Dietary fiber for optimizing gastrointestinal health (Proceedings)

The gastrointestinal tract (GIT) in dogs and cats is a very dynamic organ that performs numerous functions essential for health and well-being. For example, a critical function of the GIT is 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 immunological organ in the body and also functions as an endocrine organ.

Although the effect of various nutrients on the GIT in dogs and cats has been studied for decades, it is only in the past 10-15 years that significant knowledge and understanding of the role that dietary fiber has in maintaining health and preventing diseases has been recognized. Historically fiber has not been considered essential in the diets of dogs and cats, but more and more the critical role that fiber has in promoting a healthy GIT is being recognized. The role of fiber in the GIT can be generally summarized as having three different effects: 1) physical, 2) fermentative, and 3) bacterial.

Introduction of Dietary Fiber

Dietary fiber was originally defined as "the remnants of plant cell walls not hydrolyzed by the alimentary enzymes of man," but the definition was subsequently modified to include all plant polysaccharides and lignin which are resistant to hydrolysis by digestive enzymes.1 More recently, the definition of fiber has been modified further and is now defined as the composite of all dietary constituents that are not digested by endogenous enzyme secretions in mammals. Although not digestible, dietary fiber is considered to have nutritional value because of its importance in maintenances of the functional integrity of the GIT.

Dietary fiber consists of material of diverse chemical and morphological 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 utilized for specific effects on the GIT. Major components of dietary fiber include nonstarch polysaccharides, cellulose, hemicellulose, mixed-linkage beta-glucans, pectins, gums and mucilages. Lignins are also included in the estimates of total dietary fiber because they are plant cell wall constituents which can greatly affect the digestibility of plant-derived foods.2 Quantitatively, lignins do not make a significant contribution to total dietary fiber intake unless intact seeds are consumed.

The diverse nature of fiber has lead to numerous ways of classifying fiber, 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.3-6 These numerous classifications of fiber have lead to confusion since fibers classified in the same category in one system may be classified in entirely different categories in another system.

In the past, dietary fiber has been classified by its solubility (soluble vs insoluble). This classification is based on how fiber reacts with water.3 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 GIT (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 because the fermentability or the capacity for fiber breakdown by intestinal bacteria more accurately assesses fiber's potential beneficial effects in the GIT than does solubility.

Table 1. Dietary Fiber Fermentation in Dogs

Beneficial Effects of Fermentation of Fiber by Intestinal Bacteria

Fiber usually passes through the stomach and small intestines intact in dogs and cats because they do not endogenously produce the enzymes needed to digest fiber. Until recently, fiber fermentation was thought 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 (DER)] to these species. However, dogs and cats have a relatively short and simple structure of the large intestines, and as a result, SCFAs provide less than 5% of energy needs and therefore have little effect on energy balance.7,8

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

1. An energy source for colonocytes.9

• Colonocytes derive more than 70 percent of their energy

• needs 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 GIT turnover rapidly and must be

• replaced on average once every three 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 of dogs.

3. Maintenance of intestinal motility.

• Normal intestinal motility appears to be influenced by the

• presence of SCFA. Kamath et al. infused bolus doses of physiological 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 (eg,

• Clostridia, salmonella, Enterobacteria) can produce 1) toxins, 2) carcinogens, and 3) putrefactive

• substances. Beneficial bacteria (eg, Bifidobacteria, Lactobacilli) 1) inhibit the presence of

• harmful bacteria, 2) stimulate immune function, 3) aid in digestion and (or) absorption of food, and 4)

• synthesize vitamins. Maintenance of beneficial indigenous bacterial populations is important in

• prevention of pathogenic bacterial overgrowth in the intestine. Indigenous bacterial populations in the

• dog or cat GIT ferment certain fiber sources that result in the production of SCFA. The presence of SCFA

• inhibits the growth of pathogenic bacteria. 15,16 As a result, not only are indigenous bacterial populations

• necessary for the production of SCFA, but they can also directly inhibit pathogenic bacterial overgrowth

• in the gut. A growing body of evidence supports the theory that certain dietary fiber sources can modify

• the composition of the intestinal microflora.

5. Maintenance of optimal colonic morphology.1

• Dogs fed a fermentable source of fiber had

• increased colon weights, increase mucosal surface areas and mucosal hypertrophy compared

• to 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 nonfermentable fiber

• (cellulose) as the sole source of dietary fiber fed to dogs resulted in a higher incidence of mucus

• distension and cryptitis when compared to 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). As 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 not metabolized by intestinal bacteria to produce SCFAs. Rather they retain their structure while passing through the GIT 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, 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 production of SCFAs occur 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. Beet pulp is not derived from red beets; therefore, there is not anything in sugar beet pulp that can affect coat color of dogs. It has been widely used in the livestock industry for many years, and in the last 10 years, the use of beet pulp has expanded into the pet food industry as a source of fiber. It is a safe fiber source 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 bacterial. 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 bacteria in the colon.13,21

Fructooligosaccharides (FOS)

Certain fermentable fiber sources, such as 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. Fructooligosaccharides are found naturally in many different foods, including plants like beet root (after pulp processing), soy (in the hulls), psyllium, chicory (after hydrolysis) and numerous other fruits, vegetables and grains. 22 They can also be synthesized commercially. Beneficial intestinal bacteria (i.e. lactobacilli and bifidobacteria) use fermentable fiber as a metabolic fuel, whereas pathogenic bacteria (i.e. 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. 23 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. 24 Similarly, FOS supplementation increased numbers of beneficial bacterial and decreased numbers of potential pathogens in the large intestine of healthy cats. 25 This too prevents excessive numbers of pathogenic bacteria from invading the intestines.

Mannanoligosaccharides (MOS)

Mannanoligosaccharides are unique fiber sources similar to FOS. The difference between FOS and MOS is that fructose is the predominant sugar molecule in FOS, whereas mannose is the predominant sugar molecule in MOS. Mannanoligosaccharides are natural fibers found in yeast cells, and they use a different mechanism to prevent the growth of harmful bacteria in the GIT than do FOS. One way pathogenic bacteria establish themselves in the GIT is by attaching to the intestinal wall and colonizing the GIT. Pathogenic bacteria are able to attach to the intestinal wall because they have finger-like projections, called fimbriae, that allow them to bind to specific residues (e.g. mannose) on intestinal cells. Since MOS contains mannose, fimbriated mannose-specific pathogens can bind to MOS instead of the intestinal wall. 26 By preventing these bacteria from adhering to the intestinal wall, MOS can inhibit the growth of pathogenic organisms, reduce their effects in the GIT and aid in the excretion of these harmful bacteria. They are very effective in preventing diarrhea and contribute to the prevention of digestion-related infectious diseases.

FOS and MOS Enhance the Effectiveness of the Gastrointestinal Immune System

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

The gut contains both non-immunological barrier defenses and immunological barrier defenses. Non-immunological barriers include 1) the anatomy of the gut (intact microvilli and tight junctions between cells), 2) peristalsis and mucus, which makes it difficult for pathogens to attach and enter cells, 3) low pH of gastric secretions, 4) digestive and bactericidal enzymes secreted by the stomach, pancreas and epithelial cells to inhibit the attachment and growth of bacteria. 27

The gut is also the largest immunological organ in the body. As a result, immunological barriers play a critical role in maintaining normal health and function of not only the GIT 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). 28 Each diet contained similar amounts of fiber but differed in its fermentability. The diet supplemented with fermentable fiber significantly (P<0.05) decreased the proportion of Ig+ cells and increased the CD4/CD8 (ie 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

Fiber is not considered an essential nutrient for dogs and cats, and with the exception of insoluble fiber, such as cellulose, was largely ignored in the diet of dogs and cats until the last 10-15 years. More and more studies are supporting the important role of dietary fiber for maintaining gastrointestinal and systemic health. Specific fiber types are being used to cause specific affects on bacterial populations in the gut, as well as systemic affects on the immune system. As more studies are conducted in dogs and cats demonstrating the number of beneficial effects different fiber types (fermentable vs nonfermentable) have on GIT health, fiber may one day be classified as an essential nutrient for these species.

References available upon request