Dietary fibers are the indigestible portion of plant foods that move food through the digestive system, absorbing water and making defecation easier. 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.
Soluble and insoluble fibers
Sources of dietary fiber are usually divided according to whether they are water-soluble or not. Both types of fiber are present in all plant foods, with varying degrees of each according to a plant’s characteristics. Insoluble fiber possesses passive water-attracting properties that help to increase bulk, soften stool and shorten transit time through the intestinal tract. Soluble fiber undergoes metabolic processing via fermentation, yielding end-products with broad, significant health effects. 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. Other sources of insoluble fiber include whole wheat, wheat and corn bran, flax seed lignans and vegetables such as celery, green beans and potato skins.
The American Association of Cereal Chemists defined soluble fiber this way: “the edible parts of plants or similar carbohydrates resistant to digestion and absorption in the human small intestine with complete or partial fermentation in the large intestine”.
There are several key words in that statement that invite analysis and comment for considering fermentable fiber.
edible parts of plants — indicates that all parts of a plant we eat — skin, pulp, seeds, stems, leaves, roots — contain fiber. Both insoluble and soluble sources are in those plant components.
resistant to digestion and absorption in the human small intestine — foods providing nutrients are digested by gastric acid and digestive enzymes in the stomach and small intestine where the nutrients are released then absorbed through the intestinal wall for transport via the blood throughout the body. A food resistant to this process is undigested, as insoluble and soluble fibers are. They pass to the large intestine only affected by their absorption of water (insoluble fiber) or dissolution in water (soluble fiber).
complete or partial fermentation in the large intestine — the large intestine comprises a segment called the colon within which additional nutrient absorption occurs through the process of fermentation. Fermentation occurs by the action of colonic bacteria on the food mass, producing gases and short-chain fatty acids. It is these short-chain fatty acids — butyric, ethanoic (acetic), propionic, and valeric acids — that might have significant health properties.
Short-chain fatty acids
Short-chain fatty acids are used by the intestinal mucosa or absorbed through the colonic wall into the portal circulation (supplying the liver) that transports them into the general circulatory system. Particularly, butyric acid has extensive physiological actions that promote health effects, among which are:
- Stabilizes blood glucose levels by acting on pancreatic insulin release and liver control of glycogen breakdown
- Suppresses cholesterol synthesis by the liver and reduces blood levels of LDL cholesterol and triglycerides responsible for atherosclerosis
- Lowers colonic pH (i.e., raises the acidity level in the colon) which protects the lining from formation of colonic polyps and increases absorption of dietary minerals
- Stimulates production of T helper cells, antibodies, leukocytes, cytokines and lymph mechanisms having crucial roles in immune protection
- Increases proliferation of colonic bacteria beneficial for intestinal health — Bifidobacteria and Lactobacilli (serving a probiotic function)
- Improves barrier properties of the colonic mucosal layer, inhibiting inflammatory and adhesion irritants
Summarizing these effects, fermentable fibers yield the important short-chain fatty acids that affect blood glucose and lipid levels, improve the colonic environment and regulate immune responses. 
Regulatory guidance on fiber products
On average, North Americans consume less than 50% of the dietary fiber levels required for good health. In the preferred food choices of today's youth, this value may be as low as 20%, a factor considered by experts as contributing to the obesity crisis seen in many developed countries.  
Recognizing the growing scientific evidence for physiological benefits of increased fiber intake, regulatory agencies such as the US Food and Drug Administration (FDA) have given approvals to food products making health claims for fiber.
In clinical trials to date, these fiber sources were shown to significantly reduce blood cholesterol levels -- thus are important to general cardiovascular health -- and to lower risk of onset for some types of cancer.
Soluble (fermentable) fiber sources gaining FDA approval are
- Psyllium seed husk (7 grams per day)
- Beta-glucan from oat bran, whole oats, oatrim or rolled oats (3 grams per day)
- Beta-glucan from whole grain or dry-milled barley (3 grams per day)
Other examples of fermentable fiber sources (from plant foods or biotechnology) used in functional foods and supplements include inulin, fructans, xanthan gum, cellulose, guar gum, fructooligosaccharides (FOS) and oligo- or polysaccharides.
Consistent intake of fermentable fiber through foods like berries and other fresh fruit, vegetables, whole grains, seeds and nuts is now known to reduce risk of some of the world’s most prevalent diseases — obesity, diabetes, high blood cholesterol, cardiovascular disease, and numerous gastrointestinal disorders. In this last category are constipation, inflammatory bowel disease, ulcerative colitis, hemorrhoids, Crohn’s disease, diverticulitis, and colon cancer--all disorders of the intestinal tract where fermentable fiber can provide healthful benefits.
Insufficient fiber in the diet can complicate defecation. Low-fiber feces are dehydrated and hardened, making them difficult to evacuate -- defining constipation and possibly leading to development of hemorrhoids.
Although many researchers believe that dietary fiber intake reduces risk of colon cancer, one study, conducted by researchers at the Harvard School of Medicine of over 88,000 women, did not show a statistically significant relationship between higher fiber consumption and lower rates of colorectal cancer or adenomas.
Summary of definition and potential health benefits
‘Dietary fiber’ has been used as a collective term for a complex mixture of substances with different chemical and physical properties which exert different types of physiological effects. The use of certain analytical methods to quantify ‘dietary fiber’ by nature of its indigestibility results in many other indigestible components being isolated along with the carbohydrate components of dietary fiber. These components include resistant starches and oligosaccharides along with other substances that exist within the plant cell structure and contribute to the material that passes through the digestive tract. Such components are likely to have physiological effects. Yet, some differentiation has to be made between these indigestible plant components and other partially digested material, such as protein, that appears in the large bowel. Thus, it is better to classify fiber as a group of compounds with different physiological characteristics, rather than to be constrained by defining it chemically. Diets naturally high in fiber can be considered to bring about five main physiological consequences:
Therefore, it is not appropriate to state that fiber has a single all encompassing physiological property as these effects are dependent on the type of fiber in the diet. The beneficial effects of high fiber diets are the summation of the effects of the different types of fiber present in the diet and also other components of such diets. Defining fiber physiologically allows recognition of indigestible carbohydrates with structures and physiological properties similar to those of naturally occurring dietary fibers.
- improvements in gastrointestinal health
- improvements in glucose tolerance and the insulin response
- reduction of hyperlipidemia, hypertension and other coronary heart disease risk factors
- reduction in the risk of developing some cancers
- increased satiety and hence some degree of weight management
Guidelines on fiber intake
The American Dietetic Association (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). No guidelines have yet been established for the elderly or very ill. Patients with current constipation, vomiting, and abdominal pain should see a physician. Certain bulking agents are not commonly recommended with the prescription of opioids because the slow transit time mixed with larger stools may lead to severe constipation, pain, or obstruction.
Sources of fiber
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 American Dietetic Association recommends consuming a variety of fiber-rich foods.
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 (particularly prune juice, plums and berries)
- certain vegetables such as broccoli and carrots
- 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).
Legumes also typically contain shorter-chain carbohydrates indigestible by the human digestive tract but which may be metabolized by bacterial fermentation in the large intestine (colon), yielding short-chain fatty acids and gases (flatulence).
Sources of insoluble fiber include:
- whole grain foods
- nuts and seeds
- vegetables such as green beans, cauliflower, zucchini, and celery
- 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).
Remarkable among plant foods, the Amazonian palmberry, açaí (Euterpe oleracea Mart.), has been analyzed by two research groups reporting its content of dietary fiber is 25-44% of total mass in freeze-dried powder.  
There are many types of soluble fiber supplements available 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.
Psyllium seed husk may reduce the risk of heart disease by lowering cholesterol levels, and is known to help alleviate the symptoms of irritable bowel syndrome, though it often causes uncomfortable bloating. Psyllium husk may be used as a bulk-forming laxative.
The FDA allows foods containing 1.7 g of psyllium husk soluble fiber or .75 g of oat or barley soluble fiber as beta-glucans to claim that reduced risk of heart disease can result from regular consumption.
The FDA statement template for making this claim is: Soluble fiber from foods such as [name of soluble fiber source, and, if desired, name of food product], as part of a diet low in saturated fat and cholesterol, may reduce the risk of heart disease. A serving of [name of food product] supplies __ grams of the [necessary daily dietary intake for the benefit] soluble fiber from [name of soluble fiber source] necessary per day to have this effect.
In clinical studies approved by the FDA, the cholesterol-lowering benefit of soluble fiber from psyllium, when taken as directed and combined with a low-fat, low-cholesterol diet, was 4 to 6 percent for total blood cholesterol and 4 to 8 percent for LDL (bad) cholesterol vs. a low-fat diet alone.
Inulins are a group of oligosaccharides occurring naturally in many plants. They belong to a class of carbohydrates known as fructans. Inulin is used increasingly in prepared foods due to its favorable nutritional characteristics. Subtly sweet, it can be used to replace sugar, fat, and flour, and is often used to improve the flow and mixing qualities of powdered nutritional supplements.
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 gum fiber supplements are relatively new to the market. Often sold as a powder, vegetable gum fibers dissolve easily with no aftertaste. They are effective for the treatment of irritable bowel syndrome (Parisi, 2002). Examples of vegetable gum fibers are guar gum (example brand Benefiber reformulated to wheat dextrin in 2006) and acacia gum.
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.  The food's phytate content is mainly responsible for the reduced bioavailability of certain minerals and vitamins like calcium, zinc, vitamin C and magnesium.
- Prebiotic — indigestible matter which encourages growth of gut flora
- Low residue diet
- Resistant starch
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- US Food and Drug Administration. Health Claims: Soluble fiber from certain foods and risk of heart diseases. Code of Federal Regulations. 2001;21:101.81.
- Eastwood MA, Brydon WG. Tadesse K. Effect of fiber on colon function. In: Spiller GA, Kay RM, eds. Medical Aspects of Dietary Fiber. New York, NY: Plenum Press; 1980:1-26.
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Fiber Health Claims That Meet Significant Scientific Agreement, US Food and Drug Administration, http://www.cfsan.fda.gov/~dms/lab-ssa.html
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Higgins JA. Resistant starch: metabolic effects and potential health benefits. Journal of AOAC International 87:761-767, 2004.
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- Food Sources of Dietary Fiber, Appendix B-8, Dietary Guidelines for Americans published jointly in 2005 by the US Department of Health and Human Services and the Department of Agriculture
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