Dietary Fiber- A Simple Matter?
Despite an evolving trend toward healthier eating, individuals continue to consume significantly less dietary fiber than the daily value (DV) of 25 or 30 grams, averaging only 14-15 g/person/ day. A recent study indicates only 24.4% of U.S. adults consumed fruits and vegetables (a good source of dietary fiber) five or more times per day (JAMA 286:1195, 2001).
Chronic inadequate intake of dietary fiber has led manufacturers to supplementing food with fiber and the marketing of dietary fiber supplements. As interest has increased in fiber and its potentially beneficial effects, manufactures have isolated dietary fiber for addition to food from a wide range of carbohydrate sources. Many of these isolated materials are used as food ingredients based on technical properties, such as thickening or fat reduction with fructans.
While the public and health experts are focused on the importance of dietary fiber and recommendations are made regarding intake, there has not been a consensus as to the definition of dietary fiber. Since the 1950s, a variety of definitions for dietary fiber have been promulgated by scientific and regulatory agencies worldwide. Some definitions specifically address the physiological effects of dietary fiber, whereas others rely more on prescribed analytical methods as the sole determinant of dietary fiber. Several analytical methods for the measurement of dietary fiber have been developed by the Association of Official Analytical Chemists International (AOAC) and are widely accepted in the U.S. In 1987, the FDA adopted the AOAC method 985.29 for regulatory purposes to identify dietary fiber as a mixture of non-starch polysaccharides, lignin and some resistant starch (resistant to the enzymes protease, amylase and amyloglucosidase used in the AOAC methods), with related methods added over the years. Therefore, the regulatory definition of dietary fiber is based on labeling and has been limited to methodology – the fiber must be enzymatically non-digestible and insoluble in four parts ethanol and one part water. This method excludes non-digestible water-soluble oligo- and polysaccharides, including polydextrose and fructans. At present, FDA does not have a written definition of dietary fiber for the purpose of food labeling and health claims.
The scientific community agrees the definition of dietary fiber should not only reflect methodology, but physiological effects as well. A commission of the Food and Nutrition Board’s (FNB)/Institute of Medicine (IOM) Dietary Reference Intake (DRI) Standing Committee established the Panel on the Definition of Dietary Fiber. The proposed definitions are included in the Institute of Medicine (IOM) report entitled Dietary Reference Intakes. Proposed Definition of Dietary Fiber (2001). In the report, the Panel proposed definitions to encompass current and possible future non-digestible carbohydrates in the food supply (IOM, 2001). These definitions include:
Dietary Fiber consists of nondigestible carbohydrates and lignin that are intrinsic and intact in plants.
Added Fiber consists of isolated, non-digestible carbohydrates that have beneficial physiological effects in humans.
Total Fiber is the sum of Dietary Fiber and Added Fiber.
According to the IOM report, there are three established physiological effects of Added Fiber on human health. These include attenuation of postprandial blood glucose concentrations, attenuation of cholesterol concentrations and improved laxation.
For nutrition labeling purposes, the Nutrition Labeling and Education Act of 1990 (NLEA) declared dietary fiber a nutrient. This is also in accordance with the IOM report, indicating the proposed definitions should be based on health and physiological benefits rather than on analytical methods, and allow for the recognition of dietary fiber as a nutrient (IOM, 2001).
The added physiological benefits of Added Fiber can also suggest functionality. Functional foods have been defined as “those foods or food ingredients that may provide a health benefit beyond the traditional nutrients contained therein.” Because many of these functional foods, such as dietary fiber and antioxidants, are also “macro” ingredients, animal testing models may not be adequate for determining safety or efficacy and clinical evaluation may be needed. Clinical evaluation of dietary fiber can be difficult, as the presence of vitamins and minerals in the fiber and attempts at controlling for other substances in the food have not always been successful. These and other variables make it difficult to separate out the specific effect of fiber per se from the beneficial effects shown in epidemiological studies.