Dietary fiber, or roughage, is the part of plants that is not digested or absorbed in the stomach or small intestine. There are many different types of fiber; they are often classified as either soluble or insoluble. Most plant foods have a mix of soluble and insoluble fibers. Soluble and insoluble fibers have distinct functions and benefits, as well as some that overlap. In its position paper on the health implications of dietary fiber, the American Dietetic Association explains that the total fiber content of some foods is known to have benefits but the exact cause --- or whether it is due to soluble vs. insoluble fiber --- is unclear.
Fiber Recommendations
The Recommended Daily Allowance for total fiber is based on your calorie intake. Aim for 14g fiber per 1,000 calories you eaat. For women, this is about 25g fiber daily; for men, 38g fiber daily. The American Dietetic Association reports that usual fiber intake in the U.S. is only 15g daily. On food labels, the Daily Value used is 25g total fiber, based on a 2,000 calorie diet. The FDA defines less than 5 percent DV as a low-fiber food and more than 20 percent DV as a high-fiber food. There are no specific recommendations for soluble and insoluble fiber. Rather, the Dietary Guidelines recommend that you eat a variety of fruits, vegetables, legumes and whole grains to receive all the health benefits of different types of fiber.
Soluble Fiber Functions
Fibers that are classified as soluble attract and dissolve in water. They become soft and gel-like in the small intestine. They slow digestion and slow the rate of nutrient absorption --- especially carbohydrates --- from the stomach and small intestine. In the large intestine, naturally occurring bacteria ferment some soluble fibers. This is why you may experience gas when you eat some high-fiber foods. Oat bran, barley, legumes, brown rice, whole grains or cereals, psyllium and many fruits and vegetables contain soluble fiber.
Soluble Fiber Benefits
According to the American Dietetic Association, soluble fiber intake reduces risk of cardiovascular disease. It lowers total cholesterol and "bad" low-density lipoprotein cholesterol, decreasing risk of heart attack and death from heart disease. Additional cardiovascular benefits include lower blood pressure and reduced levels of C-reactive protein, a marker that indicates inflammation. Soluble fiber can assist in weight control by slowing digestion and increasing satiety. It aids laxation by softening the stool.
An often-cited claim is that soluble fiber stabilizes blood glucose levels for people with diabetes. However, in its nutrition recommendations, the American Diabetes Association reports that this has only been shown to be effective at an intake of 50g fiber daily. Lower intake levels --- such as the RDA of 25 g daily --- has not been shown to improve after-meal glucose levels or long-term blood sugar control.
Insoluble Fiber Functions
Insoluble fiber does not dissolve in water. It passes through the digestive tract essentially unchanged, adding bulk and form to the stool. Insoluble fiber helps waste pass more quickly through the colon. Because insoluble fiber is not fermented by bacteria in the large intestine, it is not associated with gas. Insoluble fiber is found primarily in whole grains, wheat bran and some vegetables.
Insoluble Fiber Benefits
Insoluble fiber is especially linked to digestive tract health. By moving waste quickly through the large intestine, it can decrease constipation. By giving form to the stool it can also improve diarrhea, or loose, watery stools. It also improves regularity of bowel movements. Insoluble fiber can reduce symptoms of irritable bowel syndrome and diverticulosis. It can benefit weight control efforts by increasing satiety and fullness. Insoluble fiber's role in preventing cancer --- especially colorectal --- is still being studied.
References
- "Journal of the American Dietetic Association;" Health Implications of Dietary Fiber; October, 2008
- FDA: Using the Nutrition Facts Label: A Guide For Older Adults
- National Digestive Diseases Information Clearinghouse: Gas in the Digestive Tract; January 2008
- "Diabetes Care"; Nutrition Recommendations and Interventions for Diabetes; American Diabetes Association; January 2008
- MayoClinic.com: Dietary Fiber: Essential for a Healthy Diet; Nov. 19, 2009
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