Defining dietary fiber

Defining dietary fiber

By Andrea Platzman, M.S., R.D., C.D.N.

Researchers have assigned many health benefits to fiber, but how do product developers know what is, or isn't, fiber?

Diets high in fiber, found mainly in fruits, vegetables and whole grains, have been linked to lower cholesterol and blood pressure levels, reduced cardiovascular disease and cancer risk, and prevention of constipation and other bowel problems. Additionally, it has been shown that diets high in dietary fiber can help people with diabetes keep blood sugar levels under control. A recent statement from Claude Lenfant, M.D., director of the National Heart, Lung and Blood Institute (NHLBI), Bethesda, MD, based in preliminary findings of the CARDIA study, a multi-center, population-based study: "High fiber diets may protect against obesity and cardiovascular disease in healthy young adults by lowering insulin levels." The study's findings are now being confirmed in long-term interventional studies.

Fiber often is divided into two main categories: water soluble or insoluble. Water soluble fibers include gums, hydrocolloids, most pectins, mucilages and some hemicelluoses. Insoluble fibers include cellulose, some hemicelluloses, lignin and enzyme-resistant starches. Scientists believe that a mixture of soluble and insoluble fibers is necessary to maintain a healthy gut. Including fiber into food products can offer many functional benefits such as improving texture, appearance, moisture control and shelf life. Much of their functionality comes from their ability to absorb from two to 10 times their weight in water.

Currently, the average American ingests about 13 gm of dietary fiber each day while the recommended amount of fiber to consume each day is 25 gm to 35 gm. It is, however, important to note that consuming more than 50 gm to 60 gm of dietary fiber is not a good idea. An excessive amount of dietary fiber can interfere with body's absorption of calcium, zinc and iron. In addition, huge amounts of fiber can cause intestinal blockage.

New definition
The first definition, developed by H. Trowell and his associates in 1972 defined fiber as
"that proportion of food which is derived from cellular walls of plants which is digested very poorly by human beings." In 1998, an ad hoc committee formed by the American Association of Cereal Chemists (AACC) to set a new or modify the old fiber definition if necessary. (To read more about the fiber definition project, click here). The committee developed a consensus definition and on June 1, 2000, the AACC board of directors approved the following the definition for dietary fiber:
Dietary fiber is the edible parts of plants or analogous carbohydrates that are resistant to digestion and absorption in the human small intestine with complete or partial fermentation in the large intestine. Dietary fiber includes polysaccharides, oligosaccharides, lignin, and associated plant substances. Dietary fibers promote beneficial physiological effects including laxation, and/or blood cholesterol attenuation, and/or blood glucose attenuation.

The message about fiber intake and reduced colon cancer was not included because most of the participants thought the data on this subject was too contradictory at this time. The new definition will help food formulators define some of their functional foods, still a largely growing sector of the food industry. With an expanded dietary fiber definition, more food products will probably be included. Currently, the AACC Board is in discussion regarding the desire for the definition to be used globally.

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