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Fiber is the part of the plant that is resistant to hydrolysis (A chemical decomposition in which a substance is split into simpler compounds by the addition or the taking up of the elements of water) by human digestive enzymes and, with the exception of lignin, fibers are complex carbohydrates. These include pectin, gums, mucilages, hemicellulose, polysaccharides cellulose, and nonpolysaccharide lignins.
Fibers are water-soluble except cellulose, hemicellulose, and lignin, all of which form part of cell walls. Soluble fibers are sticky and combine with water to form gel-like substances. Pectin is a water-soluble fiber found in soft fruits and vegetables. Gums that are common food additives are also water-soluble, found in stems and seeds of some tropical plants. In general, fruits are higher in pectin and vegetables are higher in cellulose. Although cellulose and hemicellulose are not hydrolyzed, intestinal bacterial can digest some fiber to produce lipid fragments known as short-chain fatty acids. These fatty acids are absorbed in the colon and yield energy when metabolized.
Water Insoluble fibers pass through the gastrointestinal track unchanged, absorbing up to 15 times their weight, important since they provide the digestive tract with ‘bulk’ that helps facilitate food through the intestines to be evacuated as solid waste; hence, fiber often is called "nature’s natural laxative". Cellulose’s ability to absorb water produces softer stools and regular bowel movements. Also, insoluble fiber may prevent colon and rectal cancer and help to control diverticulosis (A sac or pouch in the walls of a canal or organ [e.g., GI tract] that becomes inflamed and causes pain and stagnation of feces. Source: The American Dietetic Association's Complete Food & Nutrition Guide, p. 145). Water-soluble fiber, as found in beans, fruit, and oat bran lowers cholesterol by binding to the cholesterol found in liver bile, to help control diabetes.
Overall, dietary fiber does not provide much nourishment to the human diet because of the inability to break down these carb sources for energy, yet reduces available kcalories by providing a sense of satiety and by absorbing some nutrients including fat. However, fiber also absorbs and eliminates essential fatty acids, food substances that are essential for good health and energy metabolism.
Also, dietary fiber may be a detoxifier since it binds with some toxic substances before elimination. But a diet extremely high in fiber is not a good idea since it impairs calcium, iron, and zinc absorption in the intestine.
Increasing fiber should be a gradual process since the majority of fiber products may cause unpleasant bloating, cramps, gas, and other symptoms – especially if they are eaten in large amounts at once. It is best to add one high fiber food at each eating session to judge whether the food causes symptoms or not. For instance, some people who have problems with brown rice may not have problems with apples, oatmeal, whole wheat bread, or other sources of similar fiber.
Many nutrition authorities estimate that 20-35 grams of fiber daily is a desirable intake for the average individual. Note that the amount of nutrients can vary in wheat products since the refining of grains remove part of the seed (e.g., bran, endosperm, and germ). Here are some fiber-rich sources:
1 ounce dry-roasted peanuts: 2.2 g 1/2 cup cooked broccoli: 2.2 g 1 potato with skin: 2.5 g 1 slice whole wheat bread: 2.8 g 1 cup carrots: 3.0 g 1/2 large grapefruit: 3.1 g 1 apple: 3.5 g 1 cup cooked long-grain brown rice: 3.3 g 1 cup cooked instant oatmeal: 3.5 g 3 cups air-popped popcorn: 3.7 g 1 pear: 4.3 g 1 cup of whole wheat spaghetti cooked: 5 g 1 cup baked beans: 7.0 g 1 cup boiled lentils: 7.9 g 1 serving bran cereal: 11 g
Here are different sources of fiber and their uses in the body:
CELLULOSE: Fruit legumes, nuts, oat bran, seeds, whole grains, and vegetables. Adds bulk to stool to reduce constipation; oat bran lowers cholesterol; may help control blood sugar; helps weight loss by displacing kcalories.
GUMS: Algae, barley, fruits, legumes, oats, seaweed, seeds, and vegetables. Adds bulk to stool to reduce constipation; may lower blood cholesterol; helps control blood sugar; helps weight loss by displacing kcalories.
HEMICELLULOSE: Fruits, legumes, nuts, oat bran, seeds, whole grains, and vegetables. Adds bulk to stool to reduce constipation; oat bran lowers cholesterol; may help control blood sugar; helps weight loss by displacing kcalories.
LIGNINS: Woody parts of bran, fruit skins, nuts, seeds, whole grains and vegetables. Adds bulk to stool to reduce constipation; may lower blood cholesterol; may help control blood sugar; helps weight loss by displacing kcalories.
MUCILAGES: Plant seeds and secretions. Adds bulk to stool to reduce constipation; may lower blood cholesterol; helps control blood sugar; helps weight loss by displacing kcalories.
PECTINS: Algae, barley, fruits, legumes, oats, seaweed, seeds, and vegetables. Adds bulk to stool to reduce constipation; may lower blood cholesterol; helps control blood sugar; helps weight loss by displacing kcalories.
About the Author Brian D. Johnston is the Director of Education and President of the I.A.R.T. fitness certification and education institute. He has written over 12 books and is a contributing author to the Merck Medical Manual. An international lecturer, Mr. Johnston wears many hats in the fitness and health industries, and can be reached at info@ExerciseCertification.com. Visit his site at www.ExerciseCertification.com for more free articles.
A Background to Dietary Fiber by Brian D. Johnston