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 five main physiological consequences:
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.
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.
The British Nutrition Foundation has recommended a minimum fiber intake of 12-24 g/day for healthy adults.
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
bran
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.
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
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 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.
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
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 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).[verification needed] Examples of vegetable gum fibers are guar gum (example brand Benefiber reformulated to wheat dextrin in 2006) and acacia gum.
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. The food's phytate content is mainly responsible for the reduced bioavailability of certain minerals and vitamins like calcium, zinc, vitamin C and magnesium.
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