Basics of Dietary Fiber

Dietary fiber or roughage is the indigestible portion of food derived from plants. It has two main components:

• Soluble fiber, which dissolves in water, is readily fermented in the colon into gases and physiologically active byproducts, and can be prebiotic and viscous. It delays gastric emptying which in turn can cause an extended feeling of fullness.
• Insoluble fiber, which does not dissolve in water, is metabolically inert and provides bulking, or it can be prebiotic and metabolically ferment in the large intestine. Bulking fibers absorb water as they move through the digestive system, easing defecation.

Dietary fibers can act by changing the nature of the contents of the gastrointestinal tract and by changing how other nutrients and chemicals are absorbed. Some types of soluble fiber absorb water to become a gelatinous, viscous substance which is fermented by bacteria in the digestive tract. Some types of insoluble fiber have bulking action and are not fermented. Lignin, a major dietary insoluble fiber source, may alter the rate and metabolism of soluble fibers. Other types of insoluble fiber, notably resistant starch, are fully fermented. Some but not all soluble plant fibers block intestinal mucosal adherence and translocation of potentially pathogenic bacteria and may therefore modulate intestinal inflammation, an effect that has been termed contrabiotic.

Chemically, dietary fiber consists of non-starch polysaccharides such as arabinoxylans, cellulose, and many other plant components such as resistant starch, resistant dextrins, inulin, lignin, chitins, pectins, beta-glucans, and oligosaccharides. 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. The term "fiber" is something of a misnomer, since many types of so-called dietary fiber are not actually fibrous.

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 healthful compounds during the fermentation of soluble fiber, and insoluble fiber's ability (via its passive hygroscopic properties) to increase bulk, soften stool, and shorten transit time through the intestinal tract. A disadvantage of a diet high in fiber is the potential for significant intestinal gas production and bloating.

Definition of Dietary Fiber

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 starch, along with inulin and other oligosaccharides.

Sources of Fiber

Plant sources of fiber

Some plants contain significant amounts of soluble and insoluble fiber. For example, plums and prunes have a thick skin covering a juicy pulp. The skin is a source of insoluble fiber, whereas soluble fiber is in the pulp. Grapes also contain a fair amount of fiber.

The root of the konjac plant, or glucomannan, produces results similar to fiber and may also be used to relieve constipation. Glucomannan is sold in various forms, and while safe in some forms, it can be unsafe in others, possibly leading to throat or intestinal blockage.

Soluble fiber is found in varying quantities in all plant foods, including:

• legumes (peas, soybeans, lupins and other beans)
• oats, rye, chia, and barley
• some fruits (including figs, avocados, plums, prunes, berries, ripe bananas, and the skin of apples, quinces 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 also)
• psyllium seed husks (a mucilage soluble fiber) and flax seeds
• nuts, with almonds being the highest in dietary fiber

Sources of insoluble fiber include:

• whole grain foods
• wheat and corn bran
• legumes such as beans and peas
• nuts and seeds
• potato skins
• lignans
• vegetables such as green beans, cauliflower, zucchini (courgette), celery, and nopal
• some fruits including avocado, and unripe bananas
• the skins of some fruits, including kiwifruit, grapes and tomatoes

Dietary Fiber and Obesity

Dietary fiber has many functions in diet, one of which may be to aid in energy intake control and reduced risk for development of obesity. The role of dietary fiber in energy intake regulation and obesity development is related to its unique physical and chemical properties that aid in early signals of satiation and enhanced or prolonged signals of satiety. Early signals of satiation may be induced through cephalic- and gastric-phase responses related to the bulking effects of dietary fiber on energy density and palatability, whereas the viscosity-producing effects of certain fibers may enhance satiety through intestinal-phase events related to modified gastrointestinal function and subsequent delay in fat absorption. In general, fiber-rich diets, whether achieved through fiber supplementation or incorporation of high fiber foods into meals, have a reduced energy density compared with high fat diets. This is related to fiber’s ability to add bulk and weight to the diet. There are also indications that women may be more sensitive to dietary manipulation with fiber than men. The relationship of body weight status and fiber effect on energy intake suggests that obese individuals may be more likely to reduce food intake with dietary fiber inclusion.

Guidelines for Fiber Intake

Current recommendations from the United States National Academy of Sciences, Institute of Medicine, state that for Adequate Intake, adult men ages 14–50 consume 38 grams of dietary fiber per day, men 51 and older 30 grams, women ages 19–50 to consume 25 grams per day, women 51 and older 21 grams.

The AND (Academy of Nutrition and Dietetics, previously 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 AND'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.

                                      https://en.wikipedia.org/wiki/Dietary_fiber