Dietary fibres are coarse, indigestible carbohydrates. They are constituents of cell walls in plants and differ from starch and sugar by their inability to be broken down by the digestive enzymes in the gastro-intestinal tract. For that reason, dietary fibre hardly provides any energy on its way through the digestive system. On its passage through the system, fibre affects the intestinal contents and its transit time, which benefits the digestion and also provides secondary health benefits.
Dietary fibre is divided in two groups: Soluble and insoluble fibre types.
Soluble dietary fibre
Soluble fibre binds enormous amounts of liquid and forms a slimy gel, which is divided in two main groups: Pectin and beta-glucans.
Pectin is mainly found in citrus fruits (primarily in the peeling), in other types of fruit, in root vegetables, in unripe apples, and supplements. Pectin is also used to produce jelly and is able to bind an amount of liquid that represents 15 times its own weight.
Beta-glucans are found in several subtypes. Beta-glucan 1,4 that is mainly found in oats binds fat and lowers LDL cholesterol. Beta-glucan 1,3/1,6 that is found in the shiitake mushroom and in supplements strengthens the immune defence.
The soluble fibres add a lot of bulk to the intestinal content and cause the food to stay longer in the small intestine. This helps provide satiety and contributes to an improved digestion.
Insoluble dietary fibre
Insoluble fibre is not able to dissolve in water. However, it is able to bind small amounts of liquid, which adds bulk to the intestinal content. Insoluble fibre mainly affects the peristalsis of the intestinal wall (a mechanical effect), which increases the transit time of food. Part of the fibre ferments (to some extent) in the appendix and the colon, thereby serving as nourishment for the natural gut flora. This all contributes to a well-functioning digestion. Insoluble fibre is divided in cellulose, hemi-cellulose, and lignin, which is not a carbohydrate but is indigestible just like other types of fibre. Insoluble fibre is found in such things as whole-grain and unpolished rice.
Functions and importance for
- Digestion - consistency of intestinal content and peristalsis (transit time)
- Provides vegetable slime that lubricates the inside of the intestines
- Increases the amount of beneficial gut bacteria
- Fat metabolism and cholesterol levels (binds fat)
- Prevention of cardiovascular disease (reduces blood levels of unhealthy lipids)
- Carbohydrate metabolism and blood sugar (slows down the calorie uptake)
- Weight regulation (prolonged satiety and less hunger)
- Clearance of harmful substances
- Cancer prevention (protection of the intestinal mucosa and removal of cancerous compounds)
Fibre helps lower cholesterol by binding part of the fats and excreting them in the feces. Blood sugar regulation also plays a role for cholesterol levels.
Studies show that 30 grams of dietary fibre per day (compared with 15 grams of dietary fibre daily) can lower the risk of developing intestinal cancer by 40%. The risk of other cancer forms is also reduced.
Deficiencies and poor utilisation may be caused by
- Too little dietary fibre
- Consumption of highly aqueous vegetables that are low in fiber (e.g. tomatoes, lettuce, bell pepper, and cucumber) at the expense of vegetables that are rich in fiber (such as cabbage, beans, and root vegetables).
- Drinking juice of fruits and vegetables instead of eating the real source
- Eating too much white flour and white bread at the expense of whole-grain and rye bread.
- Poor digestion
- Irritable bowel
- Elevated cholesterol levels
- Increased risk of several lifestyle diseases
Mainly wheat bran, whole-grain, coarse vegetables, prunes, figs, other types of fruit, berries, mushrooms, nuts, almonds, kernels, and seeds (flaxseed in particular)
Fibre content in grams per 100 grams
|Almonds and figs||10|
|Oats and coarse
|Apples, kale and peas||5-6|
Recommended daily allowance (RDA)
According to the Nordic Nutrient Recommendations most adults need 25-35 grams of dietary fibre daily. There are no specific recommendations for each individual fibre type.
Elderly people with poor appetite and others with an increased need for more energy dense foods should consume less dietary fibre. This is also the case with small children. The reason is that dietary fibre fills the stomach and gives faster satiety.
- Low-fibre diets
- The above listed deficiency symptoms
Overdoing - side effects
Allergic reactions to certain fibre types may occur in rare cases. Fibre supplements can cause abdominal bloating and pain in the beginning because the gut flora needs to adjust to the increased fibre intake and that takes some time. This normally regulates itself and there is always the possibility of starting off with a lower dosage.
Important information about supplements
Fibre supplements should be taken with large amounts of liquids, for instance a whole glass of water, in order to provide the optimal effect on the digestion and on the blood sugar.
As fibre supplements may slow down the intestinal transit time they may affect the uptake of medicine. Therefore, fibre supplements should generally be taken 1-2 hours in advance of any type of medicine, unless other instructions have been given by the physician or healthcare professional.