Dietary Fiber: Still No Accepted Definition

Dr. Robert Atkins reportedly developed his Atkins Diet for weight reduction because he was concerned with the great reduction in fiber intake in the 20th century, which was matched by a dramatic increase in intake of refined carbohydrates after refining of grains began in the 1890s. Fiber-free sugar consumption also increased seriously with the development of soft drinks. Because long-term ingestion of low-fiber foods does not satisfy human hunger, consumers started to eat more, leading to the serious obesity problem we have today, he said.

Searching for a Definition
To provide an update on the status of dietary fiber, I conducted a search for the current definition of dietary fiber compared to any other form of edible fiber commonly utilized in food formulations. I discovered that scientists have been attempting to agree on a universally accepted dietary fiber definition since the mid-1970s. In fact, there is no official or legal definition of dietary fiber in the United States at this time.

How is it possible that after more than 30 years since the health benefits of dietary fiber were reintroduced to the world, dietary fiber has not yet been defined? In the ’70s, studies to verify benefits of dietary fiber were pioneered by Burkitt, Trowell, Walker and Painter, who stated that dietary fiber “is protective against a range of diseases. In 1976, Trowell redefined dietary fiber to be “the remnants of edible plant cells, polysaccharides, lignin, and associated substances resistant to (hydrolysis) digestion by the alimentary enzymes of humans.”

When the Nutrition Labeling and Education Act of 1990 and 1993 required mandatory nutrition labeling of foods, the Food and Drug Administration declared dietary fiber a nutrient, assigned it a Daily Value (DV) of 25 g, and required that it be measured in foods and reported on the Nutrition Facts panel. Scientists interested in developing analytical methods to measure dietary fiber established a simplified “de facto” definition that it was the residue recovered. This “residue” or de facto definition for dietary fiber was adopted by at least seven countries, with or without some qualifying statement. The allowance of including oligosaccharides and manufactured nondigestible carbohydrates resulted in significant objections to this definition. A “Novel Fibre” definition by Health Canada in 1988 required several analysis restrictions and safety considerations, and resulted in further questions on acceptance.

In 1998, the American Association of Cereal Chemists appointed a committee of cereal science experts to begin development of an acceptable definition. The finalized AACC Dietary Fiber Definition Report was issued in June 2000 (www.aaccnet.org/definitions). It offered this definition: 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 and partial fermentation in the large intestine. Dietary fiber includes polysaccharides, oligosaccharides, lignin, and associated plant substance. Dietary fibers promote beneficial physiological effects including laxation, and/or blood cholesterol attenuation and/ or blood glucose attenuation.

A lengthy minority report was subsequently prepared by the original committee chair, Dennis T. Gordon, formerly of North Dakota State University, and a group of dissenting scientists worldwide and was submitted to AACC in February 2001. Controversy resulted from the apparent fact that the first two sentences of the definition do define dietary fiber but the third sentence appears to be either a marketing claim or a health claim.

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Still another effort to resolve a definition for North America was through the combined study by the agencies of Health Canada and the Food and Nutrition Board of the U.S. Institute of Medicine/National Academy of Sciences. They proposed Dietary Reference Intakes (DRIs) for North America. From this, IOM/NAS proposed several definitions in 2002 (www.nap.edu/books/0309085373/html): “Dietary Fiber consists of nondigestible carbohydrates and lignin that are intrinsic and intact in plants. Functional Fiber consists of isolated, nondigestible carbohydrates that have beneficial physiological effects in humans. Total Fiber is the sum of Dietary Fiber and Functional Fiber.”

The recommended daily intake for total fiber for adults 50 years and younger was set at 38 g for men and 25 g for women, while for men and women over 50 it is 30 and 21 g, respectively, due to decreased food consumption.

Current recommendations for DRIs for dietary fiber are available for men and women of varied ages and during pregnancy. However, some people may have to change their eating habits to maintain optimal fiber intake recommendations. Increased fiber intake may be satisfied by eating several servings of whole grains, fruits, vegetables, and dried beans daily.

Dietary fiber has an effect on many physiological and metabolic changes in the body and appears to be most effective when soluble fiber intake and insoluble fiber intake are balanced to get the different benefits they each provide. Fiber is also only one part of a properly balanced diet. Excessive fiber intake may reduce the levels of selected minerals absorbed from foods, especially in young children.

Different types of plants provide varied types of fiber in varying amounts. Pectin and gum, for example, are water-soluble fibers that slow passage of food through the intestines, but do not increase fecal bulk. On the other hand, beans, oat bran, fruits, and vegetables contain soluble fiber. The fibers in these cell walls include cellulose, hemicellulose, and lignin, which are water-insoluble. Such fibers increase fecal bulk and speed up passage of food through the digestive tract.

Dietary fiber is found only in plant foods, not in meat, milk, or eggs. Further description of dietary fiber health benefits and charts of dietary fiber content of foods per serving size can be found in fact sheets from the Colorado State University Cooperative Extension at www.ext.colostate.edu/pubs/foodnut/09333.html.

Minority Report Discusses Problems
According to Gordon, “the problems faced in defining dietary fiber in North America for regulatory purposes in food labeling continue to run in the same closed circle. Scientists and regulators have long contended that dietary fiber should be defined both as a chemical identity and by its physiological properties. While this is a noble goal, and of importance in clinical studies for proposed health claims, it has been difficult to meet both these requirements with one convenient and affordable test procedure for the simple task of labeling foods. And although two AOAC-approved methods to measure total dietary fiber are available, they do not provide for complete recovery of all dietary fiber components in foods. A realistic definition with a concurrent analytical test to be used by food manufacturers and regulators for food labeling and international commerce is required. And finally, it must be recognized in North America that isolated, modified, and/or synthesized forms of food-safe, nondigestible carbohydrates should be defined as dietary fiber.”

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According to Gordon, “the problems faced in defining dietary fiber in North America for regulatory purposes in food labeling continue to run in the same closed circle. Scientists and regulators have long contended that dietary fiber should be defined both as a chemical identity and by its physiological properties. While this is a noble goal, and of importance in clinical studies for proposed health claims, it has been difficult to meet both these requirements with one convenient and affordable test procedure for the simple task of labeling foods. And although two AOACapproved methods to measure total dietary fiber are available, they do not provide for complete recovery of all dietary fiber components in foods. A realistic definition with a concurrent analytical test to be used by food manufacturers and regulators for food labeling and international commerce is required. And finally, it must be recognized in North America that isolated, modified, and/or synthesized forms of food-safe, nondigestible carbohydrates should be defined as dietary fiber.”

“Today, a definition of dietary fiber is needed, “ he continued. “While great attention seems to have focused on activities to define dietary fiber in North America, other countries also have an interest in this topic. A simple definition, with almost worldwide acceptance, will help in the accurate food labeling and marketing of foods in international commerce, but more important, it may help consumers to select foods to increase their fiber intakes. It can be accurately stated that dietary fiber consumption among many populations is less than one-half their optimum intakes. Other countries and the Codex Alimentarius Commission appear to be providing more realistic definitions for dietary fiber to achieve more harmonious international trade and helping the consumer find more dietary fiber–rich foods.”

The Codex Committee on Nutrition and Foods for Specialty Dietary Uses proposed in 2004 that dietary fiber consists of edible nondigestible material composed of carbohydrate polymers with a degree of polymerization (DP) not lower than 3 or of carbohydrate polymers (DP>3) processed (by physical, enzymatic or chemical means) or synthetic. Dietary fiber is neither digested nor absorbed in the small intestine and has at least one of the following properties: increases stools production; stimulates colonic formulation; reduces fasting cholesterol levels; reduces post-prandial blood sugar and/ or insulin levels.

The Codex-proposed definition of dietary fiber appears to address many of the problems associated with other definitions. In addition, the Codex recommendations list additional AOAC-approved methods (995.16 for beta-glucans, 997.08 and 999.03 for fructans, 2000.11 for polydextrose, 2001.02 for trans-oligosaccharides, 2001.03 for resistant maltodextrin, and 2002.02 for resistant starch and algal fiber) that can be used to measure those sources of dietary fiber not recovered with the two generally approved dietary fiber methods (985.29 and 991.43 for total dietary fiber). This is a positive inclusion and acknowledges that it may require a number of different methods to measure all the potential forms of dietary fiber.

Definition Should Be Based on Two Perspectives
Gordon summed up the situation as follows:
“Dietary fiber is first a chemical identity; however, it should be defined on the basis of both chemical and physiological perspectives so that the potential benefits (health claims or structure–function claims) can be considered by the food processor or consumer. Dietary fiber may be a nutrient, or it might be claimed to be an essential nutrient.

“Dietary fiber is a term that is almost universally recognized by everyone. It has many meanings. It is associated with healthy foods and diets, and the terms ‘dietary fiber’and ‘health’ are almost synonymous.”

“Scientists and regulators must next recognize the limitations of existing AOAC methods and help foster work to expand on these methods or develop newer methods,” he concluded.

by DEAN DUXBURY
Contributing Editor
Consultant, Oak Brook, Ill.
[email protected]