Dietary fiber (fibre) , sometimes called roughage , is the indigestible portion of plant foods that pushes food through the digestive system, absorbs water and eases defecation. It acts by changing the nature of the contents of the gastrointestinal tract, and by changing how other nutrients and chemicals are absorbed.
Dietary fiber can be soluble (able to dissolve in water) or insoluble (not able to dissolve in water). Soluble fiber, like all fiber, cannot be digested. But it does change as it passes through the digestive tract, being transformed (fermented) by bacteria there. Soluble fiber also absorbs water to become a gelatinous substance that passes through the body. Insoluble fiber has bulking action but is mostly unchanged by fermentation as it passes through the body.
Chemically, dietary fiber consists of non-starch polysaccharides such as cellulose and many other plant components such as dextrins, inulin, lignin, waxes, chitins, pectins, beta-glucans and oligosaccharides. The term "fiber" is somewhat of a misnomer, since many types of so-called dietary fiber are not fibers at all.
Food sources of dietary fiber are often divided according to whether they provide (predominantly) soluble or insoluble fiber. Plant foods contain both types of fiber in varying degrees according to the plant’s characteristics.
Advantages of consuming fiber are the production of health-promoting compounds during the fermentation of soluble fiber, and insoluble fiber's ability (via its passive water-attracting properties) to increase bulk, soften stool and shorten transit time through the intestinal tract.
Types of fiber
Originally, fiber was defined to be the components of plants that resist human digestive enzyme, a definition that includes lignin and polysaccharides. The definition was later changed to also include resistant starches, along with inulin and other oligosaccharides.
Soluble fibers, such as pectin, are viscous and fermented in the colon; insoluble fibers, such as wheat bran, are fermented only to a limited extent, although they do have bulking action.
Sources of fiber
Dietary fiber is found in plants. While all plants contain some fiber, plants with high fiber concentrations are generally the most practical source.
Fiber-rich plants can be eaten directly. Or, alternatively, they can be used to make supplements and fiber-rich processed foods.
The American Dietetic Association (ADA) recommends consuming a variety of fiber-rich foods.
Plant sources of fiber
Some plants contain significant amounts of soluble and insoluble fiber. For example plums (or prunes) have a thick skin covering a juicy pulp. The plum's skin is an example of an insoluble fiber source, whereas soluble fiber sources are inside the pulp.
Soluble fiber is found in varying quantities in all plant foods, including:
- legumes (peas, soybeans, and other beans)
- oats, rye, chia, and barley
- some fruits and fruit juices (including prune juice, plums, berries, bananas, and the insides of apples and pears)
- certain vegetables such as broccoli, carrots and Jerusalem artichokes
- root vegetables such as potatoes, sweet potatoes, and onions (skins of these vegetables are sources of insoluble fiber)
- psyllium seed husk (a mucilage soluble fiber).
Sources of insoluble fiber include:
- whole grain foods
- wheat and corn bran
- nuts and seeds
- potato skins
- flax seed
- lignans
- vegetables such as green beans, cauliflower, zucchini (courgette), celery, and nopal
- the skins of some fruits, including tomatoes
The five most fiber-rich plant foods, according to the Micronutrient Center of the Linus Pauling Institute, are legumes (15-19 grams of fiber per US cup serving, including several types of beans, lentils and peas), wheat bran (17 grams per cup), prunes (12 grams), Asian pear (10 grams each, 3.6% by weight), and quinoa (9 grams).
Rubus fruits such as raspberry (8 grams of fiber per serving) and blackberry (7.4 grams of fiber per serving) are exceptional sources of fiber.
Fiber supplements
These are a few example forms of fiber that have been sold as supplements or food additives. These may be marketed to consumers for nutritional purposes, treatment of various gastrointestinal disorders, and for such possible health benefits as lowering cholesterol levels, reducing risk of colon cancer, and losing weight.
Soluble fiber supplements may be beneficial for alleviating symptoms of irritable bowel syndrome, such as diarrhea and/or constipation and abdominal discomfort. Prebiotic soluble fiber products, like those containing inulin or oligosaccharides, may contribute to relief from inflammatory bowel disease, as in Crohn's disease, ulcerative colitis, and Clostridium difficile , due in part to the short-chain fatty acids produced with subsequent anti-inflammatory actions upon the bowel. Fiber supplements may be effective in an overall dietary plan for managing irritable bowel syndrome by modification of food choices.
Inulins
Main article: InulinChemically defined as oligosaccharides occurring naturally in most plants, inulins have nutritional value as carbohydrates, or more specifically as fructans, a polymer of the natural plant sugar, fructose. Inulin is typically extracted by manufacturers from enriched plant sources such as chicory roots or Jerusalem artichokes for use in prepared foods. Subtly sweet, it can be used to replace sugar, fat, and flour, is often used to improve the flow and mixing qualities of powdered nutritional supplements, and has significant potential health value as a prebiotic fermentable fiber.
Inulin is advantageous because it contains 25-30% the food energy of sugar or other carbohydrates and 10-15% the food energy of fat. As a prebiotic fermentable fiber, its metabolism by gut flora yields short-chain fatty acids (discussed above) which increase absorption of calcium, magnesium, and iron, resulting from upregulation of mineral-transporting genes and their membrane transport proteins within the colon wall. Among other potential beneficial effects noted above, inulin promotes an increase in the mass and health of intestinal Lactobacillus and Bifidobacterium populations.
Vegetable gums
Vegetable gum fiber supplements are relatively new to the market. Often sold as a powder, vegetable gum fibers dissolve easily with no aftertaste. In preliminary clinical trials, they have proven effective for the treatment of irritable bowel syndrome. Examples of vegetable gum fibers are guar gum (reformulated to wheat dextrin) and acacia senegal gum.
Mechanism
The main action of dietary fiber is to change the nature of the contents of the gastrointestinal tract, and to change how other nutrients and chemicals are absorbed. Soluble fiber binds to bile acids in the small intestine, making them less likely to enter the body; this in turn lowers cholesterol levels in the blood. Soluble fiber also inhibits the absorption of sugar and reduces sugar response after eating. Although insoluble fiber is associated with reduced diabetes risk, the mechanism by which this occurs is unknown.
Dietary fiber is has not been formally proposed or accepted as an essential nutrient. Like fluoride, fiber is a nutrient that acts against a harmful biological process; in fiber's case the harm comes from bile acids and sugars in the intestine.
Benefits of fiber intake
Eating fiber has many benefits for your health. The consumption of soluble fiber has been shown to protect you from developing heart disease by reducing your cholesterol levels. The consumption of insoluble fiber reduces your risk of developing constipation, colitis, colon cancer, and hemorrhoids.
—Medical News Today, What Is Fiber? What Is Dietary Fiber? Fiber Rich Foods
Table legend
Color coding of table entries:
- Both Applies to both soluble & insoluble fiber
- Soluble Applies to soluble fiber only
- Insoluble Applies to insoluble fiber only
Dietary fiber functions & benefits
Fiber does not bind to minerals and vitamins and therefore does not restrict their absorption, but rather evidence exists that fermentable fiber sources improve absorption of minerals, especially calcium. Some plant foods can reduce the absorption of minerals and vitamins like calcium, zinc, vitamin C and magnesium, but this is caused by the presence of phytate (which is also thought to have important health benefits), not by fiber.
Guidelines on fiber intake
Current recommendations from the United States National Academy of Sciences, Institute of Medicine, suggest that adults should consume 20-35 grams of dietary fiber per day, but the average American's daily intake of dietary fiber is only 12-18 grams.
The ADA recommends a minimum of 20-35 g/day for a healthy adult depending on calorie intake (e.g., a 2000 cal/8400 kJ diet should include 25 g of fiber per day). The ADA's recommendation for children is that intake should equal age in years plus 5 g/day (e.g., a 4 year old should consume 9 g/day).
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