Traditional and Novel Carbohydrate Sources for Dogs and Cats As the Most Important Source of Energy for Dogs and Cats, Carbohydrates Are Vital Nutrients in Pet Diets
December 2015 US$39.00
SPECIAL REPORT Traditional and Novel Carbohydrate Sources for Dogs and Cats As the most important source of energy for dogs and cats, carbohydrates are vital nutrients in pet diets. Carbohydrates are also critical to the proper manufacture of most commercial pet foods. Fortunately, an abundance of safe carbohydrates—natural and synthetic—are available to the pet food industry for use in all types of dietary formulas, even those that are grain-free.
by Heather F. Mangian, Ph.D.; Maria R.C. de Godoy, Ph.D.; and George C. Fahey Jr., Ph.D. Traditional and Novel Carbohydrate Sources for Dogs and Cats
by Heather F. Mangian, Ph.D.; Maria R.C. de Godoy, Ph.D.; and George C. Fahey Jr., Ph.D.
hen the topic of carbohydrates comes up related to pet animal nutrition, some consider this very important nutrient category as a Wtoxin to be avoided at all costs. Popular press articles and Internet chat routinely demonize carbohydrates as being harmful for the health and well-being of dogs and cats. However, a quick review of the facts about carbohydrates reveals their importance in companion animal nutrition and commercial pet food production. Carbohydrates are the major energy-containing constituent of plants and represent about 60 to 90 percent of plant dry matter. In plants, carbohydrates fall either into the “structural” category, composed of plant cell walls, or the “non-structural” category, composed of plant cell contents. The carbohydrates found in the cell contents are starch, disaccharides, oligosaccharides, fructan polysaccharides and resistant starch. Cell walls consist of beta glucans, pectins, gums, hemicelluloses, cellulose and lignin/phenolics. Lignins and phenolics are non-carbohydrate components but are covalently attached to the hemicelluloses and, thus, cannot easily be separated from them. Carbohydrates comprise a major proportion of animal diets and have many uses. Carbohydrates are contained in structural components of both plants and animals (e.g., cellulose in plants, cartilage in animals). In pet animal diets, structural carbohydrates comprise 3 to 30 percent, and non-structural carbohydrates 15 to 50 percent, of the formula. Dietary carbohydrates are used to maintain blood glucose, a key metabolite in animals, and to store energy in the form of glycogen. Carbohydrates also are involved in the redox reactions that generate adenosine triphosphate (ATP). They are used to enhance gastrointestinal and host health. Carbohydrates are important in pet food processing, particularly extrusion and retorting. But unlike the case for other nutrients, animals have no exogenous carbohydrate requirement since glucose can be synthesized from amino acids (protein) and glycerol (fat). There are two systems of categorizing carbohydrates. They can be described by their chemical or nutritional attributes. Chemical categories of carbohydrates rely on the number of sugar residues, ranging from one to many, and their arrangement. The chemical classification includes monosaccharides (glucose,
Petfood & Animal Nutrition 2.0 • Carbohydrate Sources 2 petfood2.com galactose and fructose), molecules that consist of one sugar; disaccharides (sucrose, lactose and maltose) that contain two sugars; oligosaccharides (fructooligosaccharides, galactooligosaccharides and others) containing from three to 60 sugars; and polysaccharides (starch, glycogen, cellulose, hemicelluloses, pectins and others) that contain many sugar molecules, often in complex arrangements. The nutritional classification of carbohydrates focuses on how carbohydrates provide nourishment. Absorbable carbohydrates are directly transported across the intestine into the blood stream and do not require enzymatic hydrolysis. These include the monosaccharides and sugar alcohols. Digestible carbohydrates are readily hydrolyzed by gastrointestinal tract enzymes to release monosaccharides. Sucrose, lactose and starch are digestible carbohydrates. Fermentable carbohydrates (oligosaccharides, resistant starch and dietary fibers) remain resistant to gastrointestinal enzymes but are readily fermented by the bacteria in the intestine. Finally, non-fermentable or poorly fermentable carbohydrates (wood cellulose, wheat bran, resistant maltodextrins) are resistant to gastrointestinal enzymes and not extensively fermented by bacteria.
Food sources high in carbohydrates Fruits and Vegetables
Apple Squash Tomatoes
Grains
Rice Corn Wheat Barley Oats Sorghum Pulses
Lentils Peas Beans
Tubers
Potato Sweet Cassava Potato
There are also man-made carbohydrates that usually use starch, sucrose or lactose as starting materials.
Petfood & Animal Nutrition 2.0 • Carbohydrate Sources 3 petfood2.com Polysaccharides are the most common types of carbohydrates found in pet foods. The largest single type of polysaccharide found in pet foods is starch, perhaps because starch is considered to be the most economical means of supplying dietary energy in pet food (in comparison to proteins and fats). Starch is usually supplied by feeding grains such as corn, rice and wheat. More recently, potato and sweet potato have become popular sources of starch in grain-free pet foods. In commercial pet foods, starches are not in their native state. Rather, they are processed either separately or as part of a diet using combinations of heat and moisture for various lengths of time. Uncooked starches are poorly digested by pets. For example corn is much more digestible in the cooked form compared with the uncooked form (as much as 17 percentage units higher). Cooking causes starch granules to become disordered, disrupting their crystalline structure, thus increasing the rate and extent of starch digestibility. This occurs as a result of gelatinization via hydrogen bond disruption. Resistant starch is the sum of starch and products of starch degradation that are not absorbed from the small intestine of man or animals. These molecules act as dietary fiber. There are four forms of resistant starch: RS1, RS2, RS3 and RS4. RS1 is starch that is physically inaccessible to digestive enzymes due to enclosures in structures such as intact cells in partly milled or The largest single type whole cereal grains. RS2 is a B-type starch granule occurring in of polysaccharide uncooked starch such as that in potatoes and high amylose grains. RS3, the most common type of resistant starch, is the found in pet foods amylose found in processed foods and is formed by repeated is starch, perhaps heating and cooling cycles (i.e., starch retrogradation). RS4 is because starch is chemically modified starch (e.g., esterified, etherified or with substituent group additions). Resistant starch may be beneficial to considered the most pets through increased microbial activity in the colon, resulting in economical means more short-chain fatty acid (SCFA) production, especially butyrate. of supplying dietary Other benefits may be lowered glycemic response and increased fecal output as a result of increased bacterial mass, with positive energy in pet food. implications for reduced bowel disease, including cancer. Blunting of the peak in glucose and insulin concentration after ingestion of a meal is an important health-related outcome. After a meal, there is a rapid rise in blood sugar concentration, followed by a rapid fall. A high glycemic index food has a rating of 70 and above, and a low glycemic index food has a rating of 55 or below. The impact of foods on blood glucose and insulin concentrations as described by the glycemic response is complex, depends on many factors, and the full implications are not known. Carbohydrates with low glycemic index values are sorghum, most dietary fibers, resistant starches, soluble corn fibers, resistant maltodextrins, pullulans and other novel
Petfood & Animal Nutrition 2.0 • Carbohydrate Sources 4 petfood2.com carbohydrates. However, the glycemic index of individual ingredients will not necessarily reflect the glycemic index of complete and balanced diets for dogs and cats due to their presence in a complex food matrix that has been processed in order to prepare a final product. Commonly used sources of digestible carbohydrates included in pet foods are grains such as barley, oats, corn, wheat and rice, with starch contents ranging from 40 to 80 percent of the grain dry matter. Research studies aimed at determining the usefulness of these carbohydrate sources have demonstrated they are well digested (84 to 100 percent) by both cats and dogs. Cooking improves digestibility of the carbohydrate. Raw carbohydrate sources (raw cornstarch, raw tapioca starch and raw potato starch) are not well digested by dogs (0 to 47 percent). Similar type studies show that cats also are able to digest and absorb carbohydrates (72 to 97 percent) with select sources (e.g., raw potato starch) not being well digested by the cat. For cats, cooking and use of smaller particle size of grain components improved the digestibility of the carbohydrate. Carbohydrates are important in the extrusion process used to produce dog and cat diets. The source and particle size of the carbohydrate can affect properties of dog food such as starch gelatinization and kibble quality. Recently, a study examined the impact of particle size on nutrient digestibility and gelatinization of carbohydrates in kibbled dog food. This study used a 3x3 factorial design, and diets composed of one of three different carbohydrate sources (rice, corn and sorghum) ground to three different particle sizes (300 µm - fine, 450 µm - medium, 600 µm - coarse) and fed to healthy beagle dogs (n=6 per treatment) were tested. Smaller particle size was associated with greater gelatinization of corn and sorghum diets. Grinding to different particle sizes did not change the gelatinization or the digestibility of rice-based diets, perhaps because the grinding processes used in this study reduced rice to a smaller particle size than was the case for other grains used in this study. The digestibility of corn and sorghum was indirectly related to particle size (nutrient by particle size interaction). A significant improvement in fecal scores (pertaining to dry matter and form) of dogs consuming diets with medium or fine carbohydrate particle size was noted, indicating a softer stool in dogs consuming coarsely ground carbohydrates. Fecal SCFA concentrations, used to predict SCFA in the distal intestine, were about 6 percent higher in dogs consuming diets containing the fine particle size carbohydrate compared to the medium or coarsely ground carbohydrate. In contrast, fecal butyrate, the SCFA associated with beneficial health effects, was almost doubled by consumption of coarsely compared to finely or medium ground grain sources. Structural carbohydrates include the fiber component of pet food ingredients. Traditional fiber sources used in the pet food industry are grain co-products
Petfood & Animal Nutrition 2.0 • Carbohydrate Sources 5 petfood2.com (corn gluten feed, wheat middling, soybean hulls), pomaces (apple, grape, tomato, citrus), wood cellulose, peanut hulls, beet pulp and gums. Prebiotic fibers are those associated with a benefit to the gastrointestinal tract of the animal through stimulation of the growth of beneficial intestinal bacteria. A variety of prebiotic fibers are available for feeding (e.g., galacto-, fructo-, isomalto-, gluco-, xylo-, pectic- and soybean-oligosaccharides, in addition to lactosucrose). Various fiber sources impact nutrient digestion, intestinal transit time and fermentation differently. Byproducts such as pomaces added to dog diets at ~8 percent of the diet decreased digestion by approximately 5 to 15 percent. On the other hand, beet pulp does not affect digestion up to a 7.5 percent dietary inclusion The single source of rate. Cats consuming diets containing 10 to 15 percent inclusion dietary fiber shown levels of fibrous ingredients had decreased protein (~3 percent) and lipid (~11 percent) digestibility compared to a 0 percent fiber to provide good stool diet. Fiber sources themselves vary widely in fermentability from characteristics without 0 to nearly 100 percent. Intestinal transit time is slowed by highly significantly decreasing viscous fibers. Highly insoluble fibers speed transit time by as much as 50 percent. Fibers vary in rate and extent of nutrient digestibility is fermentation. SCFA are the end-products of fiber fermentation in beet pulp, with 70 to the gut. The major SCFA are acetate, propionate and butyrate. Of 80% insoluble and 20 these, butyrate is largely metabolized by the colonic mucosa, where it provides energy, acts as a signaling molecule and is to 30% soluble fiber. associated with intestinal health. There are various strategies for incorporating fiber into pet diets. Pet owners who feed premium-type diets are most likely to be concerned about the role of supplemental fiber in maintaining intestinal health vs. those who feed generic, private label or popular brands. Other types of pet diets are formulated based on least cost and/or optimum palatability. These will likely already contain variable amounts of fiber (~4 to 8 percent) due to use of low-cost ingredients such as wheat middlings, peanut hulls, soy hulls, etc., that contain fiber. The selection of fiber source may be based on the characteristics of a single source or a blend of sources. The single source of dietary fiber shown to provide good stool characteristics without significantly decreasing nutrient digestibility is beet pulp. Beet pulp has about 70 to 80 percent insoluble fiber and 20 to 30 percent soluble fiber. Insoluble fibers alone may be too unfermentable and speed passage rate. Soluble fibers alone may result in osmotic diarrhea. Fiber “blends” can be formulated to simulate the properties of an “ideal” single fiber source. This alleviates the necessity of finding a single fiber source with the appropriate proportion of insoluble and soluble fibers, and
Petfood & Animal Nutrition 2.0 • Carbohydrate Sources 6 petfood2.com would expand the number of potentially beneficial fiber sources. In the future, it may be possible to formulate a fiber blend optimal for stool quality, SCFA production and host animal health (or all three). The ideal concentration and type of fiber in the diet has not been established. A diet containing 4 to 6 percent supplemental fiber works well. There are several disease states potentially affected by dietary fiber content (Table 1). Evidence supports inclusion of a soluble-viscous-fermentable fiber to provide desirable effects for a variety of disease states including cardiovascular disease, diabetes, small intestinal bacterial overgrowth, obesity, small bowel diarrhea, constipation and irritable bowel disease. Insoluble-nonviscous-nonfermentable fiber does not have as strong an impact on these disease states, but can prevent constipation while having a desirable effect on diabetes and irritable bowel symptoms. The benefit of prebiotic fibers for companion animal diets is derived from their ability to selectively stimulate health-promoting bacterial populations while diminishing the number/activity of the pathogenic microbiota. Prebiotic fibers are fermented by beneficial colonic bacteria (e.g., bifidobacteria, lactobacilli, eubacteria) and are not extensively used by potentially pathogenic bacterial species (e.g., select species of staphylococci, clostridia and veillonella). Table 1. Disease States Potentially Affected by Fiber Content of Companion Animal Diets
Dietary fiber type Disease state Soluble-viscous- Insoluble-nonviscous- fermentable nonfermentable Cardiovascular disease Desirable effect No effect Desirable effect if fed at high Diabetes Desirable effect levels or for prolonged duration Small intestinal bacterial overgrowth Debatable effect No effect Obesity Desirable effect Debatable effect Small bowel diarrhea Desirable effect No effect Constipation Desirable effect Prevented, but not treated Irritable bowel Desirable effect Desirable effect
Colon cancer Debatable effect Debatable effect
In conclusion, carbohydrates are important nutrients for both dogs and cats. They also are the most important source of energy for these animals. Both non-structural and structural carbohydrates play important roles in dog and cat
Petfood & Animal Nutrition 2.0 • Carbohydrate Sources 7 petfood2.com nutrition. Also, carbohydrates are critical to the proper manufacture of most commercial pet foods. An abundance of natural and synthetic carbohydrates are available to the pet food industry for use in all types of dietary formulas, even those that are grain-free. The majority of dogs and cats should not be denied carbohydrates as they are safe and serve many useful functions in pet food production and in pet animal metabolism. q
Heather F. Mangian, Ph.D., has served as a research scientist at the University of Illinois since 1979. Her area of research, published in numerous peer-reviewed publications, focuses on the impact of nutrition on health and advancing new concepts in companion animal nutrition. As a senior research specialist, Dr. Mangian manages complex projects using both animal and human models. She has guided more than 80 undergraduate, graduate and visiting scholars in their laboratory and research projects. Maria R. Cattai de Godoy, Ph.D., is assistant professor in the Department of Animal Sciences at the University of Illinois at Urbana-Champaign. Her research is in the area of companion animal and comparative nutrition, with special emphasis on ingredient evaluation, pet food technology, and the use of nutraceutical ingredients to improve health and manage clinical conditions of pet animals. Dr. Godoy's laboratory also uses “omics” technology as a tool to understand the interface between nutrition and health. George C. Fahey Jr., Ph.D., is professor emeritus of animal sciences and Kraft Foods endowed professor emeritus of nutritional sciences at the University of Illinois at Urbana-Champaign. He has served on the faculty since 1976 and has held research, teaching and administrative appointments. Dr. Fahey’s research is in the area of comparative nutrition, specifically carbohydrates such as dietary fibers, oligosaccharides, novel carbohydrates and resistant starches. His consulting business emphasizes pet food science and pet animal nutrition.
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