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2 Carbohydrates CARBOHYDRATES IN FEEDS yield more energy than carbohydrates digested by microbial action. The type of linkage between the monosaccharide Carbohydrates are the principal sources of energy in residues in the oligosaccharides and polysaccharides has a horse diets. Most of the carbohydrates in equine diets origi- great influence on the site of digestion of these compounds, nate from forages, grains, and grain byproducts. Carbohy- and thus their nutritional value to the horse. Hydrolysis of drates may be categorized by degree of polymerization the α1-6 and the α1-4 linkages of starch and maltose can (DP), and are often referred to as mono-, di-, oligo-, or poly- occur in the equine small intestine, but horses do not pro- saccharides. Monosaccharides of nutritional importance in- duce the enzymes necessary to digest the β1-4 linkages clude glucose, fructose, galactose, mannose, arabinose, and found in cellulose or the mixed linkages found in hemicel- xylose. Free monosaccharides occur in low concentrations lulose. Therefore, digestion of cellulose and hemicellulose in plants, but they are important constituents of the oligosac- must occur as a result of microbial fermentation. Stachyose, charides and polysaccharides found in horse feeds. There raffinose, β-glucans, and pectin are also resistant to enzy- are a few disaccharides of nutritional importance for horses. matic hydrolysis. Lactose, a disaccharide composed of glucose and galactose, A variety of systems have been developed to classify is an important nutrient source to the nursing foal. Maltose, plant carbohydrates. Some methods have classified carbohy- a disaccharide consisting of two glucose units, is produced drates according to their role in the plant, whereas other sys- in the gastrointestinal tract by the action of amylase on tems have attempted to classify carbohydrates in ways that starch, and may then be further digested to glucose. have significance to animal or human nutrition. The analyt- Oligosaccharides are compounds composed of short chains ical methods used to determine the carbohydrate composi- of monosaccharides (DP of 3-10). Oligosaccharides occur- tion of feeds are discussed in Chapter 10. There are readily ring in animal feeds include raffinose, stachyose, and fruc- available methods for measuring some, but not all carbohy- tooligosaccharides (FOS). The term “fructan” is often used drate components in animal feeds. Because some compo- to describe carbohydrates containing multiple fructose units. nents have traditionally been difficult to measure, many car- Therefore, fructooligogoasaccharides and inulin are types of bohydrate partitioning systems have used collective terms fructans. Some fructans may be considered polysaccharides such as nonstarch polysaccharides or total dietary fiber (Fig- by the strict definition that polysaccharides contain more ure 10-1). The most common system of analysis for feeds is than 10 monosaccharide units. However, compared to many the system initially developed by Van Soest in the late other polysaccharides, fructan polysaccharides have a lower 1960s. This system separates the feed into neutral detergent degree of polymerization (Tungland and Meyer, 2002). solubles and neutral detergent fiber (NDF). The NDF frac- Polysaccharides are the largest and most complex category tion contains cellulose, most hemicellulose, and lignin. For of carbohydrates in horse feeds. Starch and cellulose are the many years, the amount of nonstructural carbohydrate most common polysaccharides in horse diets. Pectin and (NSC) in a feed was determined by subtracting the amount hemicellulose are also polysaccharides. of NDF, protein, ether extract, and ash from total dry matter All carbohydrates contain similar amounts of gross en- (DM). More recently, the term “nonfibrous” (or nonfiber) ergy. However, when utilized by the horse, they provide carbohydrates (NFC) has been used to represent this differ- variable amounts of digestible energy (DE), metabolizable ence, whereas “nonstructural carbohydrate” has been used energy (ME), or net energy (NE). Carbohydrates digested to describe a chemically analyzed fraction of a feed (NRC, and absorbed as monosaccharides in the small intestine 2001). The NFC fraction is comprised of all carbohydrates 34

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CARBOHYDRATES 35 not found in the NDF component of a feed. The NSC frac- slowly fermentable carbohydrates (CHO-FS). They sug- tion includes mono- and disaccharides, oligosaccharides (in- gested the hydrolyzable fraction included hexoses, disac- cluding fructan) and starch (Hall, 2003). Few commercial charides, some oligosaccharides, and the nonresistant feed analysis laboratories completely fractionate the carbo- starches. Although some fermentation of these compounds hydrates that make up NSC, but in most feeds, the amount may occur in the stomach, the primary products of digestion of NSC can be approximated by summing the amount of of these compounds are monosaccharides, and thus the en- starch and the amount of water-soluble carbohydrates ergy yield is relatively high. The rapidly fermentable frac- (WSC). The extent to which the sum of starch and WSC ac- tion included pectin, fructan, and some oligosaccharides not counts for all NSC depends on the analytical procedures digested in the small intestine. Resistant starch and neutral used to measure these fractions. The quantitative difference detergent hemicellulose could also be included in the rap- between NSC and NFC is relatively small for some feeds idly fermented fraction. In human nutrition, starch that is not such as cereal grains, but can be quite large for many other easily digested in the small intestine is categorized as resist- feeds (Table 2-1). Marked differences between NSC and ant starch (Englyst and Englyst, 2005). Hoffman et al. NFC occur in feeds that contain significant amounts of (2001) suggested that lactate and propionate are the primary pectin. products of the rapidly fermented carbohydrates in horses. The system of partitioning carbohydrates into NDF and However, lactate production from pectin fermentation by NFC fractions was developed for use in ruminant nutrition. rumen bacteria is low and acetate production is high (Stro- This system is also useful for partitioning carbohydrates in bel and Russell, 1986). Also, Moore-Colyer et al. (2000) did horse feeds, particularly if additional analyses for other car- not report increased lactate concentrations in the cecum of bohydrate fractions, such as starch, are also available. Table ponies fed sugar beet feed as compared to ponies fed hay 2-2 shows representative values for the NDF, NFC, and cubes. The slowly fermented carbohydrate fraction includes starch content of a number of common horse feeds. These cellulose, hemicellulose, and ligno-cellulose that result pri- values were obtained from a commercial feed library (Dairy marily in the production of acetate in the large intestine. The One, 2005). It should be noted that the carbohydrate com- system proposed by Hoffman et al. (2001) may allow better position of a feed can vary greatly, especially for forages and understanding of the energy value of the carbohydrate por- byproducts; therefore, analysis of individual feed ingredi- tion of the feed as it separates the components that are ab- ents is the best method of estimating carbohydrate composi- sorbed as glucose from those that are fermented to volatile tion. Because NDF and NFC are heterogeneous mixtures of fatty acids (VFAs). Unfortunately, methods for analyzing all carbohydrates that vary in digestibility, additional partition- of these fractions are not readily available. A comparison of ing of carbohydrate fractions may be useful. In 2001, Hoff- the various analytical and nutritional methods of classifying man et al. proposed that a relevant system for partitioning carbohydrates is shown in Figure 10-1. carbohydrates in equine feeds would include at least three fractions. The three fractions are: (1) hydrolyzable carbohy- CARBOHYDRATE DIGESTION drates (CHO-H), which can be digested in the small intes- tine; (2) rapidly fermented carbohydrates (CHO-FR), which Structural carbohydrates are important sources of energy are readily available for microbial fermentation; and (3) in horse diets. The microbial production of VFAs in the cecum may be sufficient to meet up to 30 percent of a horse’s energy needs at maintenance (Glinsky et al., 1976). TABLE 2-1 Neutral Detergent Fiber (NDF), Nonfiber Additional VFAs are produced in the colon. Therefore, the Carbohydrate (NFC), and Nonstructural Carbohydrate contribution of VFAs to total daily energy utilization must (NSC) Composition of Selected Feedstuffsa on a Dry be great, particularly for horses consuming all-forage diets. Matter Basis (NRC, 2001) Acetate is the principal VFA produced, but propionate and Feedstuff %NDF %NFCb %NSCc butyrate production may also be significant (Hintz et al., 1971; Moore-Colyer et al., 2000). Acetate may be used di- Alfalfa hay 43.1 22.0 12.5 rectly for energy. Pethick et al. (1993) studied acetate uptake Beet pulp 47.3 36.2 19.5 Corn gluten meal 7.0 17.3 12.0 by the hind limb of resting horses and estimated that acetate Mixed, mostly grass hay 60.9 16.6 13.6 oxidation was responsible for about 30 percent of the energy Soybean meal (48% CP) 9.6 34.4 17.2 utilized by the limb. In addition, Pratt et al. (1999) reported Soy hulls 66.6 14.1 5.3 that the clearance rate of infused acetate was increased dur- aThe values shown here may vary from those shown elsewhere in this doc- ing exercise. Acetate that is not used immediately is proba- ument. The values are provided to illustrate differences among feeds and bly used for the synthesis of long-chain fatty acids, which carbohydrate categories. Actual values for individual feeds may vary by may be stored or, in the lactating mare, secreted into the stage of maturity, variety, source, etc. b%NFC = 100 – (%CP + %NDF + %EE + %Ash). milk. Doreau et al. (1992) reported that mares consuming c%NSC determined by measurement using the methods described by Smith high-forage rations produced milk with a higher concen- (1981). tration of fat than mares fed a high-concentrate ration. The

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36 NUTRIENT REQUIREMENTS OF HORSES TABLE 2-2 Carbohydrate Composition (dry matter basis) of Selected Feed Ingredientsa Feed %NDF %ADF %NFCb %Starch Grass pasture Mean (SD) 58.8 (11.9) 35.6 (7.8) 19.6 (6.7) 3.5 (2.9) Range 46.9–70.6 27.6–43.5 12.9–26.2 0.6–6.4 Observations 4,229 4,135 3,469 1,186 Mixed, mostly grass pasture Mean (SD) 51.9 (10.0) 30.8 (6.1) 20.6 (6.8) 3.1 (2.3) Range 41.9–61.9 24.7–36.9 13.8–27.3 0.8–5.4 Observations 3,649 3,455 2,395 1,536 Fresh Bermuda grass Mean (SD) 66.7 (6.1) 38.4 (4.2) 14.0 (4.5) 2.6 (2.3) Range 60.6–72.8 34.3–42.6 9.5–18.5 0.3–4.9 Observations 304 304 285 46 Alfalfa cubes Mean (SD) 43.3 (7.0) 33.6 (5.0) 26.6 (4.5) 2.0 (1.2) Range 36.4–50.3 28.6–38.6 22.1–31.1 0.8–3.2 Observations 319 319 224 144 Legume hay Mean (SD) 38.5 (5.4) 30.0 (4.1) 30.8 (3.7) 2.4 (1.2) Range 33.1–43.9 26.0–34.1 27.1–34.5 1.3–3.6 Observations 64,704 64,673 46,133 23,738 Grass hay Mean (SD) 63.8 (6.4) 39.2 (4.6) 19.5 (6.7) 2.8 (1.5) Range 57.4–70.2 34.6–43.7 14.8–24.2 1.3–4.2 Observations 19,935 19,716 15,951 9,682 Mixed, mostly grass hay Mean (SD) 60.8 (7.2) 38.8 (4.6) 20.9 (4.6) 2.7 (1.4) Range 53.7–68.0 34.2–43.5 16.3–25.5 1.4–4.1 Observations 15,666 15,661 11,319 6,721 Bermuda grass hay Mean (SD) 67.7 (5.3) 35.6 (4.1) 16.5 (4.5) 6.1 (3.0) Range 62.4–73.0 31.6–39.7 12.0–21.0 3.1–9.0 Observations 4,874 4,874 3,529 1,989 Barley Mean (SD) 19.6 (6.8) 7.7 (3.9) 63.9 (8.3) 53.9 (9.4) Range 12.8–26.4 3.7–11.6 55.6–72.3 44.5–63.2 Observations 523 511 346 218 Steam flaked corn Mean (SD) 9.1 (1.9) 3.5 (1.0) 78.4 (3.5) 72.3 (4.8) Range 7.2–11.0 2.6–4.5 74.9–81.9 67.5–77.1 Observations 139 136 135 98 Oats Mean (SD) 27.9 (9.8) 13.5 (5.4) 50.9 (9.1) 44.3 (10.3) Range 18.0–37.7 8.1–18.9 41.9–60.0 34.0–54.5 Observations 263 260 201 125 Wheat midds Mean (SD) 37.1 (6.5) 12.9 (2.8) 37.9 (7.7) 26.0 (9.1) Range 30.7–43.6 10.1–15.7 30.1–45.6 16.9–35.1 Observations 420 414 303 116 Molasses Mean (SD) 0.7 (0.7) 0.3 (0.4) 76.7 (13.9) 1.1 (1.6) Range 0–1.4 0–0.7 62.8–90.6 0–2.8 Observations 114 114 65 41 Beet pulp Mean (SD) 41.9 (5.8) 25.6 (4.0) 44.4 (6.3) 1.3 (1.0) Range 36.1–47.7 21.6–29.5 38.1–50.6 0.3–2.4 Observations 359 380 284 141 Corn gluten feed Mean (SD) 36.0 (7.3) 11.1 (3.2) 33.4 (6.7) 16.8 (8.0) Range 28.6–43.3 7.9–14.3 26.7–40.1 8.7–24.8 Observations 330 342 260 50

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CARBOHYDRATES 37 TABLE 2-2 continued Feed %NDF %ADF %NFCb %Starch Distillers dried grains Mean (SD) 33.9 (4.2) 17.1 (3.9) 24.9 (5.0) 5.7 (3.0) Range 29.7–38.1 13.3–21.0 19.9–29.9 2.7–8.7 Observations 1,427 1,453 1,134 795 Oat hulls Mean (SD) 69.9 (14.4) 39.7 (8.9) c c Range 55.6–84.3 30.7–48.7 Observations 19 18 Rice bran Mean (SD) 30.2 (11.8) 18.1 (13.0) 14.3 (10.5) 18.4 (9.8) Range 18.6–42.0 5.1–31.1 0–31.1 8.6–28.1 Observations 110 110 86 51 Extruded soybeans Mean (SD) 17.8 (4.5) 11.4 (3.1) 20.3 (4.2) 2.4 (1.5) Range 13.4–22.3 8.3–14.5 16.1–24.6 1.0–3.9 Observations 60 59 39 15 Soybean meal Mean (SD) 13.1 (5.4) 8.4 (3.0) 28.3 (4.6) 2.0 (1.1) Range 7.7–18.5 5.5–11.4 23.7–32.9 0.8–3.1 Observations 1,273 1,253 1,044 188 Soy hulls Mean (SD) 61.7 (3.3) 44.0 (8.3) 19.7 (4.0) 1.7 (1.2) Range 53.4–70.1 37.5–50.6 15.7–23.8 0.5–2.8 Observations 297 357 218 75 aData taken from Dairy One website, September 2005. The values listed here may vary from those in feed composition ta- bles elsewhere in this document. The values shown here were drawn from a commercial website and reflect the diversity of feeds submitted for analysis. bNFC as determined by difference: %NFC = 100 – (%CP + %NDF + %EE + %Ash). cFewer than 10 observations available. propionate produced by microbial fermentation may be used (Medina et al. 2002). Moore and Dehority (1993) reported for hepatic glucose synthesis. This gluconeogenic mecha- minimal differences in cecal cellulolytic bacterial concen- nism is probably very important in maintaining glucose trations in ponies receiving diets consisting of 90 percent homeostasis in forage-fed horses (Argenzio and Hintz, hay and 10 percent concentrate compared to ponies receiv- 1970; Simmons and Ford, 1991). The role of butyrate in ing 60 percent hay and 40 percent concentrate. However, the equine gastrointestinal health has not been well studied, but higher concentrate diet was associated with increased total it is generally accepted as important in other species. Al- bacterial and protozoal concentrations in the colon. The though cellulose is the most abundant carbohydrate in most composition of the carbohydrates in the diet may also affect forages, other carbohydrates contribute to the energy pro- VFA production in the large intestine. The molar percentage duced from the microbial fermentation in the gastrointesti- of acetate in the cecum decreased, and the molar percentage nal tract, including hemicellulose, pectin, fructan, and the β- of propionate increased, when ponies were fed diets with in- glucans. As noted above, not all carbohydrates are creasing amounts of corn (Hintz et al., 1971). In that study, fermented at the same rate or produce the same proportions diet did not alter the proportion of acetate and propionate in of VFAs. Differences in the amounts of various carbohy- the colon, but the total VFA concentrations were reduced in drates found in forages are discussed in depth in Chapter 8. response to the diet containing 63 percent ground corn. Diet composition, particularly carbohydrate composition, Medina et al. (2002) found lower cecal acetate concentra- may affect the microbial population of the large intestine as tions and increased cecal propionate concentrations in well as the proportions of VFAs that are produced. Feeding horses fed a high-starch diet. The effect of diet on acetate a high-starch diet (30% starch; 3.4 g starch/kg body weight and propionate in the colon was less pronounced; however, [BW]/meal) has been reported to decrease the concentration the high-starch diet increased lactic acid concentrations and of celluloytic bacteria but increase the concentration of total decreased pH in the cecum and colon (Medina et al., 2002). anaerobic bacteria, lactic-acid utilizing bacteria, lactobacilli, In ruminants, adequate consumption of structural carbo- and streptococci in the cecum (Medina et al., 2002). The hydrates is believed to be important for gastrointestinal concentrations of lactobaccilli and streptococci in the colon health (NRC, 2001). Minimum NDF recommendations for were also increased in response to the high-starch diet the total diet of dairy cattle vary with source of NDF in the

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38 NUTRIENT REQUIREMENTS OF HORSES diet and other factors such as particle size. Chewing activity, a useful tool. Enzymatic digestion of starch occurs in the and thus saliva production, has been suggested as an impor- small intestine, but starch is also susceptible to microbial fer- tant consideration for gastrointestinal health and productiv- mentation. The equine stomach contains microbes capable of ity in dairy cows. Ruminant nutritionists have attempted to fermenting carbohydrates (Kern et al., 1974). Healy et al. assess the amount and form of fiber in a diet that allows for (1995) reported increased lactate concentrations in gastric adequate chewing and saliva production. Concepts such as fluid obtained from ponies fed a large concentrate meal, and “effective fiber,” “physically effective fiber,” and “effective other researchers have reported significant dry matter and NDF” have been developed to assess the ability of various starch disappearance to occur in the stomach (de Fombelle et diets to maintain production and gastrointestinal health in al., 2003; Varloud et al., 2003). It is also possible that fructan dairy cattle (Mertens, 1997). Similar concepts have not been can be hydrolyzed to some extent by gastric acid (Van Soest, developed in horses, but research on the effect of fiber 1994) or fermented in the stomach. amount and type on digestion and gastrointestinal health is Most glucose absorption appears to occur in the proximal needed. The amount of fiber in the diet may also affect the small intestine (Dyer et al., 2002), so factors that decrease weight of the gastrointestinal tract. High-fiber diets tend to retention time in the proximal small intestine may decrease hold more water, thus increasing total weight of the gas- small intestinal glucose absorption. The type and amount of trointestinal tract (Coenen and Meyer, 1987). However, the forage in the diet may alter small intestinal starch digestibil- mass of the gastrointestinal tissue may also be increased. ity (Meyer et al., 1993), possibly by affecting digesta flow in McLeod and Baldwin (2000) reported that lambs fed a high the small intestine. The abundance of amylase in the diges- forage diet at 2-times maintenance had heavier gut tissues tive secretions, as well as the opportunity for amylase to as- than lambs fed an isocaloric high-concentrate diet. The gas- sociate with dietary starch, may also influence the variabil- trointestinal tract has a high metabolic rate; thus, an increase ity in starch disappearance from the small intestine. in gut size could account for elevated maintenance require- Relatively little is known about the factors that affect the ments in forage-fed animals. quantity of digestive enzymes secreted into the equine gas- The digestion and metabolism of structural carbohy- trointestinal tract. The ability of horses to produce the en- drates in the large intestine may be able to meet the energy zymes associated with lactose digestion decrease with age needs of many horses at maintenance, but this source of en- (Roberts et al., 1973). In other species of animals, diet com- ergy alone is unable to meet the needs of horses with higher position has been shown to have variable effects on the se- requirements. Traditionally, dietary energy density has been cretion and activity of enzymes associated with carbohy- increased by adding grains (oats, barley, corn, or others) or drate digestion (Zebrowska and Low, 1987; Flores et al., grain byproducts to the horse’s diet. Most of the energy in 1988; Swanson et al., 2000). The amount of amylase mea- grains is found as starch. sured in the pancreatic tissue of horses fed either hay or hay Total tract starch disappearance in the horse is very high and concentrate for at least 8 weeks was not affected by diet (> 90 percent), but the extent of prececal starch digestion is (Kienzle et al., 1994). However, in another experiment, the variable. Starch is often viewed as a homogenous entity, but amylase activity (on a wet weight basis) of jejunal chyme there is structural variation in the starch found in different was higher when horses received a diet with added corn, foods. There are two main components of starch: amylose oats, or barley than when they received only hay (Kienzle, et and amylopectin. Amylose is a straight chain structure of al., 1994). These authors noted that amylase activity in jeju- glucose units. Amylopectin is a branched chain of glucose nal chyme was subject to variation among individuals. Fur- units. The proportions of amylose and amylopectin in starch ther research is needed to define the factors that influence vary by cereal grain and with other factors, including the amount and activity of carbohydrate-digesting enzymes maturity and variety (Van Soest, 1994). Interrupting the in the horse. crystalline structure of starch may increase digestibility. A Grain processing may influence the extent of prececal common method of altering the crystalline structure is gela- starch disappearance by decreasing particle size and in- tinization, which occurs with moist heating. However, under creasing surface area. Gelatinization of starch may also some conditions, amylose that has been heated and allowed occur with some processing methods. The ability of various to recrystalize can be less susceptible to enzymatic digestion processing methods to enhance small intestinal starch disap- than the original starch. The starch that reaches the large in- pearance is discussed in Chapter 8. There are differences in testine is categorized as resistant starch in human nutrition prececal starch disappearance among different types of (Englyst et al., 1996; Englyst and Englyst, 2005). Resistant grains (Radicke et al., 1991; Kienzle et al., 1992; Meyer et starch includes starch that is not accessible to digestive en- al., 1993; de Fombelle et al., 2004), with oat starch gener- zymes due to its structure or encapsulation in plant cell struc- ally being more digestible in the small intestine than corn tures and starch that has been modified by certain types of starch or barley starch. The amount of starch consumed at processing (Cummings et al., 1996; Tharanathan, 2002). A one time may also affect the percentage of starch that disap- system for characterizing the availability of the starch found pears before reaching the large intestine. When a small in various horse feeds has not been developed, but could be amount of oats was fed to horses, approximately 80 percent

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CARBOHYDRATES 39 of the starch was digested and absorbed prior to reaching the limitations (Monro, 2003). A GI that compares an equivalent terminal ileum, but when a larger amount of oats was fed, amount of the test food to the standard has been suggested starch disappearance before reaching the ileum was only 58 (Monro, 2003). This system would place food substitutions percent (Potter et al., 1992). on an equal-weight basis rather than an equal-carbohydrate Starch that is not digested in the small intestine enters the basis. Monro (2003) also suggested that a GI determined large intestine, where it will be fermented. When starch is with a specific amount of carbohydrate (50 g) may not rep- fermented, its net energy value is lower than when it is ab- resent responses to other levels of intake. sorbed as glucose. Also, starch flow to the large intestine Recently, several studies have attempted to apply the GI may alter the microbial environment. In certain cases, this concept to horse feeds. The methods that have been used in may be beneficial to the horse. Kienzle et al. (2002) sug- horses have been extremely variable, making it difficult to gested that addition of concentrate to a straw diet improved interpret the results across studies. The glucose response to the utilization of the straw, possibly by providing additional the carbohydrate in test feeds has been compared to the re- nutrients that enhanced the activity of the microbial popula- sponse to an equivalent amount of glucose administered by tion. However, negative effects of starch bypass to the large nasogastric tube (Jose-Cunilleras et al., 2004) or to oats or intestine may also occur. Feeding a high-starch diet (30% corn (Pagan et al., 1999; Vervuert et al., 2003, 2004, 2005; starch; 3.4 g starch/kg BW/meal) has been reported to de- Rodiek and Stull, 2005). Using a readily consumed grain as crease the concentration of celluloytic bacteria in the cecum the standard removes any effect associated with the admin- (Medina et al., 2002). In addition, several studies have re- istration of glucose by nasogastric tube, but the chemical ported decreased colon and/or cecal pH in horses or ponies composition of a grain may vary from one study to another. fed high concentrate/starch diets (Willard et al., 1977; Another source of variation among studies with horses has Radicke et al., 1991; Medina et al., 2002). To avoid a de- been the amount of feed or carbohydrate offered. Among crease in large intestinal pH, high-starch feeds should not be studies that have determined GI for horse feeds, there has offered in amounts that result in significant starch overflow been little consistency in the amount of carbohydrate offered to the large intestine. One researcher has suggested that the to horses. In one study, the amount of hydrolyzable carbo- capacity of the small intestine for starch digestion is reached hydrate was approximately 900 g/484-kg horse (Jose-Cu- at a starch intake of 3.5 to 4 g/kg BW (Potter et al., 1992). nilleras et al., 2004). In other studies, smaller amounts of However, Radicke et al. (1991) found that cecal pH was sup- hydrolyzable carbohydrates have been fed (Vervuert et al., pressed with starch intakes between 2 and 3 g/kg BW. 2003, 2004). When the glycemic responses to several feeds were compared to oats, the relative GI values were affected by the amount of the test feeds and the oats that were offered CARBOHYDRATE METABOLISM AND STORAGE (0.75, 1.5, or 2.5 kg) (Pagan et al., 1999). Feeds have not al- Blood glucose concentrations rise when a meal high in ways been offered in equicarbohydrate amounts or equal starch is consumed. Insulin levels also increase, which en- food amounts. One study compared feeds that provided sim- hances glucose uptake by many tissues, including muscle ilar amounts of calories that resulted in large differences in and adipose. The increase in blood glucose is generally re- both total hydrolyzable carbohydrate intake and total feed lated to the amount and availability of starch consumed (or intake among feeds (Rodiek and Stull, 2005). An advantage glucose when a source of free glucose is fed). In humans, the of this method would be that feed substitutions in horse diets term “glycemic index” (GI) has been used to characterize are often made on a caloric basis. However, a disadvantage the magnitude of the blood glucose increase to various relates to the time required to consume the feed provided foods. The primary purpose of the GI in human nutrition and the effects this may have on the glucose response curve. was to provide a means of comparing carbohydrate sources With appropriate standardization, there may be some appli- in order to manage hyperglycemia. The GI has been defined cation of the GI to horse nutrition; however, many factors as the incremental area under the blood glucose response will have to be considered, including the age, breed, and curve of a 50-g carbohydrate portion of a test food, ex- physiological state of the horse, as well as the physical form pressed as a percentage of the blood glucose response curve of the feed. In addition, a GI system must account not only to the same amount of carbohydrate from a standard food for differences among feeds when they are fed separately, consumed by the same person (FAO/WHO, 1998). The 50-g but also when they are mixed with other ingredients as is carbohydrate portion should contain 50 g of glycemic or common in the horse industry. For example, Pagan et al. “available” carbohydrate, and the standard food may be ei- (1999) reported that the glycemic response of horses con- ther white bread or glucose (FAO/WHO, 1998). The suming sweet feed was reduced when vegetable oil was FAO/WHO guideline suggested testing the standard food at added to the feed. least three times in each subject in order to obtain a repre- Absorbed glucose may be used for immediate energy or sentative mean response. The guideline also discussed the it may be stored for later use. The storage carbohydrate in method of determining available carbohydrate in the test animals is glycogen, and the main storage sites are muscle food. Application of the GI in human nutrition has some and liver. Glucose that is not utilized for immediate energy

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40 NUTRIENT REQUIREMENTS OF HORSES or for glycogen synthesis may also be used for fat synthesis. also reported decreased performance when exercise was ini- Glycogen stored in the muscle is available only to the mus- tiated with reduced muscle glycogen stores. cle as a fuel source. However, glycogen stored in the liver A number of strategies have been explored to either en- may be broken down to augment glucose availability to hance initial muscle glycogen stores or enhance repletion of other tissues. Maintaining adequate blood glucose concen- glycogen stores after exercise. It has been demonstrated that tration is important for the central nervous system. The de- muscle glycogen levels in human athletes can be elevated veloping fetus is also an important consumer of glucose above normal if an individual exercises and consumes a low- (Fowden et al., 2000a,b). Carbohydrates are an important carbohydrate diet for a few days and then consumes a high- source of energy to the exercising horse. The contribution of carbohydrate diet and limits exercise just prior to an event. carbohydrate to energy production increases with increasing This practice of “carbohydrate loading” appears to be most work intensity, partly due to increasing recruitment of fast- beneficial prior to the performance of long-term, moderate twitch muscle fibers. Muscle fibers in the horse are usually exercise. Carbohydrate loading has gained limited accep- grouped into three categories: Type I, Type IIA, and Type tance for horses, possibly because many performance horses IIX. The Type I fibers are highly oxidative and are well already receive diets high in hydrolyzable carbohydrates. equipped to use fat as a substrate. Type I fibers are recruited Also, traditional horse feeding recommendations have for activities that do not require great force generation. Type warned against sudden changes in diet in order to minimize II fibers, particularly the Type IIX fibers, are more suited to digestive disturbances (Lewis, 1995), and recent research rapid contraction and high force generation and thus must be has linked the consumption of diets high in hydrolyzable used during speed or strength work. Type IIX fibers are carbohydrates to increased incidence of some muscular dis- highly glycolytic and, thus, prefer carbohydrate as an energy orders such as equine polysaccharide storage myopathy (see source over fat. Chapter 12). Finally, there is no clear evidence that carbo- The carbohydrate utilized during exercise may arise from hydrate loading has significant benefits to equine athletes. muscle glycogen stores or from blood glucose derived from Topliff et al. (1983, 1985) did not find a positive effect on the diet or from hepatic glycogenolysis or hepatic gluconeo- performance when muscle glycogen levels were elevated genesis. Muscle and liver glycogen stores can decrease dra- above normal after using a carbohydrate-loading feeding matically when strenuous exercise continues for an extended program in horses. period. When horses exercised for 1 hour at 500 m/min or The ability to rapidly replete muscle glycogen stores fol- for 4 hours at 300 m/min, muscle glycogen stores were de- lowing exercise may be important to subsequent perfor- pleted by approximately 60 percent (Lindholm et al., 1974). mance for some activities. An initial study on glycogen re- In that study, liver glycogen was depleted by approximately pletion in exercised horses suggested that the rate of 40 percent in the 1-hour exercise bout and 90 percent in the repletion was fastest in the first few hours after exercise 4-hour exercise bout. The extent of muscle glycogen deple- (Snow et al., 1987). Repletion appeared to be faster in horses tion is affected by exercise intensity and duration. Muscle fed hay and grain after exercise than in horses fed only hay. glycogen depletion increased from approximately 13 per- At 28 hours post-exercise, muscle glycogen concentrations cent during a 506-m sprint, to approximately 35 percent dur- were 90 percent of pre-exercise values when horses were fed ing a 1,025-m sprint (Nimmo and Snow, 1983). When exer- hay and grain, but only 71.7 percent of pre-exercise values cise distance was increased to 3,620 m and average speed when they received hay only. Interestingly, at 68 hours post- was decreased, glycogen depletion decreased to about 20 exercise, muscle glycogen concentrations were 84 and 76 percent. Although the amount of muscle glycogen utilized percent of initial concentrations in horses fed the hay-and- during short-term, high-intensity exercise is lower than the concentrate diet or the hay diet, respectively. Recent studies amount used during long-term, moderate exercise, the rate have demonstrated that relatively extreme procedures are of utilization is much higher. necessary to significantly enhance the rate of muscle glyco- Maintaining adequate carbohydrate availability is impor- gen repletion in horses after exercise. Continuous intra- tant for performance. When human athletes initiate exercise venous administration of dextrose/glucose at a rate of 6 g/kg with reduced muscle glycogen stores, fatigue resistance dur- BW (3 kg/500-kg horse) during the first several hours after ing long-term exercise is decreased. The effect of dimin- glycogen-depleting exercise enhances the rate of glycogen ished glycogen stores on performance of short-term, high- resynthesis in horses (Davie et al., 1995; Lacombe et al., intensity exercise is less well understood. There was no 2001). When glucose was administered at a lower rate (2 difference in the physical performance of horses that started g/kg BW) by nasogastric tube, glycogen resynthesis was not high-intensity exercise with normal muscle glycogen stores increased above the control (Nout et al., 2003). In a subse- or with slightly depleted (~ 22 percent below normal) mus- quent study, horses were given a low soluble carbohydrate cle glycogen stores (Davie et al., 1996). However, when diet (hay), a mixed soluble carbohydrate diet (hay and muscle glycogen stores were severely depleted (~ 55 per- grain), or a high soluble carbohydrate diet (grain only) for cent) prior to high-intensity exercise, run time to fatigue was 72 hours after glycogen-depleting exercise (Lacombe et al., decreased (Lacombe et al., 1999). Topliff et al. (1983, 1985) 2004). There was no difference in rate of glycogen resyn-

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CARBOHYDRATES 41 thesis when horses were fed the hay diet or the hay/grain concentrations. Providing a grain meal 2 to 3 hours prior to diet. Glycogen resynthesis was greater in horses fed only exercise (compared to hay or no food) resulted in increased grain, compared to the other treatments. However, the au- carbohydrate and decreased lipid oxidation during exercise thors noted that water consumption and the restoration of (Jose-Cunilleras et al., 2002). Insulin is a potent inhibitor of body weight were also less in horses fed only the concen- lipolysis and lipid oxidation may have been reduced due to trate, and they cautioned that the feeding of large amounts of decreased free fatty acid availability. Providing a grain meal soluble carbohydrates in the post-exercise period could pre- a few hours prior to exercise did not reduce muscle glyco- dispose horses to gastrointestinal disorders A few studies gen utilization (Lawrence et al., 1993, 1995; Jose-Cunilleras have examined the effects of exercise, training, and post- et al., 2002), although liver glycogen may have been spared exercise carbohydrate administration on glucose trans- in one study (Lawrence et al., 1993). The response to a pre- porters (GLUT) in equine skeletal muscle in order to de- exercise meal of grain may be affected by the timing of the velop a better understanding of post-exercise glycogen meal and whether forage is also fed (Pagan and Harris, repletion. After a single bout of moderate exercise, skeletal 1999). In addition, the effects of a pre-exercise meal may be muscle content of glucose transporter-4 (GLUT-4) protein attenuated during long-term exercise or when horses per- was not increased (Nout et al., 2003). However, GLUT-4 form repeated bouts of exercise (Lawrence et al., 1995). protein content and/or GLUT-4 mRNA were increased in In humans, fatigue can be delayed if carbohydrate is con- equine skeletal muscle after a 3-day regimen of strenuous, sumed during long-term, moderate exercise, possibly by off- glycogen-depleting exercise (Lacombe et al., 2003; Jose- setting the decline in carbohydrate available from endoge- Cunilleras et al., 2005). The GLUT-4 protein content was nous sources. Intravenous infusion of glucose during 90 also increased after 6 weeks of training (McCutcheon et al., minutes of exercise in horses increased total carbohydrate 2002). These studies suggest that some types of exercise can oxidation, reduced endogenous glucose production, and did modify the amount of glucose transporters present in skele- not alter glycogen utilization in horses (Geor et al., 2000). tal muscle in horses. It has also been hypothesized that post- Intravenous administration of glucose has been shown to in- exercise administration of carbohydrate would increase crease time to fatigue in horses (Farris et al., 1995, 1999). GLUT-4 protein or GLUT-4 mRNA in equine skeletal mus- However, in these studies, signs of reduced carbohydrate cle after glycogen-depleting exercise (Lacombe et al., 2003; availability, such as hypoglycemia, did not occur in control Jose-Cunilleras et al., 2005). However, intravenous adminis- horses. Nonetheless, the increased performance of horses tration of glucose after exercise did not elevate GLUT-4 pro- given additional carbohydrate is in agreement with findings tein content, even though glycogen repletion was increased. in human athletes. The authors of these studies noted that Similarly, feeding corn in the post-exercise period (as com- the intermittent feeding of horses during long-term exercise pared to no food or only hay) did not affect GLUT-4 protein might not duplicate the effects of intravenous glucose infu- content or GLUT-4 mRNA in skeletal muscle. These initial sion. Bullimore et al. (2000) administered glucose, fructose, studies did not provide evidence that the GLUT-4 trans- or a glucose-fructose combination to horses in the middle of porter in equine skeletal muscle was sensitive to carbohy- an exercise bout, but did not compare their results to a con- drate administration after exercise. It was suggested that trol condition. Because carbohydrate stores are limited, but methods that examine plasma membrane isolates might be fat stores are not, practices that enhance fat utilization and more revealing than methods that measured total GLUT-4 spare glycogen use are considered desirable. Factors affect- (Lacombe et al., 2003). It was also suggested that variables ing lipid metabolism in exercising horses are discussed in such as muscle glycogen content and insulin response may Chapter 3. be involved in regulating post-exercise carbohydrate metab- olism (Jose-Cunilleras et al., 2005; Pratt et al., 2005). Clearly, further research is needed to elucidate the mecha- REFERENCES nisms that control glycogen in repletion in horses. Argenzio, R., and H. F. Hintz. 1970. Glucose tolerance and effect of volatile The effect of providing additional carbohydrate to exer- fatty acid on plasma glucose concentration in ponies. J. Anim. 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