What's the Scoop on Dietary Fiber?

Information about dietary fiber seems to pop up everywhere you look these days and probably for good reason.  There is more research available each day to show the many benefits of dietary fiber for the health of people and animals.  Fiber comes from plant cell walls and is a type of carbohydrate that is not digested by the enzymes present in mammals. This type of carbohydrate is comprised of four major components which are distinctly different in chemical composition.   These four carbohydrate types are, cellulose, hemicellulose, lignin, and pectin/gums.

Although fiber isn’t digested by the enzymes naturally occurring in mammalian digestive systems, gastrointestinal microbes help to ferment fiber, producing short-chain fatty acids (acetate, propionate, and butyrate) that are used for energy or other biological processes. In ruminants, fiber is degraded into rumen, which is the main source of nutrition for the animal. Fermentation in monogastric species occurs in the cecum and/or colon.

Food sources of fiber are generally classified in two categories, soluble and insoluble, but there are several subtypes of fiber within these two categories.  Soluble fiber binds with fatty acids and allows blood sugars to release more slowly into the body. They also help lower LDL (bad) cholesterol.  Insoluble fibers assist with hydration of waste and control the pH levels in the intestines.  

The world of fiber continues to evolve and recently functional fibers such as oligosaccharides have been included in the definitions of food sources of fiber.  Resistant starch (RS) includes starch and starch degradation products that escape from digestion in the small intestine of healthy individuals. Resistant starch is also more recently considered a third type of dietary fiber; it exhibits benefits of insoluble fiber and some of the benefits of soluble fiber.

Measurement of fiber is a fairly complex process and requires different methodologies for the different types of fiber. Common methods at EPL BAS include measurement of Crude Fiber, Acid Detergent Fiber (ADF), Neutral Detergent Fiber (NDF), and Total Dietary Fiber.  We can also measure Lignin and Total Starch.

Crude fiber (CF) analysis removes free sugars and starch. This alkaline hydrolysis removes protein, some carbohydrates, some hemi-cellulose and lignin and does not measure soluble fiber. Crude fiber is only 1/7 to 1/2 of total dietary fiber.

Acid Detergent Fiber (ADF) refers to the insoluble fiber within a plant cell wall and is comprised of cellulose and lignin.

Neutral Detergent Fiber (NDF) is a value comprised of ADF plus insoluble hemicellulose.  The ADF value can be subtracted from the NDF value to reach a figure close, but not exactly equal to CF.

Total Dietary Fiber (TDF) allows for separate measurement of total fiber, insoluble fiber, and soluble fiber. This method is unable to measure many oligosaccharides.