Dietary fiber describes the portion of a plant-derived food, such as pectin, gum, cellulose and lignin, that the enzymes in the intestines cannot break down. Two types of fiber exist: soluble fiber and insoluble fiber. Vegetables, fruits and grains all contain fiber but each food varies in the type and amount of fiber. Because the human body needs both soluble and insoluble fiber, the National Institute of Medicine Food and Nutrition Board provides a recommendation for the daily total fiber intake, but does not specify a soluble fiber to insoluble fiber ratio.
Soluble Fiber
Increasing your intake of soluble fiber can help reduce your risk for heart disease -- the leading cause of death for both men and women in the United States. Soluble fiber changes within the body because it absorbs water, turning it into a gel-like substance. The body cannot absorb fiber through the wall of the small intestines, where other nutrients from food get absorbed. Instead, fiber binds to bile acids, the substance necessary for the breakdown of fat. This inhibits the digestion of fats, which helps to lower blood cholesterol and triglyceride levels. As the soluble fiber continues through the digestive tract, bacteria in the large intestines try to break it down. This produces fatty acids that enter the bloodstream and work to suppress the production of cholesterol by the liver cells.
Insoluble Fiber
Insoluble fiber moves through the digestive system without changing at all. Insoluble fiber adds bulk to stool and because it attracts water, it helps make stools softer and easier to pass. This promotes a healthy digestive tract and helps prevent constipation. In addition, a diet rich in insoluble fiber prevents hemorrhoids, often caused by constipation and straining, and the onset of diverticulitis -- the formation of small pouches in the lining of the colon that can become infected. Both soluble and insoluble fiber slow the process of digestion, which provides a feeling of fullness for longer and helps to stabilize blood sugar levels, taking the stress off the pancreas. This action of fiber reduces the risk for diabetes -- a potentially life-threatening disease characterized by high blood sugar levels.
Recommended Intake
The U.S. Department of Agriculture publication, "Dietary Guidelines for Americans 2010," recommends that adults consume 14 g of fiber for every 1,000 calories consumed. This correlates to the National Institute of Medicine's recommendation that adult women should get at least 25 g of fiber while adult men should consume 38 g of fiber per day. Unfortunately, the average American only consumes about 14 g of fiber per day, representing about half of the daily recommended intake, according to Colorado State University Extension. Although both types of fiber are important, these recommendations only list the amount of total fiber to consume each day. Because most plant-derived foods contain both types of fiber in varying ratios, counting total fiber is sufficient.
Sources
Many foods contain more insoluble fiber than soluble fiber, with a common ratio of 75 percent to 25 percent respectively, according to the University of California. That is not the case for all foods such as asparagus, carrots, mangoes and oranges, which all contain more soluble fiber than insoluble fiber. Good sources of soluble fiber include oats, oat bran and black beans while good sources of insoluble fiber include foods like barley, wheat bran and pinto beans. To meet the daily recommended fiber intake, choose foods with at least 2.5 g of total fiber per serving.
References
- U.S. Department of Agriculture: Dietary Guidelines for Americans 2010 -- Chapter 4; February 2011
- National Institute of Medicine Food and Nutrition Board: Dietary Reference Intakes; 2004
- Colorado State University Extension: Dietary Fiber; J. Anderson, et al.; December 2010
- University of California: Student Nutrition Awareness Campaign; 2005
- MayoClinic.com: Cholesterol -- Top Five Foods to Lower Your Numbers; May 2010
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