The recommended intake of dietary fiber is 14 g per 1,000 calories consumed. That translates into a need for 25 to 35 g of fiber per day in a typical diet. If you plan to increase your fiber intake, do so over a two to three week period to allow your body to adjust to the change. Increase your fluid intake, as well, so you are drinking six to eight cups of liquids without caffeine.
As indigestible complex carbohydrates, dietary fibers perform important biological functions, though they supply no calories or nutrients and are resistant to digestive enzymes. Dietary fiber comes from the walls of plant cells and includes cellulose, hemicellulose, lignin, pectin, mucilage and gum. Many food products list total fiber content in grams, which includes both soluble and insoluble fiber.
Crude fiber refers to one type of dietary fiber, the type that remains as residue after food receives a standardized laboratory treatment with dilute acid and alkali. The treatment dissolves all the soluble fiber and some of the insoluble fiber in a food. The residue or crude fiber is primarily composed of cellulose and lignin. Crude fiber is a nutritionally obsolete term, according to the National Research Council's Commission on Life Sciences. Crude fiber measurements, the result of lab analysis, may underestimate the actual dietary fiber in a food item by 50 percent or more.
Insoluble fiber passes through the stomach and intestines undigested, but it absorbs water and organic toxins and waste. Insoluble fiber adds bulk to stools and encourages food to pass through the digestive tract more quickly. Good sources of insoluble fiber include whole grains, wheat bran and vegetables such as celery, spinach and fresh tomatoes.
Water-soluble fiber absorbs water and becomes a gel during the digestive process. As a gel, it slows digestion. Soluble fiber, such as pectin and lignin, helps to prevent cholesterol from building up in blood vessel walls, and thus helps to prevent heart disease. It can also help to stabilize blood sugar levels, which is beneficial for individuals with diabetes. Good sources of soluble fiber include barley, oat bran, seeds, nuts, peas, beans and lentils, as well as some vegetables and fruit. Plant-based foods typically contain 25 percent to 30 percent soluble fiber, which usually is less than their insoluble fiber content.
- The National Academies Press; Carbohydrates and Fiber; National Research Council’s Commission on Life Sciences; 1989
- University of Maryland Medical Center: Soluble vs. Insoluble Fiber
- Harvard University: Fiber Content of Foods in Common Portions
- University of Illinois Extension: Apples and More
- University of California Los Angeles: Facts of Fiber
- Medical Dictionary: Crude Fiber