Deep inside your lower intestine is a 24/7 dinner party. The trillions of microorganisms that live in your colon are feasting on foodstuffs you ate but failed to digest. Their motives are selfish but they are still doing you a favour, tending to the health of your gut, brain, heart and immune system.
Meanwhile, in the background, even-more-indigestible food is quietly drifting past. Even the microbes won’t touch it, but it, too, has a positive effect on your health.
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The name of all this undigested food? Fibre. Perhaps the most unglamorous of nutrients, it has so many things going for it that it deserves to be lauded as a superfood. But while the health benefits of a fibre-rich diet have been recognised since the 1950s, only in recent years have we gotten a firmer handle on the full complexity of this diverse substance and how to maximise these positive effects.
New research is uncovering the power of different types of fibre to dampen inflammation, improve our immune function and mental health – and even act as “nature’s Ozempic” by dialing down our appetite. These studies are also revealing why the fibre often added to processed food won’t do the same trick.
Dietary fibre
Dietary fibre – also known as roughage – is defined as the portion of ingested food that cannot be broken down by our own digestive enzymes. You could be forgiven for thinking that all fibre is basically the same, just humdrum rough stuff that goes in at one end and ultimately comes out at the other. After all, food labels treat it as a single, monolithic entity; if they bother at all, most simply state how many grams of fibre a portion of the product contains.
But fibre is not just fibre. “It is much more complex than other types of food,” says at Monash University in Melbourne. Despite the monolithic label, “fibre” encompasses a hugely diverse . The majority are structural molecules from plants’ rigid cell walls. These “non-starch polysaccharides” include cellulose, lignin, various hemicelluloses, pectin and others. The second, smaller category is resistant starch, which comes from energy-storing starch granules inside the cells.
Types of fibre
Fibre thus runs the gamut from huge molecules such as cellulose, which can contain chains of carbon thousands of atoms long, to tiny sugar-like molecules with a carbon backbone of just three atoms. Different plants contain different amounts of these fibre types. Resistant starch is abundant in green bananas (see “Fibre flex”, below) and legumes, for instance, whereas pectin is common in fruits. In turn, these fibres have distinct impacts on our health. “What we are discovering is that the different types of fibre have very different effects on metabolism,” says at the University of Alberta in Edmonton, Canada.
On top of this natural bounty, we are also increasingly obtaining fibre from unnatural sources. This includes either plant material that has been purified and processed, and entirely synthetic fibre, both of which are packaged into supplements or added to food. There are also animal sources of fibre, mainly chitin and chitosan from insect and crustacean exoskeletons, but these are not a significant component of Western diets.
Given this diversity, there have been attempts to break down the classification of fibre into smaller subunits. The most widely used distinction is whether a type of fibre is soluble in water. Cellulose and lignin, for example, don’t dissolve in water and are classed as insoluble dietary fibre, while pectins, some hemicelluloses and many others are soluble. The thinking was that these two categories have different biological effects. But we now know that this distinction is neither clear-cut nor especially useful when considering their effects on human physiology, says at Newcastle University, UK. A more useful subcategorisation, he says, is whether a fibre is fermentable: is it broken down by hungry microorganisms in the large intestine?
When we eat fibre, it passes through the mouth, oesophagus, stomach and small intestine largely untouched. Chewing can physically alter it – for example, breaking apart the plant cells to release fibrous molecules from their matrix – but, by definition, there are no enzymes to break it down.
It then reaches the large intestine, or colon, and the dinner party starts. This capacious organ doesn’t secrete digestive enzymes of its own – but it is teeming with microbes that do, many of them capable of breaking down fibre. “Our microbes have over 1000 different enzymes that digest fibre,” says Marques.
Even so, these microbes can only digest , such as pectin and resistant starch, but not cellulose, which passes unfermented through the human gut. But the upshot is that the colon is essentially a fermentation vat for the chemical processing of tough plant material, producing energy for the microbes plus various interesting by-products – of which, more later.
Beyond this, exactly what happens in the colon is a moveable feast. Diet composition varies widely from person to person and from day to day, making it hard to figure out what kind of fibre is doing what. “But we can predict that the different types of fibre will have quite fundamentally different effects,” says Mathers.
If that wasn’t complex enough, “everybody is going to respond to fibre differently because of the different microbes in their colon”, says Madsen. This makes nailing down dietary fibre’s precise effects extremely difficult. As a rough approximation, however, most of the fermentable fibre types are broken down by gut microbes, while unfermentable types glide through unaltered and are ultimately passed in stool. Both of these types of fibre have important roles in human health.
Epidemiological research has consistently found that eating adequate amounts of fibre protects against a range of health conditions including cardiovascular disease, stroke, type 2 diabetes and colorectal cancer. In 2015, the UK Scientific Advisory Committee on Nutrition found that – about the amount in a portion of baked beans – to a person’s diet produces a statistically significant reduction in their risk of developing all of these conditions.
The question then is: how much is enough? In 2019, a team of researchers led by at the University of Otago in Dunedin, New Zealand, pooled the results of 185 studies and found that this risk reduction was greatest when . People hitting that target showed a reduced risk of dying from all causes and also a significantly lower risk of coronary heart disease, type 2 diabetes and colorectal cancer.
As a result of these studies and others, most countries recommend that adults eat 25 to 35 grams of fibre a day. The , and only around 20 per cent of people hit the 25-to-35-gram target, a persistent problem known as the “fibre gap”. Filling this gap is hard (see “How to eat enough,” below). “It takes an active effort,” says Marques. “I’m someone who knows where to find fibre and I struggle to get the amount that I know I should be eating.”
And while we know that fibre is good for us, epidemiological studies don’t drill down into the mechanisms by which it exerts its positive effects. In recent years, there has been a huge effort to understand the complexities of dietary fibre and its interactions with the microbiome, in part to produce a more useful classification system, but also to find ways of getting more of the most beneficial fibre into people’s diets.
Many of the positive health effects of fibre . One of the most important is that it absorbs and retains water, and can become viscous in the process. Some fibres, such as those from vegetables, can hold up to 25 times their own mass and volume of water; for fruits and grain fibres this figure drops to 13 and six times, respectively. In the stomach, this bulks up food and leads to feelings of fullness. It also delays the passage of food from the stomach into the small intestine, which extends the period of satiation after a meal. Both effects reduce the amount of food that we eat during and between meals.
The colon is a fermentation vat for the chemical processing of tough plant material
Lower down, the water-retaining fibre softens, enlarges and lubricates stools, aiding gut motility and producing a laxative effect. This guards against constipation, which is a suspected cause of colorectal cancer and which has recently been found to be a and , such as heart attacks and strokes. “Why that is the case, we are still trying to understand,” says Marques.
Fibre also soaks up fat, cholesterol and glucose to varying degrees, depending on the type – fibre from walnuts, for instance, can absorb nearly 30 times its own mass worth of oil – reducing the absorption of these potentially harmful nutrients by the body. It does the same for bile acids secreted in the small intestine to help digest fat. Our bodies like to reabsorb and recycle bile acids, but a high-fibre diet forces the liver to synthesise new ones from cholesterol, reducing cholesterol levels in the bloodstream. And fibre is a natural chelating agent, picking up toxic heavy metals such as lead and helping to purge them from the gut.
Microbiome interactions
For a long time, that was the extent of our understanding of fibre’s positive health effects, says Madsen. But we now know that there is more to fibre than its physical prowess, evident in its interactions with the gut microbiome.
One of its jobs is maintaining the gut wall – something first recognised in 2016 in . A healthy gut lining has a thick layer of mucus that helps to maintain “barrier function” – in other words, being in control of what passes from the gut into the bloodstream. “If we eat a lot of fibre, then bacteria in your gut will [eat the fibre and] leave this mucus alone,” said of Frankfurt University Hospital in Germany at a . “But if you stop doing that, they will start degrading the mucus because they need something to eat. Then, of course, the gut barrier is broken down.”
When this mucus is eroded, microorganisms can cross the gut barrier and enter the bloodstream. This jolts the immune system into action, resulting in low-level inflammation spreading throughout the body and an increased risk of high blood pressure; it also promotes obesity and metabolic dysfunction.
Microbial fermentation of fibre also produces myriad chemical by-products with wide-ranging health benefits. Chief among them are three (SCFAs) – acetate, propionate and butyrate – which increase acidity inside the gut, inhibiting the growth of harmful bacteria. But all three also bind to and activate a large and diverse class of receptors on cell surfaces called G protein-coupled receptors (GPCRs), initially within the gut but also throughout the body once they have diffused into the bloodstream.
One of the key interactions between SCFAs and receptors in the gut triggers the release of two appetite-suppressing hormones: PYY and GLP-1. The latter is mimicked by the new generation of weight-loss drugs, leading fibre to be dubbed “nature’s Ozempic”. PYY release is also stimulated in the ileum – the final segment of the small intestine – by a dietary fibre called stachyose, found mainly in legumes such as beans and lentils, according to research published last year by Gary Frost at Imperial College London and his colleagues.
Impact on inflammation
As well as this, we now also know that SCFAs stimulate a gut receptor that helps to maintain barrier function. This is another reason why low-fibre diets can lead to inflammation, says Marques.
Once in the bloodstream, SCFAs travel to all parts of the body, including the brain. “They’re wonderful metabolites,” says Madsen. Among their , they have been shown to boost immune function, further reduce inflammation and protect against dementia and metabolic diseases such as type 2 diabetes. Given the link that exists between chronic inflammation and depression, SCFAs may explain the findings of a yet-to-be-published study by Madsen and her colleagues, which showed that eating a five-day, high-fibre diet once a month can reduce symptoms of depression and anxiety.
Eating more fibre also changes the composition of the gut microbiome, possibly for the better – though exactly which types of microbes make for a healthier microbiome is the subject of much debate.
Unfortunately, translating these new discoveries into actionable dietary advice is challenging. Many nutritionists hope that we will one day be able to stop lumping all fibre together and offer more fine-tuned advice – perhaps stating what proportion of the fibre is a good source of SCFAs, much as food labels say how much of the fat is saturated and how much of the carbs are sugars. “I think we are heading towards that,” says Mathers. “But I am not sure we have enough detailed information to do it yet.”
Plant-based
For now, then, the existing nutritional advice stands – but with some useful embellishments. Thirty grams of fibre a day is still a good target, but not just comprising any old fibre. Cover as many of the bases as possible – fruits, vegetables, pulses, nuts, seeds and wholegrains (see “Fibre hacks”, below). Then you are hedging your bets on getting adequate amounts of the full range of fibres. And don’t worry about eating too much: “There is no downside,” says Marques. “You might get to a threshold where you are not getting any additional benefit, but you are not going to have a health issue because you have too much fibre.” Unless, that is, you have a bowel condition such as Crohn’s or inflammatory bowel disease, where excess fibre might trigger an episode.
It is also advisable to obtain fibre from whole foods rather than relying on supplements or processed foods fortified with fibre, which often contain just a few types. “A commercial fibre supplement doesn’t give you the same portfolio of dietary fibres, and we think that portfolio is really important,” says Frost.
Supplements also lack the complex structure of the fibres found in whole plants, which is increasingly seen as being important to their health effects. “We think that the cell structure does play an important role when the food goes into the colon,” says Frost. For one thing, it enhances the release of the satiety hormones PYY and GLP-1, he says. For these reasons, synthetic fibres, which are increasingly added to processed foods to make them appear to be a good source of fibre, are also likely to be suboptimal. “We have no idea whether they are actually beneficial,” says Mathers.
Another reason for opting for a range of fibre-rich whole foods is that these also contain lots of other plant-based benefits such as vitamins, antioxidants and unrefined (but digestible) carbohydrates. In fact, these nutrients might be part of the reason behind the epidemiological evidence showing that diets rich in fibre are so beneficial. “Whether it’s the fibre per se or whether it’s a diet which contains lots of fibre is something which has never fully been resolved,” says Mathers.
Even so, the message remains the same: “It’s clear that high-fibre diets prevent chronic diseases,” says Madsen. So if you don’t already eat 30 grams of fibre a day, it is time to get the party started.
How to eat enough
Getting enough fibre in your diet is hard. Calculating your intake is no easier, especially because cooking and ripening can alter a food's fibre content (see "Fibre flex", above).
I tracked my fibre intake for a couple of weeks. Sometimes it was easy – packaged food usually listed the fibre content in grams per 100, while supermarket websites filled in most of the gaps on things like bakery items. I weighed my raw ingredients and calculated their fibre content using an online calculator. When I ate out, I had to estimate the fibre content from the calorie count on the menu; in this way, I could roughly calculate my overall fibre intake in grams. But drilling down into the different types of fibre is practically impossible.
My diet is mostly plant-based, so I expected to be close to, or over, the 30-gram recommended intake through my regular diet. But I wasn't – and I found I had to make a conscious effort. That involved a lot of oatcakes, nuts, dried fruit, baked beans on wholemeal toast, popcorn, hummus, avocados (surprisingly fibrous) and raspberries (ditto).
For five days straight, I smashed the target. I made more visits than usual to the loo and was bloated and flatulent – though I am told that these uncomfortable side effects decline as the microbiome adapts to a high-fibre diet. I was also very thirsty, presumably because fibre absorbs a lot of water in the colon. But I also found that my urge to snack declined and I lost a bit of weight.
Fibre hacks
The word "fibre" usually evokes images of All-Bran, lentils, brown rice and other virtuous (though not necessarily tempting) foodstuffs. But some delicious and healthy foods deliver surprising amounts of fibre.
Avocados, for example, contain 6.7 grams of fibre per 100 grams, enough to get them over the 6-gram threshold needed to be called a high-fibre food. Passion fruit clocks up 10 grams and raspberries tout 9.1. Popcorn has an eye-popping 14 grams, almost half of your daily target in a 100-gram bowl (though that is admittedly a lot of popcorn). By the same token, some foods that you might assume are very fibrous are not. Celery, for instance, contains just 1.5 grams; cabbage has 1.7. In fact, from a fibre perspective, you would be better off eating potato chips (around 3.9 grams) and dark chocolate (around 6 grams).
White pasta and rice don't contain much fibre when freshly cooked, but when cooled, their starch crystallises into a form of fibre called resistant starch, which remains intact when you reheat it.
