There’s fibre … and there’s fibre

Bran Flakes; photo by micheile || visual stories on Unsplash

Fibre? That’s bran flakes, right? Doesn’t it help you poo (snicker snicker)?
Well, sort of, but there is a lot more to it than that.

One of the main reasons we should eat at least five portions of fruit and vegetables a day is because they are packed full of fibre. But, even after the last two years, when intakes went up because most of us had no choice but to cook and eat at home, the average Brit is still only eating around three portions of fruit and veg a day. We should also eat wholegrains (also full of fibre) but again, intake of wholegrains in the UK is very low.

That low intake of fruit, veg and wholegrains means that, in the UK we eat far too little fibre. UK recommendations for fibre intake are 30 grams a day for an adult. But most adults only manage around 19 grams a day, a little over 60 percent of what it should be.

What does fibre do?

Eating fibre supports our health in all sorts of ways. It reduces the risk of developing diabetes, heart disease, and some cancers, reduces inflammation, removes cholesterol from our bodies, and helps keep our weight down. Oh yeah, and it helps us poo – the speed of travel within the gut is called the transit rate. Overall, fibre helps us live longer and healthier lives.

A lot of those benefits are down to the different types of fibre and how they act inside our bodies. But strangely, human beings can’t actually digest fibre. This is because, unlike herbivorous animals, our digestive system does not have the capacity to do what is necessary to break it down. We do not have the ability to ferment things in our stomach, nor do we have a sufficiently large gut to accommodate that level of fermentation. But the huge colonies of microorganisms that inhabit our gut (the microbiome) can digest a lot of it, and what they do with fibre is quite remarkable.

Four types of fibre

Close-up of a vegetable salad; photo by Nadine Primeau, on Unsplash

All whole plant foods (fruit, vegetables, wholegrains, peas and beans, nuts and seeds) contain a mixture of different types of fibre; the level of each type varies between different foods. There is very little fibre contained in the increasingly popular ultra-processed meat alternatives.

Soluble fibre

Soluble fibre absorbs water and turns into a gel which lubricates and softens the stuff passing through our gut, making pooing much easier. Particularly good sources of soluble fibre are beans, peas and lentils, oat bran, barley, flax seeds, and sweet potatoes.

Soluble fibre gel slows down transit rate, giving us more time to absorb nutrients from food. And soluble fibre soaks up cholesterol in the small intestine which stops it being absorbed into our blood. Instead, we poo it out.

Insoluble fibre

Insoluble fibre is what most people think of as fibre – the stuff in bran flakes. It does not absorb water, instead it adds bulk to the stuff in our gut and is a bit rough and scratchy. This irritates the gut wall, which makes it shove the stuff with insoluble fibre in it along to the exit.

Clearly you don’t want your gut contents to sit around for days, but if things shoot through too fast, there isn’t time for enough absorption of nutrients to take place.

So there needs to be a balance between the soluble and the insoluble. Particularly good sources of insoluble fibre are wheat bran, the skins of fruit and vegetables, nuts, seeds, beans, peas and pulses, and whole grains.

Insoluble fibre helps to protect us from type II diabetes, can reduce inflammation and might play a part in protecting us from developing heart disease. This might be because it increases the transit rate, which speeds up the removal of substances that don’t do our health any favours.

Resistant starch

Resistant starch is formed in all starchy foods when the food is cooked. The level increases if the food is allowed to cool after cooking. If food is reheated, the level does not reduce. We can’t digest it – which means it does not provide any calories. Rice, potatoes, pasta and even puddings which are cooked and then cooled have a lower calorie count than if they were eaten freshly cooked.

Like soluble fibre, resistant starch absorbs water, forming a gel which softens and lubricates everything moving through the gut. There is some evidence that high consumption of resistant starch can reduce blood triglycerides and inflammation and may support weight loss.


Finally, there are FODMAPs. Fermentable olig-, di, mono-saccharides and polyols. Like resistant starch, these are carbohydrates which we can’t digest. Foods with high levels of FODMAPS include apples, apricots, peaches, pears, beetroot, broccoli, cabbage, leeks, mushrooms, peas and beans, whole wheat, barley and rye. The polyol sweeteners maltitol, mannitol, sorbitol and xylitol are also FODMAPs.

Beneficial Bacteria; photo by Darryl Leja, National Institutes of Health

What is the microbiome?

The microbiome consists of trillions of bacteria, viruses, yeasts, single celled protozoa, and even parasites, all living in and on every square centimetre of us. Everybody’s microbiome is as unique as their fingerprints. And the microbiome in your nose (yep), is different from the microbiome on the soles of your feet, which itself is different from the microbiome in the gut.

Some of them are good guys (probiotics), some of them are bad guys (pathogens) and some of them are neutral (commensals). Sometimes, if things get tipped out of balance, commensals can be persuaded to join the dark side, but they are far more likely to work with the good guys.

The probiotic microbiome in the gut ferments the fibre that we eat. It prefers some types of fibre over others; it quite likes soluble fibre and resistant starch, it ignores insoluble fibre, but it absolutely loves FODMAPs.

Fermenting fibre

Scientists have been studying the microbiome for a long time, but it is only in the last 20 years that they have really begun to understand just how important it is. When the microbiome ferments fibre, it uses it to produce food for itself as well as several substances that are beneficial to our health.

Some of the most important are short-chain fatty-acids which could be some of the hardest working substances that most people have never heard of. They support the health of the gut itself, support the work of the immune system, increase our absorption of calcium, iron and magnesium, help to regulate our appetite and our insulin balance, reduce the risk of heart disease, protect against colorectal cancer, and even facilitate communication between the gut and the brain.

A slight downside to fermentation

There is a bit of a drawback though. Fermentation creates all sorts of by-products, including a large amount of gas, and that gas only has one way of getting out of the body. Although flatulence is a bit of a social taboo, or the source of endless humour, too much gas can be uncomfortable.

And for anyone suffering from a gastro-intestinal disease like Crohn’s disease, IBS, diverticulitis or colitis, a rapid build up of gas can trigger a flare up of their disease.

Because FODMAPs are the thing that our gut bacteria love to ferment the most, anyone with gut issues might want to consider eating a low FODMAP diet.

To sum up

Fibre is far more than just breakfast cereal or an aid to defecation. We need all the different types, and we can only get enough of them if we eat lots of whole plant foods. Eating a fibre-rich diet helps to keep you healthy, and that can surely only be a good thing!