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