The gastrointestinal (GI) tract in dogs and cats is a very dynamic organ that performs numerous functions essential for health and well-being. Critical roles of the GI tract include digestion and absorption of nutrients, as well as elimination of potentially harmful substances and waste products. In addition, the GI tract is the most voluminous immunologic organ in the body and also functions as an endocrine organ.

The GI tract also contains a very large and diverse population of bacteria, and these bacteria affect the health of the host in many significant ways. Recognition that these bacteria play a large role in the overall health of the animal has led to research efforts focusing on methods to manipulate the GI bacterial population to improve health. One way to accomplish this is through the use of prebiotics.

What are prebiotics?

Introduction to dietary fiber as a prebiotic

Although the effects of various nutrients on the GI tract in dogs and cats have been studied for decades, it is only within the past 10 to 15 years that significant knowledge and understanding of the role of dietary fiber in maintaining health and preventing disease has been recognized. Historically, fiber has not been considered essential in the diets of dogs and cats; more and more, however, its critical role in promoting a healthy GI tract is being recognized. As a result, dietary fiber is considered to have nutritional value because of its role as a prebiotic and its importance in maintaining the functional integrity of the GI tract.

What is dietary fiber?

Dietary fiber was originally defined as "the remnants of plant cell walls not hydrolyzed by the alimentary enzymes of man"; this definition was subsequently modified to include all plant polysaccharides and lignins that are resistant to hydrolysis by digestive enzymes. This definition has been modified further, and fiber is now defined as "the composite of all dietary constituents that are not digested by endogenous enzyme secretions in mammals."

Dietary fiber consists of material of diverse chemical and morphologic structure. Large differences exist in the physical form and the physiologic effect of various classes of dietary fiber in dogs and cats, and it is now recognized that specific fiber types can be used for specific effects on the GI tract. Major components of dietary fiber include nonstarch polysaccharides, cellulose, hemicellulose, mixed-linkage ?-glucans, pectins, gums, and mucilages. Lignins are also included in the estimates of total dietary fiber because they are plant cell wall constituents that can greatly affect the digestibility of plant-derived foods. Quantitatively, lignins do not make a significant contribution to total dietary fiber intake unless intact seeds are consumed.

Classification of fiber

The diverse nature of fiber has led to numerous ways of classifying it, including by solubility in water, rate of fermentation, digestible and indigestible fractions, water-holding capacity, viscosity, fecal-bulking ability, cation exchange capacity, bile acid–binding ability, and microbial fuel value. These numerous classifications of fiber have led to confusion because fibers classified in the same category in one system may be placed in entirely different categories in another system.

In the past, dietary fiber has been classified by its solubility (soluble versus insoluble). This classification is based on how fiber reacts with water. All fibers hold water to some degree; however, the soluble fibers have a greater water-holding capacity than insoluble fibers, and they may form gels and viscous solutions in the GI tract (Table 1). In more recent years, this classification of fiber has fallen into disfavor. Categorization of fiber types based on fermentability (Table 1) is a more meaningful way to describe certain fiber sources for dogs and cats. Fermentability, or the capacity for fiber breakdown by intestinal bacteria, provides a better indication of the physiological responses resulting from fiber ingestion, and it more accurately assesses fiber's potential beneficial effects in the GI tract than does solubility. As fermentative substrates, fiber acts as a prebiotic.