Prebiotics: The drugless approach to GI health (Proceedings)

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?

Prebiotics are defined as nondigestible food ingredients that beneficially affect the host by selectively stimulating the growth and/or activity of one or a limited number of beneficial bacteria in the colon that improve host health. The most common prebiotic found in diets of dogs and cats is dietary fiber.

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.

Beneficial effects of fermentation of fiber by intestinal bacteria

Fiber usually passes through the stomach and small intestines intact in dogs and cats because dogs and cats do not endogenously produce the enzymes needed to digest fiber. Until recently, fiber fermentation was believed to be irrelevant in dogs and cats, However, once fiber reaches the large intestines, intestinal bacteria are able to ferment certain types of fiber, resulting in the production of short-chain fatty acids (SCFAs). As a result, fermentation of fiber is very important in dogs and cats. The major SCFAs produced from fermentation are acetate, propionate, and butyrate. In ruminants and herbivorous animals, SCFAs provide a significant source of energy (i.e., up to 75% of daily energy requirement). Because the large intestines of dogs and cats have a relatively short and simple structure, however, SCFAs provide less than 5% of energy needs in these species and have little effect on energy balance.

Although production of SCFAs in the large intestines of dogs and cats provides little energy, it does have a number of beneficial effects:

1. An energy source for colonocytes. Colonocytes derive more than 70% of their energy from luminally derived SCFAs. As a result, fermentation of fiber provides a readily available source of energy for colonocytes that aids in maintaining the health and function of these cells. SCFAs are also important for cell renewal and repair. Epithelial cells of the GI tract turn over rapidly and must be replaced on average once every 3 days. By providing energy for intestinal cells, SCFAs facilitate the replacement of cells that have been sloughed during the normal process of cell turnover.

2. Maintenance of normal intestinal electrolyte and fluid balance. SCFAs facilitate the absorption of sodium, chloride, and water in the colon. For example, a study in dogs showed that sodium and SCFA absorption could account for the entire osmotic absorption of water from the colon. As a result, providing fermentable fiber in the diet is essential in maintaining the normal homeostatic absorptive function of the intestine in dogs.

3. Maintenance of intestinal motility. Normal intestinal motility appears to be influenced by the presence of SCFAs. Kamath et al infused bolus doses of physiologic concentrations of SCFA into the ileum of dogs. As the dose of SCFA increased, ileal motility increased. Therefore, providing fermentable fiber in the diet may be important in maintaining normal intestinal motility.

4. Amelioration and prevention of pathogenic bacterial overgrowth. Harmful bacteria (e.g., Clostridium, Salmonella, enterobacteria) can produce toxins, carcinogens, and putrefactive substances. Beneficial bacteria (e.g., Bifidobacterium, lactobacilli) inhibit the presence of harmful bacteria, stimulate immune function, aid in digestion and absorption of food, and synthesize vitamins. Maintenance of beneficial indigenous bacterial populations is important in preventing pathogenic bacterial overgrowth in the intestine. Indigenous bacterial populations in the canine or feline GI tract ferment certain fiber sources that result in the production of SCFAs. The presence of SCFAs inhibits the growth of pathogenic bacteria. As a result, not only are indigenous bacterial populations necessary for the production of SCFAs, but they can also directly inhibit pathogenic bacterial overgrowth in the gut. A growing body of evidence supports the concept that certain dietary fiber sources are prebiotics and can modify the composition of the intestinal microflora.

5. Maintenance of optimal colonic morphology. Dogs fed a fermentable source of fiber had increased colon weights, increased mucosal surface areas, and mucosal hypertrophy when compared with dogs fed a nonfermentable fiber source (cellulose). These effects on the colon also aid recovery after intestinal surgery.

6. Amelioration of intestinal inflammation. Diets containing poorly fermentable fiber (cellulose) as the sole source of dietary fiber fed to dogs resulted in a higher incidence of mucus distension and cryptitis when compared with similar diets that contained a fermentable fiber source.

All fermentable fibers are not created equal

Fiber sources can vary in their level of fermentability (Table 1), and therefore, some fiber types function better as a prebiotic than other fiber types. As the fermentation rate of fiber increases, GI transit time decreases, fecal bulk decreases, and fecal bile acid excretion increases.

Fibers with low fermentability (e.g., cellulose, methylcellulose, oat fiber, peanut hulls, xanthan gum, locust bean gum) are poorly metabolized by intestinal bacteria to produce SCFAs. Rather, they retain their structure while passing through the GI tract intact and act as bulking agents. Highly fermentable fibers (e.g., pectin, guar gum, pectin) are rapidly metabolized by intestinal bacteria. One of the products from bacterial fermentation of fiber is SCFAs; however, less desirable substances are also produced from bacterial fermentation, including carbon dioxide, hydrogen, and methane. If a fiber source is rapidly fermentable, large amounts of gases will be rapidly produced in the colon, resulting in diarrhea and cramping. Moderately fermentable fiber sources (e.g., beet pulp, inulin, rice bran, gum arabic, xanthan gum) produce SCFAs without resulting in rapid production of gases and associated diarrhea. Therefore, moderately fermentable fiber produces the beneficial effects associated with SCFA production without the undesirable effects seen with rapidly fermentable fiber.

An ideal fiber source for dogs and cats should contain a moderately fermentable portion to facilitate SCFA generation as well as a nonfermentable portion to provide bulk and enhance peristalsis. Beet pulp fulfills both of these requirements.

What is beet pulp?

Beet pulp is the fiber material that remains after sugar is extracted from sugar beets. Because beet pulp is not derived from red beets, there is nothing in sugar beet pulp that can affect the coat color of dogs. It has been widely used as a source of fiber in the livestock industry for many years; and in the past 10 years, its use has expanded into the pet food industry. Beet pulp is safe and contains no known toxins.

Fiber as a source of prebiotics to enhance intestinal health

Prebiotics have received a lot of attention recently as a way to modulate bacterial populations in the colon to favor beneficial bacteria. Appropriate types of prebiotics can also optimize stool characteristics and reduce fecal odor because fermentation of fiber reduces fecal ammonia, indole and phenol concentrations, compounds have been implicated as the major malodorous components of feces and as toxins in the colon.

Two fermentable fiber sources that function as prebiotics in the colon are fructosooligosaccharides (FOS) and mannanoligosaccharides (MOS). Although both of the fiber sources can have a dramatic effect on the microbial population in the colon, the mechanism by which they accomplish this is different between the two sources.

Fructooligosaccharides

Certain fermentable fiber sources, such as fructooligosaccharides (FOS), are good sources of prebiotics for dogs and cats, and the proportion of different bacterial species is related to the type of fermentable substrate available. FOS are found naturally in many different foods, including plants such as sugar beet root (after pulp processing), soy (in the hulls), psyllium, and chicory (after hydrolysis) as well as numerous other fruits, vegetables, and grains. FOS can also be synthesized commercially. Beneficial intestinal bacteria (e.g., lactobacilli, Bifidobacterium) use fermentable fiber as a metabolic fuel, whereas pathogenic bacteria (e.g., Salmonella, Escherichia coli, Clostridium perfringens) cannot metabolize FOS for energy. Production of SCFAs from fiber by beneficial bacteria also lowers colonic pH, further impeding the growth of bacterial pathogens. As a result, in the presence of FOS, beneficial bacteria thrive, multiply, and crowd out pathogenic bacteria. In addition, a study by Willard et al showed that supplementing the diet of dogs with FOS resulted in a significant decrease in the number of aerobic and anaerobic bacteria in the small intestine. Similarly, FOS supplementation increased the number of beneficial bacteria and decreased the number of potential pathogens in the large intestine of healthy cats. This, too, prevents excessive numbers of pathogenic bacteria from invading the intestines.

Mannanoligosaccharides

Mannanoligosaccharides (MOS) are unique fiber sources similar to FOS. The difference between MOS and FOS is that fructose is the predominant sugar molecule in FOS, whereas mannose is the predominant sugar molecule in MOS. MOS are natural fibers found in yeast cells, and they use a different mechanism than FOS to prevent the growth of harmful bacteria in the GI tract. One way pathogenic bacteria establish themselves in the GI tract is by attaching to the intestinal wall and colonizing. Pathogenic bacteria are able to attach to the intestinal wall because they have fingerlike projections called fimbriae that allow them to bind to specific residues (e.g., mannose) on intestinal cells. Because MOS contain mannose, fimbriated mannose-specific pathogens bind to MOS instead of the intestinal wall. By preventing these bacteria from adhering to the intestinal wall, MOS can inhibit the growth of pathogenic organisms, reduce their effects in the GI tract, and aid in the excretion of these harmful bacteria. In addition, they are very effective in preventing diarrhea.

FOS and MOS enhance the effectiveness of the gastrointestinal immune system

The GI tract contains a large population of bacteria, and it is critical to the health of dogs and cats that these bacteria remain in the gut and are prevented from translocating systemically. The GI tract is constantly under antigenic stimulation from both bacteria and food, and the integrity of the GI tract is essential for maintenance of intestinal health.

The gut contains both immunologic and nonimmunologic barrier defenses. Nonimmunologic barriers include gut anatomy (intact microvilli and tight junctions between cells); peristalsis and mucus, which make it difficult for pathogens to attach and enter cells; low pH of gastric secretions; and digestive and bactericidal enzymes secreted by the stomach, pancreas, and epithelial cells to inhibit the attachment and growth of bacteria.

Because the gut is the largest immunologic organ in the body, immunologic barriers play a critical role in maintaining normal health and function not only of the GI tract but systemically as well. Gut-associated lymphoid tissue (GALT) is composed of cells residing in the lamina propria regions of the gut, intraepithelial lymphocytes interspersed between epithelial cells, and immune cells located in organized lymphatic tissue (Peyer's patches and mesenteric lymph nodes).

Fermentable fibers, such as FOS, can have a major impact on the gut's immune function. In a study by Field et al, dogs were fed isonitrogenous, isoenergetic, meat-based diets supplemented with either a combination of fermentable fibers (beet pulp, gum arabic, and FOS) or with nonfermentable fiber (cellulose). Each diet contained similar amounts of fiber but differed in its fermentability. The diet supplemented with fermentable fiber significantly decreased (P < .05) the proportion of Ig+ cells and increased the CD4:CD8 (i.e., T-helper cell:cytotoxic T-cell ratio) in peripheral blood. Therefore, adding fermentable fiber to the diet of dogs changed the composition and function of immune cells in GALT.

Conclusions

Prebiotics can have a dramatic effect on the microbial population in the colon, and fiber is the most common prebiotic found in the diet of dogs and cats. Although fiber is not considered an essential nutrient in the diets of dogs and cats, it does provide many beneficial effects that contribute to the overall health of the animal. With the exception of insoluble fiber, such as cellulose, fiber was largely ignored in the diets of dogs and cats until the past 10 to 15 years, but it is now well recognized that dietary fiber plays an important in maintaining GI and systemic health. Specific fiber types are being used to cause specific effects on bacterial populations in the gut as well as systemic effects on the immune system. As more studies are conducted on dogs and cats demonstrating the number of beneficial effects different fiber types (fermentable versus nonfermentable) have on GI tract health, fiber may one day be classified as an essential nutrient for these species.

Table 1. Dietary fiber fermentation in dogs

References available upon request