Dietary Fibre: What You Need to Know
General information
Even parts of food that the human body cannot digest can still benefit health. Dietary fibre (roughage) used to be considered worthless and superfluous, but today these natural bulking agents are seen as part of a healthy diet. Dietary fibre consists of substances in plant-based foods that the human small intestine cannot break down. Because the body's own enzymes cannot break down these substances, they are excreted undigested in the stool. (Only in the large intestine can gut bacteria convert small amounts of these substances; the resulting breakdown products, e.g. short-chain fatty acids, can be absorbed and used by the body. Even so, the energy gained from dietary fibre is negligible, if only because of the small amounts involved. More important is the swelling of the dietary fibre that remains in the intestine, which stimulates peristalsis, i.e. bowel movement, and so promotes the transport of the intestinal contents. All dietary fibres are structural or supporting substances from plants. These include polysaccharides (e.g. cellulose, hemicellulose) as well as lignin, pectin, agar-agar and guar.
Foods rich in dietary fibre
Westerners fall well short on dietary fibre: people in western industrialised nations mostly eat too much refined wheat flour and too little fruit, vegetables and wholegrain products. The average German consumes 18 to 20 g of fibre a day. However, the German Nutrition Society (DGE) recommends taking in at least 30 g of fibre a day through fibre-rich foods! To meet this requirement, fibre-rich foods should be a regular part of the daily diet. These include linseed and wholegrain bread as well as peas, lentils, beans, sprouts and Brussels sprouts. Wholegrain pasta, wholegrain rusks, fruit bread, and dried fruit and berries are also rich in fibre. 30 g of dietary fibre, for example, is found in around 350 g of wholegrain bread. Anyone who eats one to two pieces of fruit a day (ideally with the skin on), a portion of salad of around 75 g and about 200 g of vegetables has covered their daily fibre requirement.
Effects
When it comes to their effects, nutrition scientists distinguish between soluble and insoluble dietary fibre: soluble fibre such as guar, pectin and agar-agar is found particularly in fruit and vegetables as well as in oats. In the large intestine, it binds to bile acids, which are then excreted with the stool. To form new bile acids, the body uses up cholesterol, and as a result the cholesterol level in the blood falls. In addition, the bacterial breakdown of this fibre in the large intestine produces certain breakdown products that inhibit cholesterol synthesis in the liver. Soluble fibre plays a particularly important role in the diet of diabetics: after a meal, it prevents blood sugar levels from rising too quickly. Insoluble fibre such as lignin, cellulose and hemicellulose is found mainly in the outer layers of cereal grains, i.e. in wholegrain cereals and wholegrain products. In the large intestine, it binds water and swells. It also forms the food base for the bacteria in the gut, which can multiply and produce large amounts of metabolic products. This fills the intestine, which in turn stimulates bowel movement. The stool as a whole becomes softer and is excreted more quickly. A fibre-rich diet promotes regular digestion, slightly lowers the risk of colon cancer and so protects against a number of other diseases of the rectum (e.g. haemorrhoids). A possible disadvantage is that dietary fibre may inhibit the absorption of minerals into the body. Anyone who eats wholegrain products will normally still get more than enough minerals regardless.
