Fiber

Fiber

Fiber has received a great deal of research attention among animal scientists because of its importance to the ruminant. In the ruminant, it represents the plant cell wall that is utilized as an energy source by the rumen microflora, and is extensively degraded.

In the monogastric animal, fiber represents the insoluble matter of plant cell walls that is indigestible by animal enzymes, but can be partially degraded by gastrointestinal microflora.

Fiber is actually very complex. It is a combination of at least four major components which are distinctly different in chemical composition. These four major components are cellulose, hemicellulose, lignin, and pectin and gums.

Because of the complexity of defining and measuring fiber, any procedure for its measurement must strike a compromise between a complete, fractionated measurement of all of the various species, and a simplified system involving grouping in different compounds (cellulose, hemicellulose, lignin and pectin). To say that a ration has "X" amount of fiber makes no sense without some understanding of the ingredients used and/or knowledge of the amounts of each of the major fiber components that are present. The analysis of crude fiber generally includes the lignin and a portion of the cellulose and hemicellulose. Acid detergent fiber is thought to measure cell walls and is an estimate of all the lignin and cellulose. Neutral detergent fiber is an estimate of lignin, cellulose and hemicellulose. None of the above analyses include the pectin and gums.

The constituents of fiber affect the gastrointestinal tract differently, ultimately affecting the nutrition of the animal. Some fibers have a high water-holding capacity, which affects the speed at which the diet passes through the intestinal tract (transit time). Other fiber constituents have extensive cation-exchange capacity, which can bind dietary minerals.

Current research indicates that various fibers may have these physiological effects:

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

The changes produced by fiber in these and other body functions have been implicated in colon cancer, diverticular disease, diabetes, atherosclerosis, coronary heart disease, and hemorrhoids.

Different Analytical Methods

The fiber percentages provided in the our Purina TestDiet^® specification sheets are all based on very different analytical methods that measure insoluble fiber that cannot be broken down by the gastrointestinal enzymes.

“Max Fiber” (Crude Fiber – “CF”) 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; therefore, only partial recovery of fiber components is achieved. This is very different than Total Dietary Fiber that measures both soluble (which can be broken down by enzymes) and insoluble 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; whereas, Neutral Detergent Fiber (NDF) is a value comprised of ADF plus hemi cellulose, another insoluble fiber. In part, ADF can be subtracted from from NDF to reach a figure close to CF; but, it will never be exact, as the analytical methods for estimating are very different and measure different things.

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, and specifically in the metabolism of fats. 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 a derivative of beets and provides no measurable energy (kcal). At your request, we can modify any formula to substitute inulin for some or all of the cellulose.