Dietary Fiber

Dietary fiber is undoubtedly an essential part of a healthy eating plan, yet its effect as a nutrient has been somewhat elusive. The definition of fiber has evolved over the years, as have the methods used to assess it and although dietary fiber is derived from one of the macronutrients, the absence of a deficiency state prevents it from being considered an essential nutrient. There is no Estimated Average Requirement (EAR) or Recommended Dietary Allowance (RDA) for fiber, as there are for other carbohydrates. Instead, an Adequate Intake (AI) has been established, and this amount is contingent on the amount of calories consumed.

The 2020-2025 Dietary Guidelines for Americans identify dietary fiber as being one of several "dietary components of public health concern" due to underconsumption, and it's often described as being one of the nutrients that should be increased when reviewing the Nutrition Facts Label.

Traditionally, dietary fiber has been categorized as either soluble or insoluble in an attempt to assign physiologic effects to chemical types of fiber. The solubility reflects the nutrient's relationship with water. Soluble fibers are frequently credited as promoting cardiovascular health and insoluble fibers with gastrointestinal health. However, not all soluble and insoluble fibers perform these functions, and fermentation and viscosity have been proposed as being of greater value in determining the role of fiber. Although, viscosity can be difficult to assess and fermentation can vary between individuals.

Guidance from the National Academies of Science, Engineering, and Medicine (formerly the Institute of Medicine) and the U.S. Food and Drug Administration no longer recommend categorizing fiber based on solubility, but rather to look at total Dietary Fiber as the combination of the naturally occurring fibers as they exist in plants and the isolated, extracted or synthetic fibers with proven health benefits, sometimes referred to as functional fiber. Although there are still authorized health claims associated with soluble fiber which are approved for use in food labeling.

For the purpose of Nutrition and Supplement Facts labeling, a source of fiber would need to demonstrate that it possessed a physiological effect that was considered to be beneficial to human health, such as lowering blood glucose, blood pressure or calorie intake or improving laxation, which has been evident in naturally occurring sources. In the case of isolated, non-digestible or synthetic non-digestible carbohydrates, the FDA has proposed select ones be added to the definition of dietary fiber based on its review of the evidence.

Inulin, for example, is a soluble source that was previously referred to as a "functional fiber". Prior research has not demonstrated a significant decrease in serum cholesterol levels with inulin intake; although some research suggests it may be effective in reducing serum triglycerides. It may also increase calcium absorption and bone mineral density.

Although inulin is found in some fruits and vegetables, such as asparagus, onions and Jerusalem artichokes, it is more commonly obtained from commercial foods containing isolated inulin such as chicory root extract, which has Generally Recognized as Safe (GRAS) status in the United States.

Physio-chemically, inulin is a non-viscous soluble fiber that is fermented by bacteria in the colon and acts as a prebiotic. It can be used to increase bulk and palatability of foods, as well as replace fat in recipes. Isolated inulin can be identified in a wide variety of foods including: yogurt, high fiber breakfast bars, cereal, bread and ice cream.

Following the Nutrition Facts Label final rule, inulin and inulin-type fructans, along with several other isolated or synthetic non-digestible carbohydrates have been addressed in FDA guidance for use by the food industry when declaring the amount of dietary fiber.

Future studies on fiber and its various sources will help to further identify the physiological effects and the basis for its definition in relation to human health and nutrient status.

References:

• U.S. Department of Health and Human Services and U.S. Department of Agriculture. 2020 – 2025 Dietary Guidelines for Americans. 9th Edition. December 2020.
• Institute of Medicine, Food and Nutrition Board. Dietary Reference Intakes: Energy, Carbohydrates, Fiber, Fat, Fatty Acids, Cholesterol, Protein and Amino Acids. Washington, D.C.: National Academies Press; 2005.
• McRorie J, McKeown, N. Understanding the Physics of Functional Fibers in the Gastrointestinal Tract: An Evidence-Based Approach to Resolving Enduring Misconceptions about Insoluble and Soluble Fiber. J Acad Nutr Diet. 2016; 251-264.
• U.S. Food and Drug Administration. Review of the Scientific Evidence on the Physiological Effects of Certain Non-Digestible Carbohydrates. June 2018.
• U.S. Food and Drug Administration. Questions and Answers on Dietary Fiber. January 2020.