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3 Fats and Fatty Acids .INTRODUCTION the methyl end of the molecule (the omega [Ï] end), in the form of A:BÏ-C (or A:Bn-C), where A is the number of car- Fats or oils are generally used in equine diets to increase bon atoms in the chain, B is the number of double bonds, energy density and/or substitute for hydrolyzable and rap- and C is the position of the first double bond from the idly fermentable carbohydrates in the form of cereal grains. methyl terminus. For example, linoleic acid is referred to as However, fat supplementation has other potential benefits, 18:2n-6 because it contains 18 carbon atoms and 2 double including improved energetic efficiency (Kronfeld, 1996), bonds, the first between carbons 6 and 7 counted from the enhanced body condition, diminished excitability (Holland methyl terminus. Fatty acids with the first double bond in et al., 1996a), and metabolic adaptations that increase fat ox- this position are commonly referred to as omega-6 fatty idation during exercise (Dunnett et al., 2002). Dietary fats acids, whereas fatty acids with the first double bond be- also serve as carriers for fat-soluble vitamins and supply the tween carbons 3 and 4 (e.g., Î±-linolenic, 18:3Ï-3) are in the essential fatty acids (EFAs) linoleic acid and Î±-linolenic omega-3 series. acid that are not synthesized by the body, although an EFA Triglycerides exist as fats or oils at room temperature ac- requirement for horses has not been determined. All fatty cording to their physical properties of the component fatty acids serve structural functions, and several of the polyun- acids. Triglycerides with a high proportion of relatively saturated fatty acids (PUFAs) are precursors of the short-chain fatty acids or unsaturated fatty acids tend to be prostaglandins and other eicosanoids, which are important liquid (oils) at room temperature. Saturated fatty acids have for a host of cellular functions. a higher melting point and exist as solids (fats) at room tem- From a biochemical viewpoint, fats belong to a broad perature. group of compounds known as lipids that can be glycerol or nonglycerol based. Glycerol-based lipids include the glycol- ipids, phospholipids, and triglycerides (also termed triacyl- SOURCES OF DIETARY FATS AND FATTY ACIDS glycerols). Triglycerides consist of three fatty acid mole- Both animal fats and vegetable fats or oils have been fed cules linked to a glycerol backbone. Cholesterol and its fatty to horses, although use of vegetable sources is more preva- acid esters are nonglycerol-based lipids. Included in this cat- lent in part due to superior palatability. In one study that ex- egory of lipids are waxes, terpenes, cerebrosides, and vari- amined the palatability of several animal and vegetable fats ous sterols. and oils, corn oil was the most palatable (Holland et al., Fatty acids may be designated on the basis of the number 1998). In this study, diets containing up to 15 percent added of carbon atoms they contain and the number of double fat as corn oil were readily accepted by horses. Several other bonds. Long-chain fatty acids contain 16 to 20 carbon vegetable sources, including seed and fruit oils (e.g., soy, atoms, medium-chain fatty acids contain 6 to 10 carbons, canola, linseed [or flax], sunflower, safflower, coconut, and and short-chain or volatile fatty acids (VFAs), produced in peanut) and other feed byproducts with relatively high fat the intestinal tract by bacterial fermentation, contain only 2 content (e.g., lecithin, stabilized rice bran, wheat germ, and to 5 carbons. Saturated fatty acids (SFAs) contain no double copra), appear to have acceptable palatability and are widely bonds, monounsaturated (MUFAs) a single double bond, used in horse diets. Fish oil (menhaden) and medium-chain and PUFAs two or more double bonds. Fatty acids are also triglycerides (MCTs) also have been fed to horses. Use of described by the position of the first double bond relative to these different fat and oil sources can be dictated by cost and 44
FATS AND FATTY ACIDS 45 availability. Table 8-5 provides the fatty acid composition of feld et al., 2004), an effect of chain length or saturation of some of the fats and oils added to horse diets. FAs on digestibility was not detected when comparing veg- etable oils (corn, soy, soy lecithin, peanut), tallow, and vegetable-tallow blends. DIGESTIBILITY AND ENERGY VALUE OF FATS In ruminants, the addition of oils to rations decreases ru- minal fermentation of fiber (Coppock and Wilks, 1991), an In general, there is an increase in the digestibility of fat effect not observed when encapsulated or protected fat is when fats or oils are added to forages or grains (Potter et fed. There is conflicting evidence regarding the effects of al., 1992). Mean estimates of apparent digestibilities of fats added fat on the utilization of other nutrients in equine ra- in horses and ponies are 42â49 percent for forages (Fon- tions. The addition of corn oil at up to 15 percent of the total nesbeck et al., 1967; Bowman et al., 1979; Sturgeon et al., diet (DM basis) had no effect on fiber digestibility in three 2000), 55â76 percent for grains (Hintz and Schryver, studies (Bowman et al., 1979; Kane et al., 1979; Bush et al., 1989), and 88â94 percent for added fats and oils (Kane et 2001). The digestibility of calcium, phosphorus, and magne- al., 1979; McCann et al., 1987). Kronfeld et al. (2004) com- sium also were largely unchanged in horses fed fat- piled published data from digestibility studies in which five supplemented diets (Bowman et al., 1979; Rich, 1980; Mc- basal (hay and grain mixes without added fats; 23â37 g Cann et al., 1987). The preileal digestibility of protein was fat/kg dry matter [DM]) and 18 test feeds with added fats slightly lower in horses fed coconut or soybean oils at 0.5 or (76â233 g fat/kg DM) were evaluated (Rich, 1980; 1 g/kg BW/d (Meyer et al., 1997). Kronfeld et al. (2004) re- Custalow, 1992; Holland, 1994; Bowman et al., 1979). This ported that the addition of corn oil (up to 233 g/kg DM), tal- analysis demonstrated mean apparent digestibilities (Da) of low (up to 190 g/kg DM), or 50:50 soy lecithin and soy oil 55, 81, and 95 percent for fat in, respectively, forages, (100 g/kg DM) had no effect on DM, crude protein (CP), or mixed feeds with added fat, and added fats. Encapsulation acid detergent fiber (ADF) digestibility. Zeyner (2002) re- of fat within cereal grains or oilseeds may decrease the ported increased fiber digestibility of a mixed hay and con- availability of these fats for digestion in the small intestine. centrate ration with isoenergetic exchange of micronized The low true digestibility (Dt) of forage fat may be ex- corn (3 g starch/kg BW) by soy oil (up to 14 percent of DM). plained by the poor digestibility of waxes, pigments, and However, a further increase in the fat content of the ration other non-triglyceride lipid components. Kronfeld et al. (22 and 32 percent of DM) resulted in a statistically signifi- (2004) also reported that the true digestibility of added fats cant depression of gut microbial fermentative activity and approached 100 percent with an endogenous fecal fat of decreased fiber digestibility. Jansen and co-workers reported 0.22 g/kg body weight (BW)/d. Maximal fat digestibility a negative associative effect of soy oil on crude fiber (CF), occurred between 100 and 150 g/kg DM and was sustained neutral detergent fiber (NDF), ADF, and nitrogen-free ex- to 230 g/kg DM, suggesting high capacity for fat digestion tract (NFE) digestibility when horses were fed hay and in horses adapted to fat-supplemented rations. However, the isocaloric concentrates with different amounts of cornstarch, results of other studies have suggested a lower upper limit glucose, and soy oil (Jansen et al., 2000, 2001, 2002). In one in fat digestibility in horses. In an analysis of data from 225 of these studies (Jansen et al., 2001), CF digestibility was 71 digestion trials evaluating rations with varying amounts of percent and 55 percent in the control and fat-supplemented partially hydrogenated soy oil, Zeyner (2002) reported a feeds (soybean oil 158 g/kg DM), respectively. Regression strong positive relationship between the Da of fat and the analysis of ether extract (EE) intake (as soybean oil) on CF amount of fat in the diet, but only up to approximately 7 digestibility showed that an increase of 10 g/kg DM of soy- percent DM. Assuming a gross energy value of 9.5 Mcal/kg bean oil resulted in a 0.9 percent decrease in apparent total for fat, caloric values for fat in forages, mixed feeds, and tract digestibility (Jansen et al., 2001). The reason(s) for this added vegetable fats have been estimated at 5.2, 7.7, and 9 apparent adverse effect of soy oil on fiber digestibility have Mcal/kg, respectively (Kronfeld et al., 2004). More precise not been elucidated. However, one possible explanation digestible energy (DE) values can be calculated from meas- could be inadequate accommodation to the oil-supplemented ured fat content (g/kg) using the equation developed by ration. Zeyner (2002) reported decreased fiber digestibility Kronfeld et al. (2004): Energy value (Mcal/kg) = 0.095(92 with rapid substitution of cereal starch by soy oil (0.33 g soy â 92e(-ether extract/342)). oil/kg BW/d). This adverse effect was not evident after a 3- In other species, there is evidence that degree of satura- week accommodation period. Another explanation may be a tion, melting point, and possibly fatty acid (FA) chain length specific negative effect of soybean oil at high levels of in- affect the digestibility of added fats. Saturated fats are less clusion. Zeyner (2002) demonstrated no adverse effects in digestible than unsaturated fats, while the digestibility is in- horses accommodated to soy oil-supplemented rations con- versely related to the melting point of the FA. These factors taining up to 0.7 g oil/kg BW/d. However, decreased fiber di- are of minor importance in horse nutrition given the wide- gestibility was observed in horses fed 1 g soy oil/kg/d. spread use of vegetable oils containing predominantly un- Therefore, at least for soy oil, 0.7 g/kg BW/d is suggested as saturated FAs. In two studies (McCann et al., 1987; Kron- an upper limit of fat inclusion.
46 NUTRIENT REQUIREMENTS OF HORSES Kronfeld et al. (2004) reported that rapid introduction of tivities of citrate synthase (CS) and Î²-hydroxyacyl CoA de- fat-supplemented diets is associated with greasy feces hydrogenase (HAD) in skeletal (middle gluteal) muscle (steatorrhea) and increased fecal output, an indication that (Orme et al., 1997). On the other hand, the feeding of a high- some fat has escaped digestion. These adverse effects are fat diet (118 g/kg DM vs. control diet 15 g/kg DM) to Shet- apparently avoided if fat is gradually introduced to the ra- land ponies for 45 days was without effect on the activities tion, with an accommodation period of 4 to 14 days de- of CS and HAD in gluteus and semitendinosus muscle, al- pending on the level of fat supplementation. though an increase in CS activity was detected in the mas- seter muscle (Geelen et al., 2001b). Similarly, the activities of CS, HAD, hexokinase, and phosphofructokinase in mid- PHYSIOLOGIC EFFECTS OF DIETARY FAT dle gluteal muscle were unchanged in Standardbred horses fed a high-fat diet (Geelen et al., 2000). Effects on Lipid and Carbohydrate Metabolism The replacement of starch by fat or oils in concentrate Studies in humans and in rodent species have shown that feeds resulted in decreased blood glycemic and insulinemic high-fat diets result in enhanced capacity for fatty acid oxi- responses to meal feeding (Williams et al., 2001; Zeyner et dation in skeletal muscle. Enhanced expression and activity al., 2005). In mature Thoroughbred horses, the consumption of proteins and enzymes associated with the transport of free of a fat and fiber supplement (approximately 10 percent fatty acids (FFA) into muscle and Î²-oxidation underlie this nonstructural carbohydrates [NSC]; where NSC was deter- increase in fat oxidation. There is some evidence that dietary mined by difference), when compared to a sweet feed sup- fat supplementation results in similar adaptations in fat me- plement (approximately 50 percent NSC), resulted in a 65 tabolism of horses, although some reports are conflicting percent and 85 percent decrease in, respectively, glycemic and few studies quantitatively assessed fat oxidation. Fat and insulinemic response (Williams et al., 2001). The feed- supplementation has been demonstrated to increase post- ing of a sweet feed (50 percent NSC, as determined by di- heparin plasma hepatic and lipoprotein lipase (LPL) activi- rect analysis) to mature Thoroughbred geldings maintained ties, decrease plasma triglyceride (TAG) concentrations by at pasture resulted in decreased insulin sensitivity when as much as 55 percent, and increase plasma cholesterol and compared to the feeding of a high-fat, low-NSC (10 percent phospholipid concentrations (Orme et al., 1997; Geelen et fat, 10 percent NSC in DM) feed or pasture alone (Hoffman al., 1999). In Warmblood horses fed soy oil at 0, 0.33, et al., 2003). 0.67, 1, and 1.33 g/kg BW/d, there were dose-dependent in- creases in serum VFA, phospholipids, and total cholesterol Substrate Storage concentrations, as well as the proportion of Î±-lipoproteins (Zeyner, 2002). Serum TAG concentrations were decreased There is conflicting information on the effects of dietary relative to the control diet at the 0.33 and 0.67 g/kg soy oil fat supplementation on muscle glycogen storage. In several dosages, but significantly increased at the 1 and 1.33 g/kg studies, muscle glycogen content was increased after oil or dosages (Zeyner, 2002). In one study a dose-response rela- fat supplementation, while other studies reported no change tionship was detected between fat intake (3 to 10.8 percent or a moderate decrease in muscle glycogen content. Five fat, as soybean oil) and heparin-released plasma lipoprotein different studies from a single laboratory examined the lipase activity (Geelen et al., 2001a). Specifically, an in- effects of feeding a fat-supplemented concentrate (100 g crease in fat intake by 1 g/kg DM was associated with an in- tallow/kg DM in a grain-based concentrate) for 3â4 weeks crease in LPL activity by 0.98 micromoles of fatty acid re- (Meyers et al., 1987; Oldham et al., 1990; Jones et al., 1991; leased per hour. As LPL is involved in the removal of FFA Scott et al., 1992; Julen et al., 1995). In these studies, (the and glycerol from triglyceride-rich lipoprotein particles in concentrates were fed) with hay in a ratio of 65:35 to 75:25 tissue capillaries of skeletal muscle and adipose tissues, the such that the concentration of fat in the total diet was ap- increase in LPL activity with fat-supplementation indicates proximately 6â7 percent. A consistent finding was increased increased availability of FFA in these tissue beds. Geelen et glycogen storage, with muscle glycogen content increasing al. (2001b) demonstrated decreases in the activities of by as much as 50 percent. In another study, Harkins et al. acetyl-CoA carboxylase and fatty acid synthase in the liver (1992) measured muscle glycogen in Thoroughbred horses of Shetland ponies on a high-fat diet (118 g/kg DM). Thus, fed a control diet of hay and a pelleted concentrate for 3 the fat-induced decrease in plasma TAG concentrations may weeks followed by a fat-supplemented diet (corn oil, 3 per- be due to a decrease in de novo fatty acid synthesis. cent DM) for a further 3 weeks, and mean muscle glycogen There is conflicting evidence regarding the effects of fat was 15.8 percent higher after oil supplementation. However, supplementation on the skeletal muscle activities of en- as hay was not fed with the fat-supplemented diet, absolute zymes involved in oxidative metabolism. In Thoroughbred starch intake was higher for the fat-supplemented diet when horses, 10 weeks of fat supplementation (soybean oil, 80 compared to the control diet. These differences in fiber and g/kg DM; control and fat-supplemented diets supplied 0.22 starch intake confounded interpretation of the apparent and 1 g/kg BW, respectively) resulted in increases in the ac- change in muscle glycogen content. Hambleton et al. (1980)
FATS AND FATTY ACIDS 47 measured muscle (quadriceps femoris m.) glycogen in fat at 8 percent or more of total DM) has been associated horses conditioned for 3 weeks and fed diets containing with mitigation of exercise-associated decreases in plasma 4 percent, 8 percent, 12 percent, or 16 percent soybean oil. glucose concentrations and increased TAG and FFA during Muscle glycogen content was measured before and after 6 prolonged, low-intensity exercise (Slade et al., 1975; Hintz hours of exercise (trotting interspersed with walking). Al- et al., 1978; Hambleton et al., 1980; Hintz et al., 1982; though mean values for muscle glycogen were higher in the Duren et al., 1987). Maintenance of higher plasma glucose 8 percent and 12 percent diets when compared to the other concentrations has been taken as evidence of a glucose-spar- diets, statistically significant differences were not detected. ing effect of fat supplementation. This hypothesis was sup- This study has been consistently misquoted in the literature ported by a recent study in which tracer-determined blood as demonstrating an effect of dietary fat on muscle glycogen glucose utilization during 90 minutes of exercise at 30 per- content. cent of maximum aerobic capacity (VO2max) was lower Other investigators have reported either no change after 5 weeks adaptation to a diet providing 29 percent DE (Essen-Gustavsson et al., 1991; Eaton et al., 1995; Orme et from fat (corn oil) (Pagan et al., 2002). al., 1997; Hyyppa et al., 1999; MacLeay et al., 1999) or a In a small group (n = 4) of conditioned Thoroughbred moderate decrease (Pagan et al., 1987; Geelen et al., 2001b) horses, supplementation with soybean oil (20 percent of DE in muscle glycogen storage following 3â10 weeks of fat from oil) resulted in a decrease in respiratory exchange ratio supplementation. In two studies, liver glycogen content also (RER) during low- (trot at 3.2 m/s) and medium- (canter at decreased after oil supplementation (Pagan et al., 1987; 7 m/s) intensity treadmill (0Â° incline) exercise (Dunnett et Geelen et al., 2001b). A fat-supplemented diet (5 percent al., 2002). This metabolic response during low-intensity ex- DM) slowed the rate of muscle glycogen replenishment in ercise was apparent after 3 weeks of oil supplementation, horses not accommodated to fat feeding (Hyyppa et al., but was not evident at higher workloads (canter at 10 m/s), 1999). However, the rate of post-exercise muscle glycogen perhaps reflecting greater dependence on carbohydrate me- resynthesis was not different from the control diet after an tabolism during intense exercise. Two other studies have adaptation period of 3 weeks. In summary, there is no con- also reported a fat supplementation-induced decrease in the sensus on the effects of dietary fat supplementation on mus- RER of horses during low-intensity exercise (Pagan et al., cle glycogen storage in horses. The variability in results be- 1987, 2002). Overall, these findings suggest an increase in tween the different studies could be due to differences in the utilization of fat, with a concomitant decrease in carbo- diet composition, duration of fat supplementation, muscle hydrate utilization, during low- and moderate-intensity ex- sampling and analysis techniques, and breed, age, and train- ercise following oil supplementation. However, oil supple- ing state of the horses studied. mentation does not appear to alter substrate oxidation at There is also conflicting information regarding the ef- high exercise intensities (> 50â60 percent VO2max). The in- fects of a fat-supplemented diet on muscle triglyceride con- crease in fat oxidation during low-intensity exercise may be tent. Essen-Gustavsson et al. (1991) demonstrated no related to changes in plasma lipase activity and mechanisms change in the triglyceride content of middle gluteal muscle for fat utilization in skeletal muscle. In the study by Dunnett from Standardbred horses fed a fat-supplemented diet (60 et al. (2002), parallel increases in plasma total lipase activ- g/kg DM) for 5 weeks. Orme et al. (1997) also reported no ity, circulating FFA during exercise, and activity of CS change in middle gluteal muscle triglyceride content of in skeletal muscle were detected. However, whereas the Thoroughbred horses provided oil supplementation (80 g fat-supplementationâinduced decrease in RER was corre- soybean oil/kg DM) for 10 weeks. On the other hand, Gee- lated to the increase in muscle CS activity (r2 = 0.95), len et al. (2001b) found an 80 percent increase in the semi- there was no relationship to changes in FFA concentrations membranosus triglyceride content of ponies fed a diet that or to total lipase or Î²-hydroxyacyl CoA dehydrogenase provided 118 g soybean oil/kg DM. activities. The length of time required for expression of these meta- bolic adaptations to dietary oil has been debated. Some nu- Metabolic Responses to Exercise tritionists have advocated that a minimum of 10â12 weeks is Several studies have assessed the effects of fat supple- required for full adaptation (Kronfeld and Harris, 2003). mentation on metabolic responses to exercise. The focus of However, metabolic adaptations to oil supplementation have earlier studies was the effects of fat supplementation on the been observed as early as 3â5 weeks after the start of sup- concentrations of various plasma substrates or metabolites plementation (Pagan et al., 1987, 1995; Orme et al., 1997; (e.g., glucose, FFA, TAG, and lactate) during exercise (Slade Pagan et al., 2002). Thus, whereas a 2â3 month period may et al., 1975; Hintz et al., 1978; Hambleton et al., 1980; Hintz be required for complete adaptation to an oil-supplemented et al., 1982; Duren et al., 1987; Ferrante et al., 1994). More diet, some of the metabolic responses are evident much ear- recent studies sought to obtain quantitative measures of sub- lier. Importantly, the metabolic response to oil supplementa- strate oxidation (Pagan et al., 1987, 2002; Dunnett et al., tion is apparently transient and dependent on its continued 2002). In general, fat supplementation (animal or vegetable use. In one study, the effects on RER were abolished within
48 NUTRIENT REQUIREMENTS OF HORSES 5 weeks of withdrawal of the oil-supplemented diet (Dun- This research group (Ferrante et al., 1994; Taylor et al., nett et al., 2002). 1995) also reported higher plasma lactate concentrations Whereas the aforementioned adaptations in lipid metab- during repeated sprints or incremental exercise in horses fed olism (e.g., decreased RER during low-intensity exercise) an oil-supplemented diet. The mechanism of this response is might imply a sparing of muscle glycogen, the available data unclear. One hypothesis is that âfat adaptationâ confers im- do not support this view. Essen-Gustavsson et al. (1991) and proved metabolic regulation of glucose utilization in mus- Pagan et al. (1987) reported no effect of fat supplementation cle, with glycogen sparing at work of low intensity but en- (approximately 7 percent of total DM intake) on muscle hanced glycolysis and lactate production during exercise glycogen utilization of Standardbred horses during 60â100 requiring higher power output (Kronfeld et al., 1998). An al- minutes of submaximal exercise. Similarly, Harkins et al. ternative explanation is down regulation in the activity of (1992) and Eaton et al. (1995) reported skeletal muscle pyruvate dehydrogenase in response to oil no change in net muscle glycogen utilization in fat- feeding, as has been observed in humans (Spriet and Watt, supplemented Thoroughbred horses undertaking exercise 2003). It should also be noted that other researchers have that simulated race performance. Other researchers (Oldham reported lower plasma lactate accumulation during sub- et al., 1990; Jones et al., 1991; Scott et al., 1992; Hughes et maximal incremental exercise when horses are fed an oil- al., 1995) have reported higher net muscle glycogen utiliza- supplemented (11.8 percent fat, DM basis) diet (Sloet van tion and plasma lactate concentrations and improved perfor- Oldruitenborgh-Oosterbaan et al., 2002). mance during exercise tests consisting of multiple sprints (e.g., 4 Ã 600 m). The improved performance was attributed Athletic Performance to the higher glycogen stores and maintenance of anaerobic energy transduction during sprint exercise. A number of researchers have attempted to determine the A single study has examined the effects of dietary fish effects of fat supplementation on athletic performance. It (menhaden) oil supplementation on metabolic response to has been hypothesized that adaptation to fat supplementa- incremental treadmill exercise in horses (OâConnor et al., tion results in an improvement in the athletic performance of 2004). Horses were assigned to either a fish oil (n = 6) or horses. Several theoretical explanations have been put forth corn oil (n = 4) treatment in which the oil (324 mg/kg BW) as explanations for enhanced athletic performance with the was supplemented to a hay and concentrate ration for 63 feeding of higher-fat diets, including: (1) an improved days. The horses underwent moderate physical conditioning power-to-weight ratio due to a reduction in DM intake and 5 days/week and completed an incremental exercise test bowel ballast; (2) decreased metabolic heat production as- after 63 days. During the test, horses that received fish oil sociated with feeding and exercise (Kronfeld, 1996); (3) en- had significantly lower heart rate and serum free fatty acid, hanced stamina as a result of muscle glycogen sparing glycerol, and cholesterol concentrations when compared to (Kronfeld et al., 1998); (4) improved sprint performance due the corn oil treatment. The physiological importance of to increased energy transduction from anaerobic glycolysis these findings remains to be determined. (Oldham et al., 1990; Kronfeld et al., 1998); and (5) mitiga- tion of acidemia during high-intensity exercise (Kronfeld et al., 1998). However, the results of studies examining the ef- Acid-Base Responses fects of fat supplementation in horses on athletic perfor- Adaptation to a fat-supplemented diet influences blood mance are equivocal. Some authors have reported improved acid-base responses to exercise. During a treadmill exercise performance (Oldham et al., 1990; Harkins et al., 1992; test, venous pH was higher (~ 7.44â7.46) in Arabian horses Eaton et al., 1995), while others have found no change fed a diet supplemented with corn oil (100 g/kg DM) when (Topliff et al., 1983; Pagan et al., 1987; Essen-Gustavsson et compared to the control diet (~ 7.40â7.42) at speeds be- al., 1991; Hyyppa et al., 1999). Harkins et al. (1992) re- tween 3 and 6 m/s (Taylor et al., 1995; Kronfeld et al., ported that 14 of 15 horses ran a 1,600-m simulated race 1998). There also was a decrease in venous blood pCO2 dur- faster when fed a fat-supplemented diet (corn oil, 3 percent ing repeated sprints, which was attributed to a lower RER DM for 3 weeks) compared to the control ration. Mean race associated with enhanced fat oxidation, although RER was time improved 2.5 s (2.1 percent) after consuming the fat- not measured (Kronfeld et al., 1998). Graham-Thiers et al. added diet. However, as discussed above, this experiment (2001) showed that dietary protein restriction (crude protein was inappropriately designed. A longitudinal design was 7.5 g/100 g with supplemental L-lysine [0.5 percent] and used with all horses completing the control-diet race first, L-threonine [0.3 percent]) had an additive effect with fat with the fat-diet race undertaken after a further 3 weeks of supplementation (corn oil, 100 g/kg DM) in the mitigation training. It is possible, therefore, that the observed decrease of the acidogenic effects of exercise. The biological signifi- in simulated race time was due to training rather than oil cance of these small alterations in acid-base response to ex- supplementation. Furthermore, as the horses were not fed ercise is unclear. hay during the second diet period, it is possible the faster race time was associated with a reduction in gut weight.
FATS AND FATTY ACIDS 49 Four weeks of fat supplementation (12 percent DE from Holland et al. (1996b) found lower plasma cortisol con- corn oil) was associated with a small but statistically signif- centrations in 4- to 5-month-old foals fed a fat-and-fiber icant increase in run time to fatigue in Thoroughbred horses supplement when compared to a starch and sugar (sweet undertaking treadmill exercise at an intensity equivalent to feed) supplement. Subjectively, the foals fed the fat-and- 120 percent of VO2max (Eaton et al., 1995). Earlier studies fiber supplement were less stressed after weaning. In a lon- of the effects of fat supplementation found enhanced perfor- gitudinal study of growing horses, supplementation with a mance during exercise protocols consisting of repeated fat and fiber concentrate was associated with lower apparent sprints (Meyers et al., 1987; Oldham et al., 1990; Scott et al., distress at weaning and diminished responses to standard- 1992) or efforts during protocols that simulated exercise un- ized temperament tests when compared to a starch-and- dertaken by cutting horses (Webb et al., 1987). Improved sugar concentrate (Nicol et al., 2005). work performance during high-intensity exercise was attrib- uted to increases in resting muscle glycogen content and the Lactation, Reproductive Performance, and Growth rate of glycogen utilization. However, in the study by Eaton et al. (1995), there was no effect of diet on glycogen utiliza- Davison et al. (1987, 1991) added 5 percent animal fat to tion rate during exercise. concentrates fed to pregnant and lactating Quarter horse Several factors could account for the variability in results mares. Feed consumption was lower during gestation, between studies, including the type and amount of fat sup- while growth rate tended to be higher in foals of mares fed plemented, duration of fat feeding, variation in the intensity the supplemented diet compared to mares and foals fed the and duration of exercise tests, small number of horses per unsupplemented diet (Davison et al., 1987, 1991). Fat sup- treatment (most often fewer than six), and differences in the plementation increased milk fat concentration at days 10 conditioning status of the horses. and 60 of lactation, but protein and total solids content were unaffected. The feeding of fat to mares during late gestation and early lactation did not affect reproductive performance Behavior (Davison et al., 1991). Growth of Thoroughbred weanlings There are limited published data on the effects of fat sup- kept at pasture and fed a 10-percent fat supplement was plementation on behavior of horses. Holland et al. (1996a) not different when compared to weanlings fed an isocalo- evaluated the effects of fat supplementation on spontaneous ric supplement rich in starch and sugar (Hoffman et al., activity (distance moved per day, measured by use of a pe- 1999). dometer) and reactivity (responses to pressure, loud noise, and sudden visual stimuli) in eight horses. The control diet Health Effects of Dietary (n-3) vs. (n-6) Fatty Acids (CON) contained chopped hay, corn, oats, beet pulp, and molasses, while the three test diets contained an additional In other species, the two recognized EFAs are linoleic 10 percent (by weight) corn oil (CO), soy lecithin-corn oil acid (18:2, n-6) and Î±-linolenic acid (18:3, n-3), both of (SL-CO), or soy lecithin-soy oil (SL-SO), with small de- which are PUFAs. Mammals lack the desaturase enzymes creases in hay, grains, and molasses. Horses were fed each necessary to introduce double bonds into fatty acid chains diet in random order for four 3-week periods. When com- that are more than nine carbons from the carboxyl terminus. pared to CON, spontaneous activity was less in horses fed Therefore, mammals are unable to synthesize n-3 and SL-CO (~ 4 km/day vs. ~ 5 km/day in CON) but unaffected n-6 fatty acids, and they must be supplied in the diet (Dun- by the CO or SL-SO diets. Similarly, there was no consistent bar and Bauer, 2002). effect of fat supplementation on measures of reactivity, al- Soy, corn, sunflower, and safflower oils are rich in though startle response to the sudden opening of an um- linoleic acid, while Î±-linolenic acid is the major fatty acid in brella was decreased in all three diets. In studies examining linseed and flaxseed oils. Soy and canola oils also contain the effect of dietary energy source on the clinical expression Î±-linolenic acid. The fatty acids in oils obtained from cold- of recurrent exertional rhabdomyolysis in Thoroughbreds, water fish (e.g., menhaden oil) are rich in the n-3 fatty acids researchers have reported decreased excitability, nervous- eicosapentaenoic acid (EPA) and docosahexaenoic acid ness, and resting heart rate when the horses were consuming (DHA). When supplied in the diet, EFAs can be converted to a fat-supplemented ration when compared to a high-grain long-chain PUFAs by desaturase and chain-elongation en- ration (MacLeay et al., 2000; McKenzie et al., 2003). These zymes located in the cellular endoplasmic reticulum. authors suggested that the effect of fat supplementation Linoleic acid is desaturated and elongated to yield arachi- on the behavior of RER-affected horses was due to the ex- donic acid (20:4, n-6) and other long-chain PUFAs that are clusion of dietary starch rather than a specific effect of di- incorporated into cell membrane phospholipids, while me- etary fat. Other researchers have reported a reduction in the tabolism of Î±-linolenic acid yields eicosapentaenoic acid exercise-associated increase in plasma cortisol concentra- (EPA; 20:5, n-3) and docosapentaenoic acid (22:5, n-3). Do- tion in horses fed a fat-supplemented ration (Crandell et al., cosapentaenoic acid may be retroconverted to EPA (20:5, 1999; Graham-Thiers et al., 2001). n-3) or further metabolized to DHA (22:6, n-3).
50 NUTRIENT REQUIREMENTS OF HORSES In human and animal nutrition there is interest in the po- either 3 percent (by weight) corn oil or fish oil for 14 weeks. tential health benefits of n-3 fatty acid supplementation (or At 12 weeks, horses fed fish oil had increased plasma con- alterations in the dietary n-6:n-3 ratio). The dietary n-6 vs. centrations of EPA (27-fold; 8.5 vs. 0.3 g/100g fatty acids), n-3 PUFA ratio affects the fatty acid composition of cell DHA (34-fold; 5.1 vs. 0.1 g/100 g fatty acids), and arachi- membranes (Simopoulos, 1999). When the diet of humans is donic acid (8.3-fold; 4.1 vs. 0.5 g/100 g fatty acids). Neu- supplemented with fish oil, the ingested EPA and DHA par- trophils from horses fed fish oil produced 17.6-fold more tially replace n-6 fatty acids (especially arachidonic acid) in leukotriene B5 (LTB5) compared with horses fed corn oil, cell membranes, particularly those of platelets, erythrocytes, and the ratio of LTB5 to leukotriene B4 was 4-fold high in neutrophils, and monocytes. This alteration in membrane horses fed fish oil. In addition, the quantity of prostaglandin fatty acid composition reduces the quantity of arachidonic E2 by endotoxin-stimulated bronchoalveolar fluid (BALF) acid available for eicosanoid synthesis, alters the spectrum cells was lower in fish oil-fed when compared to corn oil- of eicosanoids synthesized, and lessens the inflammatory fed horses. However, the production of TNF-alpha by BALF and prothrombotic responses to physical or chemical stimuli cells did not differ between diets and cell- mediated immu- (Hulbert et al., 2005). In addition, EPA competitively in- nity, assessed by the skin response to injection of the key- hibits the activity of the enzyme cyclooxygenase, which is hole limpet hemocyanin, was similar in the corn oil- and fish necessary for eicosanoid synthesis (Weber et al., 1986). oil-fed horses (Hall et al., 2004b). Several studies in horses have examined the effects of Supplementation of horses with recurrent seasonal pruritis n-3 fatty acid supplementation on plasma fatty acid compo- (âsweet itchâ) with large amounts of flaxseed (454 g flaxseed/ sition and various physiologic responses. Henry et al. (1990) 450 kg BW) was associated with a significant decrease in the fed horses a complete pelleted ration containing 8 percent allergic skin response to Culicoides extract, suggesting a pos- (by weight) raw linseed oil vs. a control ration without sible benefit of flaxseed in the management of horses with added linseed oil. After 8 weeks on the ration, the mean this condition. However, in another study, supplementation procoagulant activity and thromboxane B2 production by with flax oil did not alter clinical signs in horses with Culi- endotoxin-stimulated monocytes from horses consuming the coides hypersensitivity (Friberg and Logas, 1999). linseed oil ration decreased by 51 percent and 71 percent, In summary, the data from studies in which horses were respectively, compared with cells from horses consuming fed diets enriched with omega-3 (n-3) fatty acids (linseed, the control ration. Fatty acid analysis of membrane phos- flaxseed, or fish oils) have demonstrated modulation of in- pholipids demonstrated a decrease in the n-6:n-3 ratio in flammatory mediator synthesis by cells harvested from monocytes from horses fed the linseed oil ration (Henry et blood, peritoneal fluid, or respiratory secretions. However, al., 1990). In a companion study (Morris et al., 1991), it was physiological importance of these findings is unclear, and shown that endotoxin-induced synthesis of tumor necrosis further research is needed to determine the effects of n-3 factor by peritoneal macrophages was lower in horses fed fatty acid supplementation in the treatment and prevention the linseed oil ration when compared to cells from horses of inflammatory diseases in horses (e.g., recurrent airway fed the control ration. This research group also demon- obstruction) (McCann and Carrick, 1998). strated that intravenous administration of a single dose of a 20 percent lipid emulsion enriched with n-3 fatty acids to REQUIREMENTS, DEFICIENCIES, AND EXCESSES healthy horses resulted in decreased inflammatory mediator synthesis (thromboxane, TNF-alpha) by monocytes and a Dietary fats or oils are required to facilitate absorption of lower n-6:n-3 fatty acid ratio in cell membranes when com- the fat-soluble vitamins A, D, E, and K, and as a source of pared to the infusion of a lipid emulsion enriched with n-6 the EFAs, linoleic and Î±-linolenic. There have been no re- fatty acids (McCann et al., 2000). However, the feeding of ports of EFA deficiency in horses. In other species, an EFA an 8 percent linseed oil ration for 8 weeks did not alter the deficiency causes dry coat, scaly skin, hair loss, and, with in vivo response of horses to the infusion of Escherichia coli prolonged deficiency, development of exudative dermatitis 055:B5 endotoxin (0.03 mg/kg BW, infused over 30 min- (Connor, 2000). Decreased reproductive efficiency and fetal utes) (Henry et al., 1991). In another study, there were sta- abnormalities are other potential consequences of an EFA tistically significant increases in the plasma concentrations deficiency. No clinical abnormalities were observed in of Î±-linolenic acid (18:3, n-3) and EPA, but not DHA, in ponies fed extremely low-fat diets (0.05 percent and 0.22 horses fed a 10 percent flaxseed oil-enriched complete pel- percent total fat containing, respectively, 0.03 and 0.14 per- let (80 percent of the ration) and hay (20 percent) for 16 cent linoleic acid) for 7 months (Sallmann et al., 1991). weeks. However, in vitro measures of platelet aggregation However, a substantial decrease in plasma and tissue vita- were not altered by the supplementation of flaxseed oil min E concentration was observed in ponies fed the 0.05 (Hansen et al., 2002). More marked increases in plasma con- percent fat diet, suggesting that there was inadequate intes- centrations of EPA and DHA were observed when healthy tinal absorption of this fat-soluble vitamin. horses were fed ration supplemented with fish (menhaden) A suggested dietary minimum for linoleic acid is 0.5 per- oil. Hall et al. (2004a) fed horses diets supplemented with cent of DM. This minimum is easily achieved when supple-
FATS AND FATTY ACIDS 51 mental fat is fed, as fats and oils added to equine rations are Coppock, C.E., and D. L. Wilks. 1991. Supplemental fat in high energy ra- high in linoleic acid (see Table 8-5). Because fat is energy tions for lactating cows: effects on intake, digestion, milk yield and composition. J. Anim. Sci. 69:3826â3837. dense, one concern with the feeding of fat-supplemented Crandell, K. G., J. D. Pagan, P. Harris, and S. E. Duren. 1999. A compari- diets is weight gain associated with the provision of DE in son of grain, oil and beet pulp as energy sources for the exercised horse. excess of needs. The promotion of weight gain in hard Equine Vet. J. Suppl. 30:485â489. keeper horses is a common rationale for the addition of fat Custalow, S. E. 1992. Lactate and Glucose Responses to Exercise in the to equine diets. For less active or easy keeper horses, how- Horse: Influence of Interval Training and Dietary Fat. M.S. Thesis. Vir- ginia Polytechnic Institute and State University, Blacksburg. ever, use of fat-supplemented energy concentrates may lead Davison, K. E., G. D. Potter, J. W. Evans, L. W. Greene, P. S. Hargis, C. D. to undesirable weight gain. Metabolic utilization of ab- Corn, and S. P. Webb. 1987. Growth and nutrient utilization in weanling sorbed fat is highly efficient. In ponies fed supplemental horses fed added dietary fat. Pp. 95â100 in Proc. 11th Equine Nutr. corn oil, the DE to net energy (NE) conversion efficiency Physiol. Soc. Symp., Stillwater, OK. was about 85 percent (Kane et al., 1979). The comparative Davison, K. E., G. D. Potter, L. W. Greene, J. W. Evans, and W. C. Mac- Mullan. 1991. Lactation and reproductive performance of mares fed efficiency for a hay/grain diet is less than 60 percent. Ac- added dietary fat during late gestation and early lactation. J. Equine Vet. cordingly, the NE of fat-supplemented feeds is higher than Sci. 11:111â115. is predicted by estimation of the DE content. Dunbar, B. L., and J. E. Bauer. 2002. Metabolism of dietary essential fatty In Shetland ponies, there is evidence that fat supplemen- acids and their conversion to long-chain polyunsaturated metabolites. J. tation (soybean oil, 10 percent DM) results in glucose intol- Am. Vet. Med. Assoc. 220:1621â1626. Dunnett, C. E., D. J. Marlin, and R. C. Harris. 2002. Effect of dietary lipid erance (Schmidt et al., 2001). When ponies were fed to meet on response to exercise: relationship to metabolic adaptation. Equine DE requirement (15.5 MJ DE/100 kg BW), mean plasma Vet. J. Suppl. 34:75â80. glucose concentrations during an oral glucose tolerance test Duren, S. E., S. G. Jackson, J. P. Baker, and D. K. Aaron. 1987. Effect of (1 g/kg BW) were approximately 40 percent higher in the dietary fat on blood parameters in exercised Thoroughbred horses. Pp. fat-supplemented diet when compared to the control diet. 674â685 in Equine Exercise Physiology 2, J. R. Gillespie and N. E. Robinson, eds. Davis, CA: ICEEP Publications. Furthermore, the feeding of a hypercaloric (18.5 MJ DE/100 Eaton, M. D., D. R. Hodgson, and D. L. Evans. 1995. Effect of diet con- kg BW) fat-enriched diet resulted in a 25-fold increase in taining supplementary fat on the effectiveness for high intensity exer- plasma insulin concentrations after oral glucose loading. cise. Equine Vet. J. Suppl 18:353â356. Thus, glucose intolerance and insulin resistance may occur Essen-Gustavsson, B., E. Blomstrand, K. Karlstrom, A. Lindholm, and S. in ponies fed fat-supplemented diets, particularly when en- G. B. Persson. 1991. Influence of diet on substrate metabolism during exercise. Pp. 288â298 in Equine Exercise Physiology 3, S. G. B. Pers- ergy intake exceeds requirements. As insulin resistance is son, A. Lindholm, and L. B. Jeffcott, eds. Upsala: ICEEP Publications. considered a risk factor for laminitis in ponies, some caution Ferrante, P. L., D. S. Kronfeld, L. E. Taylor, and T. N. Meacham. 1994. in the feeding of fat-supplemented diets is warranted. Blood lactate concentration during exercise in horses fed a high-fat diet Most of the studies evaluating the effects of fat supple- and administered sodium bicarbonate. J. Nutr. 124:2738Sâ2739S. mentation in horses have been of short duration (less than 3 Fonnesbeck, P. V., R. K. Lydman, G. W. Vander Noot, and L. D. Symons. 1967. Digestibility of the proximate nutrients of forage by horses. J. months). However, Pagan et al. (1995) observed no adverse Anim. Sci. 26:1039â1045. effects in 2-year-old Thoroughbreds fed supplemental fat Friberg, C. A., and D. Logas. 1999. Treatment of culicoides hypersensitive (soybean oil, 12 percent DE) for 7 months. Harris et al. horses with high-dose n-3 fatty acids: a double-blinded crossover study. (1999) reported no apparent adverse effects of feeding a diet Vet. Dermatol. 10:117â122. supplemented with either an unsaturated or saturated veg- Geelen, S. N. J., M. M. Sloet van Oldruitenborgh-Oosterbaan, and A. C. Beynen. 1999. Dietary fat supplementation and equine plasma lipid me- etable oil for 6 months at 20 percent DE after 10 months at tabolism. Equine Vet. J. Suppl. 30:475â478. 12 percent DE. In Warmblood-type horses, no detrimental ef- Geelen, S. N. J., W. L. Jansen, M. J. H. Geelen, M. M. Sloet van Ol- fects on blood biochemical variables were observed after druitenborgh-Oosterbaan, and A. C. Beynen. 2000. Lipid metabolism in feeding a mixed diet containing 14 percent and 16.3 percent equines fed a fat-rich diet. Int. J. Vitam. Nutr. Res. 70:148â152. partially hydrogenated soy oil for, respectively, 168 (Zeyner Geelen, S. N. J., W. L. Jansen, M. M. Sloet van Oldruitenborgh-Oosterbaan, H. J. Breukink, and A. C. Beynen. 2001a. 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FATS AND FATTY ACIDS 53 Oldham, S. L., G. D. Potter, J. W. Evans, S. B. Smith, T. S. Taylor, and W. Spriet, L. L., and M. J. Watt. 2003. Regulatory mechanisms in the interac- S. Barnes. 1990. Storage and mobilization of muscle glycogen in exer- tion between carbohydrate and lipid oxidation during exercise. Acta. cising horses fed a fat-supplemented diet. J. Equine Vet. Sci. Physiol. Scand. 178:443â452. 10:353â359. Sturgeon, L. S., L. A. Baker, J. L. Pipkin, J. C. Haliburton, and N. K. Chi- Orme, C. E., R. C. Harris, D. J. Marlin, and J. Hurley. 1997. Metabolic rase. 2000. The digestibility and mineral availability of matua, bermuda adaptation to a fat-supplemented diet by the Thoroughbred horse. Br. J. grass, and alfalfa hay in mature horses. J. Equine Vet. Sci. 20:45â48. Nutr. 78:443â458. Taylor, L. E., P. L. Ferrante, D. S. Kronfeld, and T. N. Meacham. 1995. Pagan, J. D., B. Essen-Gustavsson, A. Lindholm, and J. Thornton. 1987. 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