Dietary fiber ( Engl. "Dietary fiber" rare "non nutritive carbohydrates") are largely indigestible food ingredients, mostly carbohydrates , predominantly in plant foods occur. They are mainly found in cereals , fruit , vegetables , legumes and, in small quantities, in milk . For the sake of simplicity, fiber is divided into water-soluble (such as locust bean gum , guar , pectin and dextrins ) and water-insoluble ( e.g. cellulose ). In contrast to what the name suggests, fiber is now an important part of human nutrition. The EU regulation on nutrition labeling assigns them a flat rate of 8 kJ / g.
The fiber content of a food given in the nutritional table is subject to legal regulations. No. 11 of the Nutrition Value Labeling Ordinance (NKV) and Annex I No. 12 of the Food Information Ordinance define and a .:
- "Dietary fiber" means carbohydrate polymers with three or more monomer units that are neither digested nor absorbed in the human small intestine and belong to the following classes:
- edible carbohydrate polymers naturally found in foods when consumed;
- Edible carbohydrate polymers that are physically, enzymatically or chemically obtained from raw food materials and that have a positive physiological effect according to generally accepted scientific evidence.
- edible synthetic carbohydrate polymers that have generally accepted scientific evidence to have beneficial physiological effects;
Differentiation from crude fiber
The term crude fiber ( Engl. "Crude fiber") was founded more than 100 years in the feed analysis coined. Since some fiber also has a fibrous structure, it is often mistakenly equated with this. The fiber content in food always exceeds the raw fiber content, which consists almost exclusively of cellulose. Conversion factors between 2 and 6 are given in the literature, e.g. B. Crude fiber content × 6 = fiber content. For cereals and pulses, conversion values of 4 to 6 apply, for fruit and vegetables around 2 to 3.
Types and occurrences
Dietary fiber is found in different amounts in plant-based foods. Dietary fiber is roughly divided into water-insoluble and water-soluble; Because they can be used as thickeners (see also mucilage ), some are specially produced for use as food additives ( alginates as salts of alginic acid from various algae , agar also from algae, xanthan , etc.).
|nutrient||E number||Occurrence / extraction|
|Cellulose||E 460||Cereals, fruits, vegetables (all plants)|
|Chitin||-||in mushrooms , exoskeleton of insects and crustaceans|
|Hemicelluloses||Grain, bran , wood , legumes|
|Hexosanes||-||Wheat , barley|
|Pentosans||-||Rye , oats|
|Arabinoxylan||-||some representatives are insoluble|
|lignin||-||Fruit kernels, vegetables (threads in green beans ), cereals|
|Xanthan gum||E 415||Extraction with Xanthomonas bacteria from sugar-containing substrates|
|Water soluble fiber|
|Lichenine||-||Oats & barley = 6-8%; Wheat & rye <2%|
|Hemicelluloses||-||Grain, bran , wood , legumes|
|Pentosans||-||Rye , oats|
|Arabinoxylan||-||some representatives are soluble|
|Fructans||replace or supplement in some plant taxa , the strength as a storage carbohydrate|
|Inulin||-||in different plants, e.g. B. Yacon , Jerusalem artichoke , chicory etc.|
|pectin||E 440||in the fruit bowl (especially apples , quinces ), vegetables|
|Alginic acid (alginates)||E 400 - E 407||in algae|
|Sodium alginate||E 401|
|Potassium alginate||E 402|
|Ammonium alginate||E 403|
|Calcium alginate||E 404|
|Propylene glycol alginate (PGA)||E 405|
|Carrageenan||E 407||Red algae|
|Raffinose||-||replace or supplement starch as a storage carbohydrate in legumes|
|Polydextrose||E 1200||synthetic polymer, approx. 1 kcal / g|
Dietary fiber levels in various foods
The fiber content in foods varies widely. In addition to the absolute content, the ratio to the carbohydrate content is of nutritional and physiological interest.
The following tables give some examples. A more detailed table is given in the web links. According to the National Consumption Study II published by the Max Rubner Institute , grain products are the most important source of fiber for Germans at 41%, ahead of fruit (21%) and vegetables (16%). According to the restrictive EU guidelines, all German type flours can be designated as a source of fiber, as they contain more than 3% fiber.
Dietary fiber can bind up to 100 times its own weight in water. Products that contain a lot of fiber, such as flaxseed or wheat bran, should therefore be accompanied by sufficient fluids, as otherwise the digestive pulp will harden in the intestine and encourage constipation instead of counteracting it.
|> 10%||Rye , rye crispbread , wholemeal rye flour / meal, wheat bran|
|5% ... 10%||Datteln , spelled , peanuts , figs , barley , pearl barley , oats hulled, rolled oats , hazelnuts , elderberries , corn , almonds , nuts , Pumpernickel , quince , rye flour: all types of flour , rye bread , black currants , raisins , whole grain bread , whole wheat pasta , walnuts , Wheat , wheat semolina , wheat flour type 1050|
|2% ... 4.9%||Apples , apricots , artichokes , avocados , bananas , pears , cauliflower , beans , peas , fennel , kale , blueberries , raspberries , squash , lentils , carrots / carrots , Brussels sprouts , cabbage , toasted bread , wheat bread, mixed wheat bread, wheat flour: type 405 and 550 , Onions|
|<2%||Pineapples , eggplants , strawberries , cucumbers , potatoes , cherries , lettuce , mandarins , melons , peaches , plums , asparagus , spinach , tomatoes , grapes , zucchini|
- Total dietary fiber in g per 100 g of the respective food (all information relates to the ready-to-eat fresh weight, usual form of consumption)
Properties and effects
Dietary fiber is completely or partially indigestible because either no enzyme is formed in the digestive tract to cleave the existing (glycosidic) bond or no transport protein for active transport through the cell membrane from the intestine into the intestinal mucosa . Humans, for example, have enzymes to cleave glycosidic bonds of the α-1 → 2 ( sucrose ) or α-1 → 4 (e.g. maltose ) type, but none for compounds of the β-1 → 4 type ( cellulose ). Humans also have a number of glucose transporters . In the case of isomalt, there is a bond that can be cleaved; the glucose, which makes up 50%, is absorbed through the intestinal wall and metabolized in the body cells, while the sorbitol and mannitol (25% each) cannot be absorbed through the intestinal wall.
Dietary fiber in food increases the volume of food without significantly increasing the energy content. Some fiber, such as bran or psyllium husk, can retain a lot of water. Unless they are sufficiently swollen before ingestion, they will absorb more water in the stomach . The resulting increase in volume leads to a further stretching of the gastric sac after the meal, which in turn leads to a lowering of the appetizing ghrelin level and thus to an increase in the feeling of satiety .
Dietary fiber increases the length of time the food remains in the stomach . On the one hand, the swelling takes a certain time; on the other hand, water must be drunk or secreted by the stomach afterwards in order to set the minimum fluidity or maximum viscosity required for the gastric passage of the food pulp .
The fiber present in the chyme ensures an increase in volume through their ability to bind water. The pressure that high-fiber chyme exerts on the intestinal wall stimulates the peristalsis , which shortens the time that high-fiber food stays in the intestine (in contrast to the stomach).
No higher animal has its own enzymes to break down water-insoluble fiber, especially cellulose. Ruminants can still break down cellulose enzymatically with the help of the microorganisms that colonize their rumen . In the small and large intestines, on the other hand, there are no corresponding bacteria, so that water- insoluble fiber passes through the digestive tract practically unchanged.
In contrast, some of the water- soluble fiber is fermented by the intestinal flora of the large intestine . This creates different amounts of odorless gases such as As carbon dioxide , methane and hydrogen , as well as short chain fatty acids (engl. SCFA (short chain fatty acids)), such as acetate , propionate and butyrate which is opposite to the medium and long-chain fatty acids have a number of features ( see fat digestion ). They are largely absorbed by the lining of the large intestine and contribute to the nutrition of the cells of the mucous membrane.
Some fiber is produced by plants to ward off predators. Such poorly digested roughage can be converted into toxic fermentation alcohols and biogenic amines , which damage the intestinal mucosa and the immune system .
In addition to water, fiber also binds minerals, toxins, bile acids and microorganisms, which are then excreted in the stool. This is not a problem with a balanced mixed diet, but with additional fiber (e.g. as a dietary supplement ) a mineral deficiency can occur in the long term.
The idea that a high-fiber diet is beneficial to health and serves to prevent diseases of civilization is based u. a. on an epidemiological study by Burkitt and Trowell in the 1970s, which suggested that Africans who eat high-fiber diets are significantly less likely to develop certain diseases of civilization than Europeans and Americans who eat modern, low-fiber diets. Due to methodological deficiencies, this study is no longer valid today as proof of the health-promoting effect. Control studies carried out since then have been able to support the hypothesis in part, and in some cases there have been contradicting results.
A high-fiber diet may have a cholesterol-lowering effect.
The chair is the only way the human body, cholesterol excrete. Dietary fiber increases the excretion of bile acid in the stool by binding bile acids or their salts and thus preventing their reabsorption in the ileum . This in turn leads to a compensatory increase in bile acid synthesis, which in turn consumes cholesterol.
But there are also studies that do not confirm a cholesterol-lowering effect.
Coronary heart disease
A possible mechanism for this could be the cholesterol-lowering effect of dietary fiber.
In an animal experiment, researchers fed mice with high blood pressure with propionate , which is produced naturally in the intestine (see above ). After that, the animals had less pronounced heart damage or abnormal enlargement of the organ, which made them less prone to cardiac arrhythmias . Vascular damage, such as B. atherosclerosis decreased in mice. The research team now hopes to confirm their results by studying the effects of the substance on humans.
Cholecystolithiasis (gallstone disease)
Blood sugar level
Dietary fiber lowers the glycemic load of the food pulp .
The carbohydrates in the intestine are absorbed more slowly from foods rich in fiber . This causes a slower rise in blood sugar after eating and, accordingly, a less steep fall in blood sugar after the breakdown of the starch. That is why diabetics are advised to eat a diet rich in fiber.
Insoluble fiber (cellulose, hemicellulose, lignin) improves blood sugar levels in people with prediabetes , especially when there is a combination of impaired fasting sugar and impaired glucose tolerance.
A diet high in fiber encourages extensive chewing. It massages and firms the gums and mechanically cleans parts of the tooth surface. Abundant chewing also increases the amount of saliva. The saliva acts as a pH buffer and the calcium phosphate contained in the saliva ensures remineralization of the tooth enamel.
Diverticulosis / diverticulitis
There are different studies on the effects of dietary fiber on patients with diverticulosis and its inflammatory form, diverticulitis , some of which come to completely opposite results. A study says that a high-fiber low- fare favors the occurrence of these diseases and that the diverticulosis can be treated by high-fiber diet. This could be proven by the fact that a high pressure inside the colon was found in diverticulosis patients, which could be significantly reduced by long-term treatment with wheat bran compared to placebo. This high pressure, among other factors, is responsible for the development of colon diverticula (protrusions).
However, there is also a study that comes to the conclusion that a high-fiber diet actually increases the risk of developing diverticulosis or diverticulitis.
It can be assumed that the individual risk of colon cancer depends on the genetic predisposition , the carcinogen contamination of the food, the composition of the food and the overall diet. The extent to which the individual factors contribute to an increase or decrease in risk is disputed. It is assumed that the accelerated intestinal transit through high-fiber food allows the carcinogens, which are more or less abundant in the food, less time to act on the intestinal wall.
Experimental results in vitro show that the butyrate formed during fiber fermentation (see above) prevents impaired cell proliferation and thus inhibits the development of cancer. However, these findings cannot simply be transferred to the in vivo environment in the human intestine.
Biopsies show that around 90 percent of all colon cancer cases develop either from colon polyps or from adenomas . Avoidance of polyps or adenomas through a diet rich in fiber has not yet been proven. There are also no known studies that prove that a high-fiber diet lowers the risk of degeneration from benign to malignant tumors.
The study situation is inconsistent: the meta-analysis of five intervention studies showed no protective effect against colon cancer. In contrast, the EPIC study shows that a high-fiber diet lowers the risk of colon cancer by around 40 percent. The reason for the discrepancy could be the inconsistent execution of the clinical studies . The EPIC study can e.g. B. Do not rule out disruptive factors .
The German Nutrition Society (DGE) recommends consuming at least 30 grams of dietary fiber daily, preferably through whole grain products , vegetables , fresh or dried fruit and nuts . Care should be taken to ensure sufficient fluid intake at the same time. The National Consumption Study II showed, however, that 68 percent of men and 75 percent of women consume significantly less dietary fiber.
The Association for Independent Health Advice (UGB) recommends increasing your dietary fiber intake only gradually. This can be done by increasing the consumption of vegetables cooked firm to the bite and then slowly increasing the consumption of raw vegetables. White flour can also be gradually replaced by wholemeal flour. A high fiber intake is achieved through whole foods .
The FoodDrinkEurope (FDE) recommends 25 grams per day in its Guideline Daily Amounts .
The Harvard School of Public Health recommends a daily intake of at least 20 grams, ideally in the form of whole grains, fruits, vegetables, legumes and nuts. The American Heart Association recommends 25 grams daily.
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