Dietary Fiber: Definitions I. Introduction a. Dietary Fiber Description
Many consumers today have heard of dietary fiber and may even take supplements for it, but few know what fiber actually is. Over the years the definition of dietary fiber has changed and fluctuated between describing attributes from a plant source to those including the physiological effects (Pyler and Gorton 2008]. Fiber as a source is found in the cell wall of plants. The cell walls of plants are made out of nonstarch polysaccharides (Pyler and Gorton 2008). These polysaccharides are linked together in a matrix or supramolecular structure (Spiller 2001). Most dietary fiber comes from plant cell wall materials; however, shellfish and some fungi and yeasts are also sources (Pyler and Gorton 2008).
The term dietary fiber is observed mainly for nutritional purposes and not for its physical nature (Pyler and Gorton 2008). It is widely known that fiber is indigestible in the small intestine for humans. For this reason, it has attracted a lot of attention for its physiological benefits (Pyler and Gorton 2008). This has ignited a multitude of research pertaining to this field (Pyler and Gorton 2008) as well as a continuous debate on one accepted definition of dietary fiber.
b. Dietary Fiber Definitions
“Dietary fiber” was a term first used in 1953 to describe components of plant cell walls that were indigestible for humans (DeVries et al 1999). Over the years the definition has been refined from basing it on the chemical analysis, to yield measurements of “crude fiber”, “acid detergent fiber”, “neutral detergent fiber”, soluble dietary fiber”, insoluble dietary fiber” and “total dietary fiber” (Pyler and Gorton 2008). All of these terms are the results of various official methods from the American Association of Cereal Chemists (AACC International).
Various organizations over the years have tried to define dietary fiber. It is widely considered a biological, not chemical, component of food. Pyler and Gorton (2008) define it as the “component of grains, vegetables, fruits and nuts that resists digestion in the gastrointestinal tract of man.”
The cell wall material of plants has been the major source of indigestible polysaccharides in human diets, however, there has been some dispute as to whether all indigestible components of foods should be considered a part of the dietary fiber definition (Spiller 2001). These indigestible components include non-carbohydrate materials associated with the plant cell wall structure (Spiller 2001). These components resist digestion from either being intrinsically non-digestible or from the cell wall structure inhibiting or preventing enzymatic attack (Spiller 2001).
Some microorganisms found in the human body’s gastrointestinal tract can digest fiber (Pyler and Gorton 2008). This leaves by-products that can be absorbed for energy. Because of this, the definition of dietary fiber changed to “the sum of all plant polysaccharides plus lignins that are not digested by human digestive enzymes, but not excluding possible digestion by bacteria residing in the large intestine” (Ranhotra and Gelroth 1985).
The AACC International defines dietary fiber as the “edible parts of analogous carbohydrates that are resistant to digestion and absorption in the human small intestine with complete or partial fermentation in the large intestine” (AACC 2001). The AACC (2001) includes polysaccharides, oligosaccharides, lignin and associated plant substances in its category of dietary fiber.
The Institute of Medicine (IOM) sums up dietary fiber to consist of “nondigestible carbohydrates and lignin that are intrinsic and intact in plants” (IOM 2001). It has a separate definition for “functional fiber” that consists of “isolated, nondigestible carbohydrates that have beneficial physiological effects in humans” (IOM 2005). Therefore, to attain “total fiber” one must consume dietary fiber and functional fiber (IOM