionate and butyrate. It's these metabolic products that provide critical energy for cells and are linked to significant reductions in mortality, explains Cummings. 

Minding your meals: How fibre protects the brain

A high-fibre diet is also now believed to be particularly important for brain health, explains Scott. The presence of the fatty-acid butyrate helps maintain the lining of the gut, she says, thus reducing the risk of harmful substances entering the bloodstream and affecting the brain.

That's why the gut microbiota can improve cognition. "The more fibre you eat, the more butyrate is produced, then the better your cognition can be retained." 

A 2022 study involving over 3,700 adults found that high dietary fibre intake was linked to a lower risk of dementia among individuals who had the highest fibre intake. Those who ate the least showed an increased risk. Similarly, another study among adults over 60 for instance, found that those who had diets higher in dietary fibre showed increased cognitive function. 

While the above findings were correlations, a more recent randomised control trial of twin pairs also identified a causal impact on fibre and cognition. Those who consumed a daily prebiotic fibre supplement showed improved results in cognitive tests in three months compared to those who had a placebo. Prebiotics are simple fibres that benefit bacteria in the gut and can be consumed as supplements. Analysis of stool samples revealed that the fibre supplement changed the participants' gut microbiome, with increased levels of beneficial bacteria, such as Bifidobacterium.

Mary Ni Lochlainn, clinical lecturer in geriatric medicine from Kings College London, led the study, and says it holds promise of using diet to help enhance brain health and memory in the older population. "The exciting thing about the microbiome is that it's malleable and certain microbes seem to be associated positively with health."

Serenity Strull/ BBC

Ni Lochlainn is therefore interested in learning how we can better harness the gut microbiome to improve age-related decline, both cognitive and physical. "It's an untapped resource and an underexplored area that we're learning a lot more about," she says, adding that it could "make growing older easier".

Research also shows that higher levels of butyrate production have a positive effect on depression, improved sleep and better cognitive function.Butyrate producing bacteria, for instance, have been linked to greater wellbeing, as well as a reduction in mental ill health.

Additionally, Scott's team has recently found that patients with Alzheimer's disease had higher numbers of pro-inflammatory markers in their faecal samples, and lower numbers of the bacteria that produce butyrate, as well as less butyrate overall.

"It ties together with that link between butyrate and the brain," she says. This was a correlational study, she says, but also notes that it supports the growing body of evidence of changes to our gut microbiota are linked to brain health.

How to eat more fibre

Research has shown that individuals with long healthy lives have diverse gut microbiomes. A diet with varied forms of fibre helps encourage this diversity, says Cummings. 

As there are so many different sources – including nuts, fruits and vegetables – it's relatively easy to increase your intake.

Increasing plant-based foods is an obvious step, especially pulses, as peas, beans and lentils are high in fibre. Some easy additions could include blending canned chickpeas into pancake batter or adding peas into pasta dishes.

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Swapping white bread and pasta for brown varieties are other simple tweaks. If you prefer the taste of white pasta, mixing the two makes the difference barely noticeable. Making sure your breakfast cereal is wholegrain helps. 

Snacking on popcorn, apples, seeds and nuts will then further improve your fibre intake, as different foods have different health effects.

Plus fibre can be found in supplements, which is especially useful for those who struggle to chew or swallow, as can be the case for individuals with Parkinson's disease.

The benefits of fibre are so large that "increasing your fibre intake is really the single most beneficial thing" people can do for their overall health, says Scott.

* Melissa Hogenboom is BBC Global's Senior Health Correspondent and author of Breadwinners.

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