Redefining Fiber

Increasing one’s dietary fiber may be as simple as changing a definition.

Prior to the 1970s, dietary fiber was primarily viewed as “roughage” and generally considered as a non-nutrient and unimportant in maintaining health. Fast-forward a decade or so and we see a marked increase in scientific information suggesting a change in this view. The scientific community has recanted its “no value” opinion and acknowledged that dietary fiber is a positive nutrient, a deficiency of which can lead to adverse health consequences. Moreover, in 1988, the Surgeon General’s Report on Nutrition and Health and the National Research Council’s Diet and Health: Implications for Reducing Chronic Disease Risk concluded that there is a relationship between the consumption of certain forms of dietary fiber and the onset of certain types of chronic disease.

In response to public awareness of the relationship between diet and health, Congress passed the Nutrition Labeling and Education Act in 1990. The NLEA requires that meaningful information about foods be conveyed in a simple, clear, consistent, and uniform nutrition label format—including the amount of dietary fiber.

The legal requirement to provide on the food label a quantitative declaration concerning fiber forced the Food and Drug administration to develop a procedure by which it could test the accuracy of a manufacturer’s efforts to comply. Accordingly, FDA promulgated a regulation providing that unless the agency specifies a particular method of analysis for quantifying a nutrient such as dietary fiber, foods are to be analyzed by “appropriate methods as given in the official methods of analysis of the AOAC International . . . or if no AOAC method is available or appropriate, by other reliable and appropriate analytical procedures.” FDA has never specified a particular method for assessing total dietary fiber but has, however, relied on several methods AOAC developed in the mid-1980s. It is important to note that none of these AOAC total dietary fiber methods recovers dietary fructans. The agency’s reliance on these methods resulted in a de facto definition for dietary fiber: if the AOAC method identifies a substance as dietary fiber, the substance may be labeled as fiber. Otherwise, the substance may not, for labeling purposes, be considered dietary fiber.

It is interesting to note that FDA’s approach to dietary fiber is independent of physiological function or dietary significance. In the mid-1970s, the then-available scientific knowledge base permitted Burkitt, Painter, and Trowell to conclude that “the hallmark of all substances included in the term ‘dietary fiber’ was that they were not digested at all by the alimentary enzymes of man.” Accordingly, more recently Gibson, Roberfroid, and others proffered definitions of dietary fiber that included non–cell wall plant material. Because many nondigestible plant materials, e.g., certain polysaccharides and mucilages, were not associated with plant cell walls, many investigators later recommended that the definition of dietary fiber be expanded to cover these types of substances, including nondigestible oligosaccharides.

Approximately 15 months ago, the American Association of Cereal Chemists established a committee to reevaluate FDA’s de facto definition of dietary fiber and to develop a consensus description. Meanwhile, new fibers and fiber extracts continued to be identified, defined, and marketed worldwide. The committee concluded that an appropriate definition of dietary fiber included “edible parts of plants with analogous carbohydrates that are resistant to digestion in the small intestine with complete or partial fermentation in the large intestine” and stated that such fibers “promote beneficial physiological effects such as laxation, blood cholesterol attenuation, and blood glucose attenuation.”

AACC’s new definition considers the growing scientific evidence concerning the physiological effects of fiber. It would define dietary fiber as comprising the edible parts of plants or analogous carbohydrates that are resistant to digestion and absorption in the human gastrointestinal tract. Thus, the definition includes dietary fructans and other oligosaccharides, as well as those plant components discussed by scientists during the past 30 years. It would go beyond FDA’s current definition derived from the current methods for measuring total dietary fiber for nutrition labeling. It would account for the current expanding knowledge about the physiological properties of dietary fiber, and would enhance the ability of consumers to more accurately distinguish dietary fiber from other nutrients.

Oligosaccharides, including those extracted from chicory, artichokes, and bananas, prepared through fermentation technology, and even those naturally found in breast milk, fall within the broad definition of “dietary fiber.” These oligosaccharides, many of which are known as fructans, are not digested by human enzymes and are readily fermented by colonic microorganisms. Extensive studies with dietary fructans confirm that these oligosaccharides in general are consistent with physical properties and physiological effects of dietary fiber. These nondigestible carbohydrates resist hydrolysis by human enzymes; are fermented by gut micro-flora; decrease gastrointestinal transit time; increase fecal weight; reduce fecal pH; present a low caloric value; attenuate plasma cholesterol; and delay or reduce glucose absorption.

Redefining dietary fiber provides an opportunity for greater human exposure to sources of dietary fiber and a boost to overall fiber consumption, which continues to fall short of recommended levels in the U.S. This redefining of dietary fiber would not only result in new nutrition and food science research, but also ultimately contribute to the development, promotion, and consumption of more healthful foods.

by Roger A. Clemens is Adjunct Professor, Food, Nutrition & Consumer Sciences, California State Polytechnic University, Pomona.