Can Too Much Fibre Make Your Gut Worse?

Fibre is indispensable for gut health, but when the amount is increased all at once it can lead to wind, bloating and abdominal pain. We explain the difference between soluble and insoluble fibre, why fibre does not always correct constipation and how to increase it the right way.

You complain of bloating, and everywhere you read the advice is the same: more fibre. So you added bran flakes to breakfast, a large bowl of raw salad to lunch and beans to dinner, and put a fibre bar in between. A week later your abdomen is more swollen than before, you have more wind and you feel worse.

You did nothing wrong; you simply went too fast. Fibre is one of the cornerstones of gut health, but the bowels do not work on the principle of “the more, the better”. As well as the amount of fibre, its type, the speed at which it is increased and the water drunk with it decide the outcome.

What is fibre and why does it matter?

Fibre is the part of plant foods that we cannot break down with our digestive enzymes. Because it is not digested, it is of little use as a source of energy; and that is exactly where its value lies. As it moves along the bowel it gives the stool bulk and softness, becomes food for the beneficial bacteria in the large intestine, slows the absorption of sugar and fat and lengthens the time you stay full.

A diet rich in fibre is associated with the prevention of constipation, control of blood sugar and cholesterol, cardiovascular health and a lower risk of bowel cancer. So cutting down on fibre is not a solution; the goal is to eat it in the right way.

How much fibre do you need a day?

For adults, around 25–30 grams of fibre a day is recommended. Most people get about half of that. Reaching this amount does not take special products; ordinary foods spread through the day are enough: oats or wholegrain bread and greens at breakfast, a portion of a vegetable dish and salad at lunch, pulses several times a week, two portions of fruit a day and a handful of nuts.

The problem usually lies not in the target but in the speed of getting there. For someone who takes in ten grams of fibre a day to go up to thirty grams within a week is a sudden load for the bowels.

The difference between soluble and insoluble fibre

Fibre is not a single substance, and its types behave differently from one another in the bowel.

• Soluble fibre: It takes on a gel consistency when it combines with water. It softens the stool, slows stomach emptying and has a favourable effect on blood sugar and cholesterol. Oats, barley, pulses, apples, citrus fruit, carrots, courgettes, flaxseed and psyllium husk are its main sources.
• Insoluble fibre: It does not hold water and works like a broom; it increases the bulk of the stool and speeds its passage. It is found in wheat bran, whole grains, the skins of vegetables and fruit, and nuts.

There is also a third property: how quickly the fibre is fermented by bacteria. The fibres obtained from chicory root and added to “added fibre” yoghurts, bars and protein products ferment very quickly and produce a lot of gas in a short time. Psyllium husk, by contrast, ferments very little and so is generally among the fibres that cause least gas. Most people who have bloating after packaged products labelled as high in fibre are consuming these rapidly fermenting fibres without realising it.

Why does fibre cause wind and bloating?

The bacteria in the large intestine produce gas as they ferment fibre. This is a natural part of digestion, and a certain amount of gas is a sign of a healthy gut. When the amount of fibre rises suddenly, the gas produced rises suddenly too; the community of bacteria has not yet been able to adapt to the new situation.

The good news is this: the bowels get used to it. When fibre is increased gradually, the balance of bacteria changes within a few weeks and gas decreases in most people. In people with a sensitive gut, on the other hand, the problem is not the amount of gas but the bowel’s sensitivity to it; the same amount of gas creates more pain and a greater feeling of bloating in these people.

Why is your constipation not improving although you eat a lot of fibre?

Fibre is the first solution that comes to mind for constipation, but not all constipation responds to fibre. In some situations, increasing fibre makes the complaint worse.

• If there is not enough water: Fibre works by holding water. When not enough fluid is taken, fibre turns the stool into a hard, bulky mass instead of softening it.
• If bowel movement is slow: In people whose bowel moves stool along slowly, added fibre gives bulk to contents that are already moving slowly; bloating and distension increase.
• If the problem is at the outlet: In constipation that comes from poor coordination of the muscles that should work during a bowel movement, fibre is not the solution; an assessment and a different treatment are needed.
• If the type of fibre is not suitable: In people with a sensitive gut, insoluble fibres such as bran can make the complaint worse; soluble fibres are better tolerated.
• If the cause lies elsewhere: Inactivity, putting off going to the toilet, some medicines, iron supplements and an underactive thyroid are not put right by fibre.

If your constipation continues although you have been getting enough fibre and water for weeks, you need to see a doctor instead of increasing fibre still further. We explain the overlooked signs of constipation in the article does a daily bowel movement mean a healthy gut?

Who needs to be more careful with fibre?

• People with irritable bowel syndrome (a sensitive gut)
• People in a flare-up of inflammatory bowel disease and those with a narrowing in the bowel
• People with slow stomach emptying
• People who have recently had bowel surgery
• People whose fluid intake is restricted and older people who cannot drink enough water

In these situations the amount and type of fibre are decided individually by the doctor and dietitian.

How should fibre be increased?

1. Start slowly. Add just one fibre-rich food to your diet each week: first oats at breakfast, the next week a vegetable dish at lunch, the week after that a portion of pulses.
2. Increase water along with it. As fibre rises, so does the need for fluid.
3. Spread it through the day. Instead of loading all the day’s fibre into one meal, divide it between meals.
4. Start with cooked vegetables. Cooked vegetables are tolerated more easily than a large bowl of raw salad.
5. Prepare pulses properly. Soaking them overnight and throwing away the soaking water, cooking them thoroughly and starting with a small portion reduce gas. Lentils are generally digested more comfortably than beans and chickpeas.
6. Chew well. Digestion begins in the mouth; fibrous foods that are not chewed well make more work for the bowel.
7. Move. Walking helps gas to move along and be passed.
8. Observe. Noting which food causes trouble, and in what amount, is far more useful than cutting everything out at once.

Fibre supplements and products with added fibre

When enough fibre cannot be reached through diet, a supplement may be an option. Psyllium husk is a soluble fibre that can regulate consistency in both constipation and loose stools, and is generally well tolerated. It must be taken with plenty of water; taken without water it can lead to difficulty swallowing and to blockage. Because it can reduce the absorption of some medicines, a gap needs to be left between it and medication.

Bars, cereals and drinks with added fibre, however, do not take the place of vegetables, fruit and pulses. The vitamins, minerals and plant compounds that come with those foods are not found in packaged products; and the rapidly fermenting fibres they contain can increase bloating.

In brief

• Fibre is indispensable; the problem is not fibre itself but increasing it quickly and without water.
• Soluble fibres are generally better tolerated; added fibres that ferment quickly cause more gas.
• Not all constipation is put right by fibre; in some situations fibre makes the complaint worse.
• Increase fibre over a period of weeks, and do not neglect water and movement.
• For constipation and bloating that do not improve, see a doctor so that the cause can be investigated.

Frequently asked questions

Should I cut out fibre completely while I am bloated?

No. Although bringing fibre down to zero gives short-lived relief, it increases constipation and leaves the gut bacteria without food. The right approach is to reduce the amount for a while, change the type and then increase it gradually again.

Does fibre reduce the absorption of minerals?

Very high amounts of fibre, and some compounds in the outer layers of grains, can reduce the absorption of iron, zinc and calcium to some extent. In someone who eats a balanced and varied diet this causes no problem in practice.

Does fibre help with weight loss?

Because fibre-rich foods keep you full for longer and are low in energy density, they support weight management. But taking a fibre supplement alone does not bring weight loss unless the eating pattern changes.

To build an eating pattern your bowels can tolerate, see the nutrition for digestive and gut health page.