Dietary fiber is a group of food components, mainly carbohydrates, that resist digestion by human digestive enzymes in the small intestine. It includes structural materials in plant foods and certain isolated or manufactured ingredients. Fiber reaches the large intestine, where it may remain relatively intact or undergo microbial breakdown. Its physiological effects vary with its chemical composition, water-holding capacity, viscosity, and fermentability. (fao.org)
Composition and definition
Dietary fiber is not a single substance. Its components include cellulose, hemicelluloses, pectins, resistant starch, and other nondigestible carbohydrates. Definitions also commonly include lignin, a structural plant polymer that is not a carbohydrate. Resistant starch is the fraction of starch that escapes digestion in the small intestine. (fao.org)
Definitions used in food regulation specify which substances may be counted as fiber on labels. In the United States, the Food and Drug Administration includes nondigestible carbohydrates containing at least three monomeric units, together with lignin, when they are naturally present and intact in plants. Isolated or synthetic nondigestible carbohydrates must have a beneficial physiological effect recognized by the agency to qualify. Consequently, resistance to digestion alone does not automatically make an added ingredient legally declarable as dietary fiber. (fda.gov)
Physical properties and classification
Fiber is commonly divided into soluble and insoluble fractions according to its behavior in water. However, solubility does not reliably indicate how extensively a fiber will be fermented or what physiological effects it will produce. Two additional properties are especially relevant:
- Viscosity: the ability of some fibers to thicken intestinal contents or form gels, affecting the movement and absorption of nutrients.
- Fermentability: the extent to which intestinal microorganisms can break down a fiber.
These properties overlap but are not interchangeable. Water-holding capacity, particle structure, and food processing also influence fiber’s behavior in the digestive tract. (fao.org)
Physiological effects
Fiber can increase the bulk and water content of intestinal material. Some fibers increase stool bulk or alter intestinal transit, while others form viscous solutions that slow the digestion and absorption of nutrients. These effects depend on the particular fiber rather than applying uniformly to all fiber-containing foods or ingredients. (fao.org)
Viscous fibers can slow the appearance of glucose in the bloodstream after a meal. Certain soluble fibers can also reduce blood cholesterol concentrations. The FDA recognizes effects such as reduced blood glucose or cholesterol and increased bowel-movement frequency when evaluating isolated or synthetic fibers. (fao.org)
In the large intestine, bacteria break down fermentable fiber through fermentation, producing short-chain fatty acids and gases. These fatty acids can provide energy to the intestinal lining and enter the body’s metabolism. Thus, “nondigestible” does not necessarily mean that fiber supplies no usable energy: microbial fermentation can make some of its energy available indirectly. (fao.org)
Food sources and processing
Naturally occurring fiber is found in whole grains, beans, peas, lentils, vegetables, fruits, nuts, and seeds. Different foods provide mixtures of fiber components rather than a single uniform type. Fiber may also be added to foods as an isolated ingredient, including cellulose, pectin, psyllium husk, or other recognized nondigestible carbohydrates. (accessdata.fda.gov)
Processing can change both fiber quantity and function. Refining cereal grains removes the bran and germ and reduces their fiber content. Other processes can alter fiber’s molecular structure, hydration, or viscosity without necessarily producing an equally large change in its measured amount. Processing can also change the proportion of resistant starch in a food. (accessdata.fda.gov)
Intake reference values and measurement
In its 2023 carbohydrate guideline, the World Health Organization recommends at least 25 grams per day of naturally occurring dietary fiber in foods for adults. The United States Nutrition Facts label uses a Daily Value of 28 grams, based on a 2,000-calorie diet. These figures have different purposes: the former is a population-level dietary recommendation, while the latter is a standardized labeling reference. (who.int)
Measured fiber content depends partly on the analytical method, because different methods capture different nondigestible components. Regulatory declarations may also require information beyond laboratory measurement. The FDA notes that analytical methods cannot, by themselves, distinguish qualifying dietary fiber from added nondigestible carbohydrates that do not meet its definition. (fao.org)
References
- Questions and Answers on Dietary Fiberfda.gov
- Interactive Nutrition Facts Label - Dietary Fiberaccessdata.fda.gov
- CHAPTER 2: METHODS OF FOOD ANALYSISfao.org
- Physiological Effects of Dietary Fibrefao.org
- Quality and Accessibility of Food-Related Datafao.org
- Effects of Food Processing on Dietary Carbohydratesfao.org
- Carbohydrate intake for adults and childrenwho.int
- Recommendations and supporting information - Carbohydrate intake for adults and childrenncbi.nlm.nih.gov