Defining Dietary Fiber

  Fiber is classified into two main categories - soluble and insoluble. Soluble fibers include gums, hydrocolloids, most pectins, mucilages and some hemicelluloses. Insoluble fibers include cellulose, some hemicelluloses, lignin and enzyme-resistant starches. Scientists believe that ingesting a mixture of soluble and insoluble fibers helps maintain a healthy gut. Additionally, soluble fiber is linked to reduced cholesterol levels, and insoluble fiber increases transit time in the gut, thus reducing the risk of colon cancer, diverticulitis, colitis and other gastrointestinal ailments.

  Oligosaccharides such as inulin and oligofructose are harder to classify as simply soluble or insoluble. They exhibit many of the benefits of fiber consumption (increased stool weight, lowered triglycerides, increased HDL cholesterol and controlled blood-glucose), although they do not test as soluble dietary fiber under the current AOAC methods. These carbohydrate compounds are termed prebiotics because they selectively stimulate the activity of bacteria already in the colon. Inulin and oligofructose can cut calories and fat in food products, as well as enhance taste and texture. Currently, there are more than 400 prebiotic food products on the market, mainly in Europe and Asia.

  In the United States the accepted fiber definition for nutrition-labeling purposes encompasses remnants of edible plant cells and polysaccharides that are resistant to enzyme hydrolysis. To fit in this definition, adequate dissolution of a fiber with a mixture of four parts ethanol (95%) and one part water must be exhibited, according to the official AOAC method. This method captures all dietary fiber components except polyfructans, a category that includes inulin and oligofructose.

  The American Association of Cereal Chemists (AACC), St. Paul, MN, has set out to create a standard definition of dietary fiber that meets the needs of consumers, food manufacturers, food scientists, chemists and processors. "To be a part of the definition, the food should be resistant to digestion and absorption in the small intestine," stated Julie Miller-Jones, Ph.D., professor at the College of St. Catherine's in St. Paul, MN at the 1999 Institute of Food Technologist's Annual Meeting & Expo® in Chicago, IL. Under this definition, oligosaccharides would be included.

  Many questions still need to be answered to determine a standard definition of dietary fiber, however. For more information, visit AACC's website at www.scisoc.org/aacc/DietaryFiber/top.htm, or see "Dietary Fiber - a New Definition?" in the December 1999 Food Product Design catalog-showcase supplement.

  "Some advantages to changing the dietary fiber definition include increasing fiber in the diet, fortifying foods with dietary fiber, and including more food products in the new definition," says John De Vries, Ph.D., principal scientist at General Mills Corporation, Minneapolis. "However, some of the disadvantages include changes in common databases, alterations in nutrition labeling, and consumer education, since many consumers may not change their perceived definition of dietary fiber."

  New functional foods are arriving in the marketplace every day, many of which contain various forms of fiber. Based on the Daily Reference Values (DRVs), a "good" source of fiber contains at least 10% of the DRV for fiber, or 2.5 grams. A product labeled "high fiber" contains at least 20% of the DRV for fiber, or 5 grams. In addition, products must also be low in total fat. (For more information on health claims and regulatory issues, see "Dietary Fiber: A Healthy Discussion" in the January 1999 issue of Food Product Design.)

  While the average American ingests about 13 grams of dietary fiber per day, the recommended amount is 25 to 30 grams. It's important to note that more is not necessarily better in the case of fiber; consuming more than 50 to 60 grams of dietary fiber is not a good idea. An excessive amount of fiber can interfere with the body's absorption of calcium, zinc and iron, and huge amounts can cause intestinal blockage.

  Of course, fiber's benefits far outweigh any potential complications, especially since most Americans' current consumption is so low. Additionaly, fiber can provide product benefits - using the right type of fiber in food products can often improve texture, appearance and moisture control, as well as lengthen shelf life. Much of fiber's functionality comes from its ability to absorb and, in some cases, bind water at a rate of two to ten times its own weight. Developing more functional foods that contain dietary fibers is a good way for food designers to provide excellent nutrition and functionality to their products.