Almost all diets contain fiber components, such as cellulose, hemicellulose, pectins, gums, and lignins that are found within plant cell walls.  These materials are not truly digested by the animal and therefore do not contribute energy when consumed.   Although fiber is not digested and absorbed by the dog and cat, some fiber is still essential in their diet.  Fiber plays a role in the regulation of the time in which food travels through the intestinal tract, fiber aids in fat absorption, and fiber effects the water holding capacity of the stools to aid in the prevention of diarrhea and constipation.  Food ingredients commonly used in the pet food industry as fiber sources include beet pulp, rice bran, powdered cellulose, peanut hulls, oat bran, soybean hulls, and tomato pomace.

Crude fiber is the form of fiber that is presented to the pet owner when purchasing their pet's diet.  It is the format in which legally; fiber must be listed on the food package.  Crude fiber represents what remains after plant material has been extracted with acid and alkali solvents.  However, crude fiber values actually underestimate the level of true fiber in the diet.  It has been estimated that while up to 80% of the cellulose is recovered during crude fiber analysis, less than 50% of the hemicellulose and lignin are recovered.  When comparing the fiber levels of pet foods based on packaging information, it is important that the crude fiber percentage is used only as a starting point in this comparison.

Other means of analysis are available to allow for the determination of total dietary fiber.  Although these methodologies are long and costly, a good estimation of true fiber level in a diet can be obtained.  This is extremely important as the fiber level of a diet can provide information regarding the quality of ingredients in a product, the ability of the product to support proper gastrointestinal function, the digestibility of a product, and the fecal output obtained upon consuming a particular product.   Dietary fiber values will always exceed those numbers obtained when analyzing for crude fiber, as the dietary fiber analysis will result in the true fiber content of the food.  For example, beet pulp contains approximately 17% crude fiber but 59% dietary fiber.  When it is essential that the pet owner understand the level of fiber in a particular diet, such as in the case of an overweight pet or in certain specific clinical conditions, the total dietary fiber content of the diet should be investigated.