Dietary fiber

Dietary fiber (sometimes called roughage) is the indigestible portion of plant foods having two main components:

• soluble (prebiotic, viscous) fiber that is readily fermented in the colon into gases and physiologically active byproducts, and
• insoluble fiber that is metabolically inert, absorbing water throughout the digestive system and easing defecation.^[1]

It acts by changing the nature of the contents of the gastrointestinal tract, and by changing how other nutrients and chemicals are absorbed.^[2] Soluble fiber absorbs water to become a gelatinous, viscous substance and is fermented by bacteria in the digestive tract. Insoluble fiber has bulking action and is not fermented,^[3] although a major dietary insoluble fiber source, lignin, may alter the fate and metabolism of soluble fibers.^[1]

Chemically, dietary fiber consists of non-starch polysaccharides such as arabinoxylans, cellulose and many other plant components such as resistant dextrins, inulin, lignin, waxes, chitins, pectins, beta-glucans and oligosaccharides.^[1] A novel position has been adopted by the US Department of Agriculture to include functional fibers as isolated fiber sources that may be included in the diet.^[1] 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 salubrious compounds during the fermentation of soluble fiber, and insoluble fiber's ability (via its passive hydrophilic properties) to increase bulk, soften stool and shorten transit time through the intestinal tract.

History of definition

Originally, fiber was defined to be the components of plants that resist human digestive enzymes, a definition that includes lignin and polysaccharides. The definition was later changed to also include resistant starches, along with inulin and other oligosaccharides.^[3]

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

Legumes such as soybeans contain dietary fibers.

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.^[4]

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 tubers and root vegetables such as sweet potatoes and onions (skins of these 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
• some fruits including avocado, and bananas
• the skins of some fruits, including tomatoes^[5]

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), bran (17 grams per cup), prunes (12 grams), Asian pear (10 grams each, 3.6% by weight), and quinoa (9 grams).^[6]

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.^[7]

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.^[8] Prebiotic soluble fiber products, like those containing inulin or oligosaccharides, may contribute to relief from inflammatory bowel disease,^[9] as in Crohn's disease,^[10] ulcerative colitis,^[11][12] and Clostridium difficile,^[13] due in part to the short-chain fatty acids produced with subsequent anti-inflammatory actions upon the bowel.^[14][15] Fiber supplements may be effective in an overall dietary plan for managing irritable bowel syndrome by modification of food choices.^[16]

Inulins

Chemically 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.^[17] 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.^[18]

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,^[19] magnesium,^[20] and iron,^[21] 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.^[22] Examples of vegetable gum fibers are guar gum 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.^[1][2] 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.^[3] Soluble fiber also attenuates the absorption of sugar, reduces sugar response after eating, normalizes blood lipid levels and, once fermented in the colon, produces short-chain fatty acids as byproducts with wide-ranging physiological activities (discussion below). Although insoluble fiber is associated with reduced diabetes risk, the mechanism by which this occurs is unknown.^[23]

Not yet formally proposed as an essential macronutrient, dietary fiber is nevertheless regarded as important for the diet, with regulatory authorities in many developed countries recommending increases in fiber intake.^{[1][2][24][25]}

Effects of fiber intake

Research has shown that fiber may benefit health in several different ways. Lignin and probably related materials that are recalcitrant to enzymatic degradation, diminish the nutritional value of foods.^[26]

Table legend

Color coding of table entries:

• Both Applies to both soluble and insoluble fiber
• Soluble Applies to soluble fiber only
• Insoluble Applies to insoluble fiber only

Dietary fiber functions and 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.^[32][33][34] 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.^[6]

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.^[6][35]

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 25g 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.

The British Nutrition Foundation has recommended a minimum fiber intake of 18 g/day for healthy adults.^[36]

Fiber recommendations in North America

On average, North Americans consume less than 50% of the dietary fiber levels recommended 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.^[37][38]

Recognizing the growing scientific evidence for physiological benefits of increased fiber intake, regulatory agencies such as the Food and Drug Administration (FDA) of the United States 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, an important factor for general cardiovascular health,^[39] and to lower risk of onset for some types of cancer.^[40]

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, resistant dextrins, 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^{[41][42][43][44]}—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.^[41]

Insufficient fiber in the diet can complicate defecation.^[45] Low-fiber feces are dehydrated and hardened, making them difficult to evacuate—defining constipation^[45] and possibly leading to development of hemorrhoids^[45] or anal fissures.

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.^[46]

Fiber recommendations in the UK

In June 2007, the British Nutrition Foundation issued a statement to define dietary fiber more concisely and list the potential health benefits established to date:^[47][48]

‘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 several main physiological consequences:

• helps prevent constipation
• reduces the risk of colon cancer
• 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

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.^[48]

Fiber and calories

Calories or kilojoules (as used on nutrition labels) are intended to be a measure of how much energy is available from the food source. This energy can be used immediately, for example allowing the body to move during exercise, or to make the heart beat. Energy that is not used immediately is stored as sugars in the short term and later converted to fats, which act as energy reserves.

Energy is extracted from food in a chemical reaction. Because of the principle of conservation of energy, energy can only be extracted when the chemical structure of food particles is changed. Since insoluble fiber particles do not change inside the body,^[49] the body should not absorb any energy (or Calories/kilojoules) from them.

Because soluble fiber is changed during fermentation, it could provide energy (Calories/kilojoules) to the body. As of 2009 nutritionists have not reached a consensus on how much energy is actually absorbed, but some approximate around 2 Calories (8.5 kilojoules) per gram of soluble fiber.^[50]

Regardless of the type of fiber, the body absorbs fewer than 4 Calories (16.7 kilojoules) per gram of fiber, which can create inconsistencies for actual product nutrition labels. In some countries, fiber is not listed on nutrition labels, and is considered 0 Calories/gram when the food's total Calories are computed. In other countries all fiber must be listed, and is considered 4 Calories/gram when the food's total Calories are computed (because chemically fiber is a type of carbohydrate and other carbohydrates contribute 4 Calories per gram). In the US, soluble fiber must be counted as 4 Calories per gram, but insoluble fiber may be (and usually is) treated as 0 Calories per gram and not mentioned on the label.^[50]

Short-chain fatty acids

When soluble fiber is fermented, short-chain fatty acids (SCFA) are produced. SCFA are involved in numerous physiological processes promoting health, including:^[51]

• stabilize blood glucose levels by acting on pancreatic insulin release and liver control of glycogen breakdown
• stimulate gene expression of glucose transporters in the intestinal mucosa, regulating glucose absorption^[52]
• provide nourishment of colonocytes, particularly by the SCFA butyrate
• suppress cholesterol synthesis by the liver and reduce blood levels of LDL cholesterol and triglycerides responsible for atherosclerosis
• lower 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
• stimulate production of T helper cells, antibodies, leukocytes, cytokines, and lymph mechanisms having crucial roles in immune protection
• improve barrier properties of the colonic mucosal layer, inhibiting inflammatory and adhesion irritants, contributing to immune functions

SCFA that are absorbed by the colonic mucosa pass through the colonic wall into the portal circulation (supplying the liver), and the liver transports them into the general circulatory system.

Overall, SCFA affect major regulatory systems, such as blood glucose and lipid levels, the colonic environment, and intestinal immune functions.^[53][54]

The major SCFA in humans are butyrate, propionate, and acetate, where butyrate is the major energy source for colonocytes, propionate is destined for uptake by the liver, and acetate enters the peripheral circulation to be metabolized by peripheral tissues.

FDA-approved health claims

The FDA allows producers of foods containing 1.7g per serving of psyllium husk soluble fiber or 0.75g of oat or barley soluble fiber as beta-glucans to claim that reduced risk of heart disease can result from their regular consumption.^[55]

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.^[55]

Eligible sources of soluble fiber providing beta-glucan include:

1. Oat bran
2. Rolled oats
3. Whole oat flour
4. Oatrim
5. Whole grain barley and dry milled barley
6. Soluble fiber from psyllium husk with purity of no less than 95%

The allowed label may state that diets low in saturated fat and cholesterol and that include soluble fiber from certain of the above foods “may” or “might” reduce the risk of heart disease.

As discussed in FDA regulation 21 CFR 101.81, the daily dietary intake levels of soluble fiber from sources listed above associated with reduced risk of coronary heart disease are:

• 3g or more per day of beta-glucan soluble fiber from either whole oats or barley, or a combination of whole oats and barley
• 7g or more per day of soluble fiber from psyllium seed husk.^[56]

Soluble fiber from consuming grains is included in other allowed health claims for lowering risk of some types of cancer and heart disease by consuming fruit and vegetables (21 CFR 101.76, 101.77, and 101.78).^[55]

Soluble fiber fermentation

The American Association of Cereal Chemists has 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.”^[57] In this definition:

edible parts of plants

indicates that some parts of a plant we eat—skin, pulp, seeds, stems, leaves, roots—contain fiber. Both insoluble and soluble sources are in those plant components.

carbohydrates

complex carbohydrates, such as long-chained sugars also called starch, oligosaccharides, or polysaccharides, are sources of soluble fermentable fiber.

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, acetic (ethanoic), propionic, and valeric acids—that scientific evidence is revealing to have significant health properties.^[51]

As an example of fermentation, shorter-chain carbohydrates (a type of fiber found in legumes) cannot be digested, but are changed via fermentation in the colon into short-chain fatty acids and gases (which are typically expelled as flatulence).

According to a 2002 journal article,^[41] fibers compounds with partial or low fermentability include:

• cellulose, a polysaccharide
• hemicellulose, a polysaccharide
• lignans, a group of phytoestrogens
• plant waxes
• resistant starches

Fiber compounds with high fermentability include:

• beta-glucans, a group of polysaccharides
• pectins, a group of heteropolysaccharides
• natural gums, a group of polysaccharides
• inulins, a group of polysaccharides
• oligosaccharides, a group of short-chained or simple sugars
• resistant dextrins^[58]

See also

• Methylcellulose
• Prebiotic—indigestible matter which encourages growth of gut flora
• Resistant starch

Footnotes

References

• Fiber, Harvard School of Public Health
• Fiber Health Claims That Meet Significant Scientific Agreement, US Food and Drug Administration
• Higgins JA (2004). "Resistant starch: metabolic effects and potential health benefits". J AOAC Int. 87 (3): 761–8. PMID 15287677. 
• Tungland BC, Meyer D. Nondigestible oligo- and polysaccharides (dietary fiber): their physiology and role in human health and food. Comprehensive Reviews in Food Science and Food Safety 1:73-92, 2002.
• Parisi GC, Zilli M, Miani MP, et al. (August 2002). "High-fiber diet supplementation in patients with irritable bowel syndrome (IBS): a multicenter, randomized, open trial comparison between wheat bran diet and partially hydrolyzed guar gum (PHGG)". Dig Dis Sci. 47 (8): 1697–704. doi:10.1023/A:1016419906546. PMID 12184518. http://www.kluweronline.com/art.pdf?issn=0163-2116&volume=47&page=1697. 
• Marlett JA (2001). "Dietary fiber and cardiovascular disease". In Dreher ML, Cho SS. Handbook of dietary fiber. New York: M. Dekker. pp. 17–30. ISBN 0-8247-8960-1. 
• 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 (1980). "Effect of fiber on colon function". In Kay R, Spiller GA. Medical aspects of dietary fiber. New York: Plenum Medical Book Co. pp. 1–26. ISBN 0-306-40507-5. 
• Prynne CJ, Southgate DA (May 1979). "The effects of a supplement of dietary fiber on faecal excretion by human subjects". Br J Nutr. 41 (3): 495–503. doi:10.1079/BJN19790064. PMID 465439. 

External links

• 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
• Anatomy and Function of the Human Digestive System