Fiber is from plant cell wall material.  It is a combination of at least four major components which are distinctly different in chemical composition including cellulose, hemicellulose, lignin, and pectin and gums. More recently, fiber has also been defined to include functional fibers such as oligosaccharides.

Fiber cannot be degraded by mammalian enzymes, but can be used by gastrointestinal microbiota. In all species, fiber fermentation by bacteria creates short-chain fatty acids (acetate, propionate, and butyrate) that the animal may then use for energy or other biological processes. In ruminants, fiber is degraded in the rumen, which provides the main fuel for the animal. In monogastric animals, is fermentation in the cecum and/or colon, depending on the species.

Current research indicates that various fibers may have these physiological effects, but is highly dependent on the type of fiber used:

  1. A decrease in the absorption of minerals
  2. The binding of bile acids which are integral to cholesterol homeostasis and fat absorption
  3. A change in the potency of intestinal toxins and carcinogens
  4. The production of volatile fatty acids which are used for energy or for inhibiting pathogens
  5. Changes in transit time
  6. Alterations in gastrointestinal bacteria

Different Analytical Methods

Measuring fiber is highly complex. The fiber percentages provided in the our Purina TestDiet® specification sheets are all based on calculated values for crude fiber (CF), neutral detergent fiber (NDF), and acid detergent fiber (ADF). Other common fiber determination methods include nonstarch polysaccharide (NSP) and total dietary fiber (TDF)

“Max Fiber” (Crude Fiber) is an average of crude fiber from all the ingredients in the formula.  Briefly, the sample is first boiled in dilute acid and then in dilute alkali. The acid hydrolysis removes free sugars and starch. The alkaline hydrolysis removes protein and some carbohydrates. This process also removes some hemi-cellulose and lignin nor will it measure soluble fiber; therefore, only partial recovery of fiber components is achieved.  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; whereas, 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 to CF; but, it will never be exact.

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

Nonstarch polysaccharide (NSP) methodology is most similar to the TDF method, but is unable to measure lignin.

Fiber in Purified Diets

Traditionally most, of not all, of the fiber in a purified diet has been supplied by cellulose, an insoluble fiber. At Purina TestDiet® we recognize the growing attention to the function of fiber in digestion and general lab animal health. Consequently, our new Purina TestDiet® DIO (diet-induced obesity) series provides equal parts of the insoluble fiber traditionally used in purified diets (cellulose) and soluble fiber (inulin), to more closely resemble a natural ingredient diet. Inulin is an oligosaccharide made from beets and artichoke and provides no measurable energy (kcal). At your request, we can modify any formula to substitute inulin, or another fiber source.