Fiber Analytical Schemes

Besides food manufacturers, epidemiologists, scientists, and dietitians need data on the fiber content of foods (8). Yet attempts to define and standardize methods to measure dietary fiber remain contentious. Dietary fiber is essentially the undigested carbohydrates in the diet (9). These carbohydrates may be fermented in the large intestine, although some resistant fibers, such as purified cellulose, escape any fermentation, whereas other fibers, such as inulin or pectin, are completed broken down by bacteria in the colon.

Most analytical schemes to measure dietary fiber are chemical and enzymatic extraction procedures. The TDF method, or Prosky Method, has become the standard method for the measurement of dietary fiber in the United States. Because the TDF method does not isolate all undigested carbohydrates, especially short-chain oligosaccharides, other methods have been developed and accepted to quantitate these compounds.

Determining the solubility of fiber was an attempt to relate physiological effects to chemical types of fiber (9). Soluble fibers were considered to have beneficial effects on serum lipids and insoluble fibers were linked with laxation benefits. This division of soluble and insoluble fiber is still used in nutrition labeling. However, despite these commonly used generalizations, scientific evidence supporting that soluble fibers lower cholesterol and insoluble fibers increase stool weight is inconsistent. Many fiber sources are mostly soluble but still enlarge stool weight, such as oat bran and psyllium.

Also, soluble fibers such as inulin do not lower blood lipids. Most fruits and vegetables are concentrated in insoluble fiber, not soluble fiber. Exceptions to this generalization include cooked potatoes, oranges, and grapefruit. The USDA Nutrient Database includes only total fiber; there are no official databases that include soluble and insoluble fiber. Lists of content of total, insoluble, and soluble fibers are compilations of data from the USDA, the published literature, and estimated values (10). Often, the values for soluble and insoluble fiber do not add to total fiber or the values for soluble fiber were estimated by subtracting a literature value for insoluble fiber from a USDA value for total fiber. Not surprisingly, there is much discrepancy in the fiber concentrations for fruits and vegetables.

Processing can either increase or decrease the fiber content of a fruit or vegetable. Peeling fruits or vegetables will lower the fiber content (11). A serving of grapefruit without any associated membrane contains much less fiber than a grapefruit serving with membranes (0.4 vs. 1.4 g/serving). Home cooking generally has a negligible effect on fiber content. Cooking, in general, may even increase the fiber content of a product if water is driven out in the cooking process. Baking or other heat treatments (e.g., extruding) used in food processing will also increase the fiber content of the product, either by concentrating the fiber by removal of water or producing Maillard products that are captured as fiber in gravimetric methods. Fruit juices are not devoid of fiber.