Dietary fibre has a naming problem. The German term «Ballaststoffe» – literally «ballast substances» – dates from a time when these food components were thought to be worthless ballast, something the body cannot make use of anyway. In Swiss technical usage and in the course, they are therefore given the more fitting name «Nahrungsfasern», food fibres. And they are anything but superfluous: they regulate digestion, dampen the rise in blood sugar, satiate you and intervene in cholesterol metabolism via a surprisingly elegant detour. In this article you will learn what dietary fibre is, what distinguishes soluble from insoluble fibre, how much you need each day and how to increase the amount without your gut rebelling.
What dietary fibre is
Dietary fibres are substances in plant-based foods that the human small intestine cannot make use of, because the body's own enzymes to break them down are missing. Chemically, most of them belong to the carbohydrates. They are without exception structural or supporting substances of plants: polysaccharides such as cellulose and hemicellulose, plus lignin, pectin and guar.
Because they remain undigested in the small intestine, they travel on to the large intestine. There, gut bacteria convert a small proportion of them; the breakdown products include short-chain fatty acids, which the body can absorb and use. The energy gained from this is negligible, however – dietary fibre supplies you with virtually no calories. Its benefit lies elsewhere.
Soluble and insoluble: two profiles of action
In nutritional science, dietary fibres are distinguished by their solubility in water. That is not hair-splitting but explains why they act so differently.
Soluble dietary fibre
Guar, pectin and agar-agar belong here. They are found particularly in fruit and vegetables as well as in oats. Their two central effects: they intervene in cholesterol metabolism (more on that in a moment) and prevent blood sugar values from rising too rapidly after a meal – which is why they play an important role in the diet of people with diabetes. Fibre content is one of the factors that decide how strongly a meal makes blood sugar rise, alongside the degree of processing, the consistency and the quantity.
Insoluble dietary fibre
Lignin, cellulose and hemicellulose are found above all in the outer layers of grains, that is, in wholegrain cereals and wholegrain products. In the large intestine they bind water and swell up. In addition they form the food basis for gut bacteria, which multiply and produce large quantities of metabolic products. In this way the bowel fills, bowel movement is stimulated, and the stool becomes looser and is excreted more quickly. This swelling and the peristalsis it stimulates are the most important single effect of dietary fibre.
The cholesterol mechanism: the elegant detour
The most interesting effect runs via the bile acids. In the large intestine, soluble dietary fibres bind to bile acids, which are thereby excreted with the stool instead of being recycled as they otherwise would be. The body is now short of bile acids – and in order to form new ones, it uses up cholesterol. Consequently the cholesterol level in the blood falls.
On top of that comes a second route: the bacterial breakdown of these dietary fibres in the large intestine produces cleavage products that inhibit cholesterol synthesis in the liver. So the body builds less new cholesterol and at the same time uses more of it. The article on lowering cholesterol goes deeper into these connections, and the article on nutrition for a strong heart gives the wider perspective. The mechanism is well described, but it does not replace a medical assessment if your blood lipid values are abnormal.
Satiety: more volume, less energy
Dietary fibre delays gastric emptying and thereby increases the feeling of fullness. At the same time it increases the volume of the food, which lowers the energy density – you eat more but take in less energy while doing so. Both together make it one of the most effective tools for losing weight.
The course is clear here: when reducing fat, a redistribution of the macronutrients towards fewer carbohydrates and instead more protein, more vegetables and fruit as well as sufficient dietary fibre is recommended – in order to lower body fat percentage, preserve muscle mass and improve blood lipid and blood sugar values. Because sticking with a diet is decided by palatability and satiety, the fibre content of a diet is not a side issue. How to combine this with a calculated calorie deficit is described in the corresponding article; the Mediterranean diet brings plenty of dietary fibre with it as standard.
The reference value: 30 grams per day
The recommendation is 30 grams of dietary fibre per day. It is achieved through wholegrain products, fruit, vegetables, salad as well as nuts, seeds and kernels. In western industrialised nations, actual intake at around 18 to 20 grams is considerably below that – too much refined wheat flour, too little fruit, vegetables and wholegrain.
Two worked examples from the course material make the quantity tangible:
- 30 grams of dietary fibre are found, for example, in around 350 grams of wholegrain bread.
- Anyone who eats one to two pieces of fruit daily – with the skin where possible – plus a portion of salad of around 75 grams and about 200 grams of vegetables has covered their daily requirement.
Particularly rich in fibre are also linseed and wholegrain bread, peas, lentils, beans, sprouts, Brussels sprouts as well as wholegrain pasta and berries. Anyone who cooks in advance has an easier time of it: meal prep with pulses as the basis brings the 30 grams together almost in passing.
Increasing the amount without discomfort
Anyone who jumps from 15 to 30 grams will feel it – bloating, a feeling of fullness, pressure in the abdomen. The reason is simple: the gut bacteria that process these fibres first have to multiply, and the fibres need water in order to swell. Three rules help.
- Increase step by step. Increase in small steps over two to four weeks rather than overnight. Swap one food first – white bread for wholegrain bread – before you tackle the next.
- Drink enough. Insoluble fibres bind water. If it is missing, the effect turns into its opposite and the stool becomes harder instead of looser. Basic fluid intake for healthy adults is roughly two to three litres of water per day, of which only about half comes in through drinking – the rest is contained in solid food and produced in metabolism. Per hour of sport, another 0.4 to 0.8 l is added depending on intensity. More on this in the article Drinking in everyday life.
- Mix both. Soluble and insoluble fibres have different jobs. Oats and fruit alongside wholegrain and pulses – not just one of them.
If the discomfort persists despite a gentle increase, a closer look is worthwhile: with irritable bowel syndrome, many people tolerate certain fibre sources less well than others. The article on irritable bowel syndrome puts this in context – and persistent digestive complaints should be investigated medically before you start adjusting your diet.
When less makes sense
There is one situation in which dietary fibre is deliberately reduced: immediately before exercise. High-fibre products lengthen digestion time, and the more intense the exercise, the more important a well-tolerated pre-exercise meal becomes. What is recommended there is easily digestible carbohydrates such as pasta, rice, maize or potatoes with only a small side dish. The same applies to carbohydrate loading before a competition, where highly satiating foods are deliberately restricted. For everyday life this expressly does not apply.
A second note: dietary fibre can inhibit the absorption of minerals. Anyone who eats wholegrain products normally still gets more than enough minerals – the disadvantage does not weigh heavily against the benefit. More relevant is the other direction: ways of eating that cut out grain products, pulses and dairy products bring with them a lack of dietary fibre, with constipation as the typical consequence – as described in the article on the paleo diet.
Course tip from update Akademie: If you are interested in how nutrients actually work in the body and how to turn that into counselling that works in everyday life, the Ernährungscoach programme at update Akademie is the right route. It covers the nutrients in detail – carbohydrates, fats, proteins, water, dietary fibre, vitamins and minerals – plus energy balance, ways of eating and the structure of a consultation from the dietary history to agreeing on goals.
Frequently asked questions about dietary fibre
«Ballaststoffe» or «Nahrungsfasern» – what is the difference?
They are the same thing. «Nahrungsfasern», food fibres, is the term customary in Switzerland and the factually more fitting one, because these substances are precisely not ballast. «Ballaststoffe» dates from a time when they were considered worthless.
Does dietary fibre count towards calories?
Hardly. A small portion is broken down bacterially in the large intestine into short-chain fatty acids, which can be used. Because the quantities are small, however, this energy gain is negligible.
Are fibre supplements from the pharmacy enough?
They can have their place with certain complaints, but they cover only part of the picture. High-fibre foods additionally supply vitamins, minerals, secondary plant compounds and water – no powder reproduces that package.
Isn't 30 grams a great deal?
It only seems that way because most people's starting point is considerably lower. One to two pieces of fruit, a portion of salad and around 200 grams of vegetables already cover the daily requirement – so the amount is achievable with ordinary food, not with special products.
Does dietary fibre help against bowel cancer?
A high-fibre diet slightly lowers the risk of colon cancer and protects against a range of further conditions of the rectum, such as haemorrhoids. «Slightly» is to be taken literally here: this is about a shift in risk, not about protection, and certainly not about treatment.
Conclusion
Dietary fibre supplies hardly any energy and is nonetheless one of the most effective building blocks of a good diet: it regulates digestion by swelling in the bowel, delays gastric emptying and thereby keeps you full for longer, dampens the rise in blood sugar after a meal and lowers cholesterol levels via the detour of the bile acids. The reference value is 30 grams per day, achievable through wholegrain, fruit, vegetables, salad, nuts and pulses. Two things decide how well you tolerate it: increase step by step and drink enough.
