The indigestible portions of plant-based food that moves through the digestive system is called dietary fiber. Dietary fiber absorbs a lot of water and thus makes digestion easier. Dietary fiber consists of non-starch polysaccharides like cellulose and many other plant components including insulin, waxes, lignin, chitins, dextrins, beta-glucans, pectins, and oligosaccharides.
Soluble and Insoluble Fibers
Sources of dietary fiber are categorized according to whether they are water-soluble or not. All kinds of plant food contain both kinds of fiber, but to varying degrees. Insoluble dietary fiber has passive water-attracting properties that help to soften stool and shorten transit time through the intestinal tract. Soluble fiber is metabolized through the process of fermentation, resulting in end products that have significant health effects. For example, the skin of plums or prunes is very thick, and covers a juicy pulp. The plum's skin is an insoluble fiber source, whereas the inside juicy pulp of the plum is soluble fiber source. Whole wheat, corn bran and vegetables like such as celery, green beans and potato skins are other some common sources of insoluble fiber.
Soluble fiber has been defined by the American Association of Cereal Chemists as the edible parts of plants that are difficult to digest and is absorbed partially in the human small intestine, with complete or partial fermentation in the large intestine. The edible parts of plants includes parts we can eat such as the skin, pulp, seeds, stems, leaves, roots. Carbohydrates such as oligosaccharides or polysaccharides, are additional sources of dietary fiber.
Dietary fiber is a collective term for a complex mixture of substances based on soluble fiber with different chemical and physical properties that cause different types of physiological effects. Dietary fiber improves gastrointestinal health, improves glucose tolerance and the insulin response in the body, reduces the risk of hyperlipidemia, hypertension and other coronary heart disease, reduces the risk of developing some cancers, and increases satiety (and therefore induces weight loss).
Guidelines on Dietary Fiber intake
It is recommended by the American Dietetic Association (ADA) that an individual should ingest a minimum of 20-35 grams of dietary fiber per day, depending on one’s overall caloric intake (e.g., a 2000 calorie per day diet should include 25 g of fiber per day). For children, ADA recommends that intake should equal age in years plus 5 g/day (e.g., a 4 year old should consume 9 g/day). Patients with constipation, vomiting, and abdominal pain should see a physician. In comparison, the British Nutrition Foundation suggested a minimum fiber intake of 12-24 g/day for healthy adults.
Sources of Dietary Fiber
Sources of soluble fiber are found in all plant-based foods, including peas, soybeans, oats, rye, chia, and barley, some fruits and fruit juices (particularly prune juice), plums and berries, vegetables like broccoli and carrots, root vegetables such as potatoes, sweet potatoes, and onions. Sources of insoluble fiber are whole grain foods, bran, nuts and seeds, vegetables like green beans, cauliflower, zucchini, and celery, and the skins of some fruits and tomatoes. Rubus fruits such as raspberry and blackberry are also excellent sources of dietary fiber.
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