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, nopal, green beans, potato skins and tomato peel.
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 this statement that invite analysis and comment for considering fermentable fiber.
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.
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.
To summarize, fiber components with partial or low fermentability are
Fiber components with high fermentability are
Short chain fatty acids (SCFA) 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. 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. Collectively, SCFA have extensive physiological actions promoting health effects, among which major functions are:
Summarizing these effects, upon fermentation, prebiotic fibers yield SCFA that affect major regulatory systems, such as blood glucose and lipid levels, the colonic environment and intestinal immune functions.
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
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.
‘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.
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.
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 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:
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.
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:
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:
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 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.
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.
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 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. 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.