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Dietary fiber
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Dietary fiber, sometimes called "roughage", is the indigestible portion of plant foods that pushes food through the digestive system, absorbing water and easing defecation.
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.
Dietary fiber can be soluble (able to dissolve in water) or insoluble (not able to dissolve in water).
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Encyclopedia
Dietary fiber, sometimes called "roughage", is the indigestible portion of plant foods that pushes food through the digestive system, absorbing water and easing defecation.
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.
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 (fermentated) by bacteria there. Soluble fiber also absorbs water to become a gelatinous substance that passes through the body
.
Insoluble fiber, however, passes through the body largely unchanged
.
Food sources of dietary fiber are often divided according to whether they provide (predominantly) soluble or insoluble fiber. To be precise, both types of fiber are present in all plant foods, with varying degrees of each according to a plant’s characteristics.
Potential 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.
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, such as those made by the All Bran, Fiber One and Quaker Oats companies. For example, Fiber One's granola-bar like "chewy bars" contain 9 grams of dietary fiber and have chickory root, a fiber-rich plant, as their main ingredient
.
The American Dietetic Association 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:
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.
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.
Psyllium husk
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.
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
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. 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. 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.
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 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.
The British Nutrition Foundation has recommended a minimum fiber intake of 12-24 g/day for healthy adults.
Fiber recommendations in North America
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 U.S. 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, an important factor for general cardiovascular health, and to lower risk of onset for some types of cancer.
Soluble (fermentable) fiber sources gaining FDA approval are
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 — 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 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.
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:
‘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.
Fiber and calories
Calories or kilojoules (as used on nutrition labels) are intended to be a measure of how much energy the body absorbs from food. 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 fat and muscle, which the body can break down and use for energy later.
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
, 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.
Regardless of the type of fiber, the body absorbs less than 4 Calories (16.7 kilojoules) per gram of fiber, which can create inconsistancies 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 (and usually is) treated as 0 Calories per gram and not mentioned on the label.
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:
- stabilize blood glucose levels by acting on pancreatic insulin release and liver control of glycogen breakdown
- 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 not 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.
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 1.7 g per serving of psyllium husk soluble fiber or 0.75 g of oat or barley soluble fiber as beta-glucans to claim that reduced risk of heart disease can result from their 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..
Eligible sources of soluble fiber providing beta-glucan include:
- Oat bran
- Rolled oats
- Whole oat flour
- Oatrim
- Whole grain barley and dry milled barley
- 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:
- 3 g or more per day of beta-glucan soluble fiber from either whole oats or barley, or a combination of whole oats and barley
- 7 g or more per day of soluble fiber from psyllium seed husk.
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).
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.”
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 excellent 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, ethanoic (acetic), propionic, and valeric acids — that scientific evidence is revealing to have significant health properties.
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,
fibers compounds with partial or low fermentability include:
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
Misconceptions
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.
See also
External links
- - Extensive information about dietary fiber
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