Fiber – Part II: Conceptual Confusion and Controversy In Search Of A Definition
Suggested Citation: Garko, M.G. (2013, July). Fiber – Part II: Conceptual confusion and controversy in search of a definition. Health and Wellness Monthly. Retrieved (insert month, day, year), from www.letstalknutrition.com.
Fiber – Part II: Conceptual Confusion and Controversy In Search Of A Definition
Michael Garko, Ph.D.
Producer and Host of Let’s Talk Nutrition
Surprisingly, there is no universally agreed upon definition of dietary fiber. There are any number of good reasons to have a standard definition of dietary fiber accepted by scientists and others working in the world of medicine, health, and nutrition while being useful and understandable to laypeople.
For one thing, there has been an inextricable connection of plant-derived fiber to the human diet over the course of human evolution. Having a universal definition of dietary fiber would help to better document, study and understand the nature of that connection and how it contributed to human development over millions of years. Furthermore, a universally accepted definition would provide (1) guidance in the scientific establishment of the health benefits associated with the daily consumption of dietary fiber, (2) direction in establishing the recommended daily intake of dietary fiber, (3) clarity for food labeling and inspection purposes, (4) direction in the development of accurate methods to analyze fiber-based foods, (5) and guidance in educating health consumers about what foods they need to eat to achieve an adequate intake of dietary fiber and importance of dietary fiber in creating, sustaining and reclaiming health and wellness, among other reasons.
The July, 2013, issue of Health and Wellness Monthly will focus on the conceptual confusion and controversy surrounding the effort since the 1950s to define dietary fiber and what sort of definition of dietary fiber would be useful not only for the scientific community but also for laypeople.
The Debate in Defining Dietary Fiber
Hipsley (1953) first introduced the phrase “dietary fiber” into the nomenclature and discourse on the scientific study of fiber. Hipsley (1953) used the term within the context of his research and hypothesis on “dietary fibre” deficiency and eclampsia (also known as toxaemia of pregnancy), which is a life-threatening complication causing pregnant women, who are typically diagnosed with preeclampsia (i.e., high blood pressure and protein in the urine) to suffer seizures or coma.
Defining dietary fiber would seem to be a simple task. Yet, such does not seem to be the case. There are more than a few definitions of dietary fiber worldwide based on biological, chemical, nutritional, physiological and analytical concerns and factors (see Champ, et al., 2003). Paradoxically, the more that has been learned scientifically about the nature of dietary fiber in terms of its constituent elements, health/physiological benefits, different fractions and mechanisms of action the more contentious the debate has become and the more difficult it has become to reach some sort of consensus, although some progress has been made internationally to reach agreement on a definition of dietary fiber. It is as if the experts cannot get out of the way of their own knowledge and insight into dietary fiber.
The major areas of contention in defining dietary fiber have centered on (1) whether nondigestible animal carbohydrates, carbohydrates not recovered by alcohol precipitation, nondigestible mono- and di-saccharides, lignin, and resistant starch should be included in a definition, (2) whether a definition should include only fiber that is intact and occurring naturally in food sources, (3) whether the fiber is resistant to digestive enzymes, (4) whether specific physiological-health benefits should be part of a definition and (5) whether one of the two general classes of methods or analytical techniques (i.e., gravimetric or chemical) is better suited for the dietary fiber analysis of foods and should be included in a definition (see Champ et al., 2003; Institute of Medicine, 2001).
Below is DeVries, Prosky & Cho’s (1999) historical account on defining dietary fiber involving different researchers and research/scientific organizations, such as the Association of Official Analytical Chemists (ASOC) and American Association of Cereal Chemists (AACC). Their account of events from 1953 to 1999 provides a sense of how different issues (especially, fiber components/fractions or methods to analyze or otherwise quantify fiber components/fractions affecting physiological functioning and health) have been implicated in defining dietary fiber:
DeVries, Prosky & Cho’s Historical Account of Defining Dietary Fiber
Hipsley coins term “dietary fiber.”
Trowell and co-workers define constituent makeup as part of their “dietary fiber hypotheses.” This definition is used to describe the remnants of plant cell wall components that are resistant to hydrolysis by human alimentary enzymes.
Trowell and co-workers broaden definition to include all digestion-resistant polysaccharides (mostly plant storage saccharides), such as gums, modified celluloses, mucilages, oligosaccharides, and pectins. The broadened definition includes cellulose, hemicellulose, lignin, gums, modified celluloses, mucilages, oligosaccharides, and pectins, and associated minor substances, such as waxes, cutin, and suberin.
Researchers Asp, Schweizer, Furda, Theander, Baker, and Southgate, among others, develop methods aimed at quantifying food components included in the definition.
Prosky begins process of developing an international consensus on definition of and methodology for dietary fiber.
Consensus on dietary fiber definition and analytical approach at AOAC Spring Workshop in Ottawa, Ontario, Canada
Prosky, Asp, Furda, Schweizer, DeVries, and Harland validate consensus methodology in multinational collaborative studies.
1985 AOAC Official Method of Analysis 985.29, Total Dietary Fiber in Foods—Enzymatic-Gravimetric Method Adopted. Method and the equivalent AACC Approved Method 32-05 become de facto working definition for dietary fiber.
1985–1988 Methodology developed and collaboratively studied for insoluble and soluble dietary fiber.
1991 AOAC Official Method of Analysis 991.42, Insoluble Dietary Fiber in Foods and Food Products, Enzymatic-Gravimetric Method, Phosphate Buffer and the equivalent AACC Approved Method 32-07 first adopted.
1988–1994 Taking a variety of approaches, Lee, Mongeau, Li, Theander and co-workers develop, validate, and bring to official or approved method status other methods fitting the definition of dietary fiber.
1992 International survey reaffirms consensus on physiological dietary fiber definition.
1993 Second international survey reaffirms consensus on physiological dietary fiber definition and reaffirms inclusive components.
1995 AOAC International Workshop on Definition of Complex Carbohydrates and Dietary Fiber reaffirms consensus on physiological dietary fiber definition and inclusive components.
1999 Definition of dietary fiber remains as “dietary fiber consists of the remnants of edible plant cells, polysaccharides, lignin and associated substances resistant to (hydrolysis) digestion by the alimentary enzymes of humans.” This definition identifies a macroconstituent of foods that includes cellulose, hemicellulose, lignin, gums, modified celluloses, mucilages, oligosaccharides, and pectins and associated minor substances, such as waxes, cutin, and suberin. AOAC 985.29/AACC 3205, AOAC 991.43/AACC32-07, and equivalent methods are being used as de facto defining methods for total dietary fiber.
Source: Adapted from DeVries, Prosky & Cho (1999).
Definitions of Dietary Fiber
As it was alluded to earlier and as suggested by DeVries and Prosky & Cho’s (1999) historical account of efforts to define dietary fiber, a number of independent researchers, scientific organizations and countries have created a definition of dietary fiber since the early 1950s, reflecting their perspectives on the issues surrounding the dietary fiber debate. Following is the Institute of Medicine’s (IOM) (2001) account of important definitions of dietary fiber which emerged in the literature from 1976 to 2000:
Table 1 Definitions of Dietary Fiber
|Trowell et al., 1976||Dietary fibre consists of the plant polysaccharides and lignin which are resistant to hydrolysis by digestive enzymes of man.|
|Health and Welfare Canada, 1985||Dietary fibre is the endogenous components of plant material in the diet which are resistant to digestion by enzymes produced by humans. They are predominantly non-starch polysaccharides and lignin and may include, in addition, associated substances.|
|U.S. Food and Drug Administration (USFDA), 1987||Dietary fiber is the material isolated by AOAC method 985.29 (see Table 2).|
Life Sciences Research Office (LSRO), 1987
Dietary fiber is the endogenous components of plant materials in the diet which are resistant to digestion by enzymes produced by humans.
|Health Canada, 1988||A novel fibre source is a food that was manufactured to be a source of dietary fibre, and that (1) had not traditionally been used for human consumption to any significant extent, or (2) had been chemically processed (e.g., oxidized) or physically processed (e.g., finely ground) so as to modify the properties of the fibre, or (3) had been highly concentrated from its plant source.|
|Anonymous, 1989 (Germany)||Dietary fiber is substances of plant origin, that cannot-be broken down to resorbable components by the body’s own enzymes in the small intestine. Included are essentially soluble and insoluble non-starch polysaccharides (cellulose, pectin, hydrocolloids) and lignin and resistant starch. Substances like some sugar substitutes, organic acids, chitin and so on, which either are not or are incompletely absorbed in the small intestine, are not included.|
|Anonymous, 1992 (Belgium)||Dietary fiber is the components of the foods that are normally not broken down by the body’s own enzymes of humans.|
|Anonymous, 1993 (Italy)||Dietary fiber is the edible substance of vegetable origin which normally is not hydrolyzed by the enzymes secreted by the human digestive system.|
|FAO/WHO, 1995 (Codex Alimentarius Commission)||Dietary fibre is the edible plant or animal material not hydrolysed by the endogenous enzymes of the human digestive tract as determined by the agreed upon method. (The Codex also approved AOAC methods 985.29 and 991.43 [see Table 2]).|
|Jian-xian, 1995 (China)||Dietary fiber is the sum of food components that are not digested by intestinal enzymes and absorbed into the body.|
|Denmark, 1995 a||Dietary fiber is the material isolated by AOAC methods 985.29 and 997.08 (see Table 2).|
|Ministry of Health and Welfare, 1996 (Japan)||Dietary fiber is the material isolated by the AOAC method 985.29. In addition, non-digestible, low molecular weight carbohydrate determined by high performance liquid chromatography is classified as dietary fiber.|
|Committee on Medical Aspects of Foods (COMA), 1998 (United Kingdom)||Dietary fibre is non-starch polysaccharide as measured by the Englyst method.|
|Finland, 1998 a||Dietary fiber is part of the carbohydrate obtained when using AOAC methods 985.29 and AOAC 997.08 (see Table 2).|
|Norway, 1998 a||Dietary fiber is the material isolated by AOAC method 985.29 (see Table 2) and inulin and oligofructose.|
|Sweden, 1999 a||Dietary fiber is edible material that cannot be broken down by human endogenous enzymes. Dietary fiber is determined with AOAC method 985.29. In addition, the fructan AOAC method 997.08 may be used (see Table 2).|
|American Association of Cereal Chemists (AACC), 2000||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.|
Hignett, 2000 (U.K. Food Standards Agency)
Dietary fiber is the material isolated by AOAC methods 985.29 and/or 991.43, combined with 997.08 (see Table 2).
|Australia New Zealand Food Authority (ANZFA) (Proposed), 2000||Dietary fibre is that fraction of the edible part of plants or their extracts, or analogous carbohydrates, that are resistant to digestion and absorption in the human small intestine, usually with complete or partial fermentation in the large intestine. The term includes polysaccharides, oligosaccharides (degrees of polymerization >2), and lignins. Dietary fibre promotes one or more of these beneficial physiological effects: laxation, reduction in blood cholesterol, and/or modulation of blood glucose.|
Source: Institute of Medicine (2001).
Subsequent to the last definition proposed by the Australia New Zealand Food Authority in 2000, major health and scientific organizations have offered other definitions of dietary fiber.
American Association of Cereal Chemists
The American Association of Cereal Chemists (2001) offered the following 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 or partial fermentation in the large intestine. Dietary fiber includes polysaccharides, oligosaccharides, lignin, and associated plant substances. Dietary fibers promote beneficial physiologic effects including laxation, and/or blood cholesterol attenuation, and/or blood glucose attenuation(American Association of Cereal Chemists, 2001, p. 112).
Codex Alimentarius Commission
The Codex Alimentarius Commission (2009), an intergovernmental body to implement the Joint Food and Agriculture Organization and World Health Organization Food Standards Programme, provided the following definition in its Report of the 30th Session of the Codex Committee on Nutrition and Foods for Special Dietary Uses held in Cape Town, South Africa November 3 – 7, 2008:
Dietary fibre means carbohydrate polymers1 with ten or more monomeric units2 , which are not hydrolysed by the endogenous enzymes in the small intestine of humans and belong to the following categories:
- Edible carbohydrate polymers naturally occurring in the food as consumed,
- carbohydrate polymers, which have been obtained from food raw material by physical, enzymatic or chemical means and which have been shown to have a physiological effect of benefit to health as demonstrated by generally accepted scientific evidence to competent authorities,
- synthetic carbohydrate polymers which have been shown to have a physiological effect of benefit to health as demonstrated by generally accepted scientific evidence to competent authorities(Codex Alimentarius Commission; Food and Agriculture Organization; World Health Organization, 2009, p.46).
The Commission of the European Communities issued a directive regarding a definition of “fibre.” Its definition focuses on fiber fractions, methods to analyze the material constituting “fibre” and scientifically established beneficial physiological effects.
For the purposes of this Directive “fibre” means carbohydrate polymers with three or more monomeric units, which are neither digested nor absorbed in the human small intestine and belong to the following categories:
— edible carbohydrate polymers naturally occurring in the food as consumed;
— edible carbohydrate polymers which have been obtained from food raw material by physical, enzymatic or chemical means and which have a beneficial physiological effect demonstrated by generally accepted scientific evidence;
— edible synthetic carbohydrate polymers which have a beneficial physiological effect demonstrated by generally accepted scientific evidence (European Union, 2008, p.12).
Institute of Medicine
After an extensive review of the literature on definitions of fiber spanning over two decades, the Institute of Medicine (2001) proposed the following definition of dietary fiber:
- Dietary Fiber consists of nondigestible carbohydrates and lignin that are intrinsic and intact in plants.
- Added Fiber consists of isolated, nondigestible carbohydrates that have beneficial physiological effects in humans.
- Total Fiber is the sum of Dietary Fiber and Added Fiber (Institute of Medicine, 2001, p. 22).