Dietary Control of Equine Plasma Triacylglycerols
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View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Utrecht University Repository DIETARY CONTROL OF EQUINE PLASMA TRIACYLGLYCEROLS Voeding en plasma triacylglycerolen bij paarden en ponies (met een samenvatting in het Nederlands) PROEFSCHRIFT Ter verkrijging van de graad van doctor aan de Universiteit Utrecht, op gezag van de Rector Magnificus, Prof. Dr. W.H. Gispen, ingevolge het besluit van het College voor Promoties in het openbaar te verdedigen op donderdag 7 maart 2002 des middags te 12.45 uur. Door Johanna Maria Hallebeek Geboren op 1 juni 1964 te Driebergen-Rijsenburg Promotor Prof. Dr. Ir. A.C. Beynen Department of Nutrition, Faculty of Veterinary Medicine, Utrecht University, The Netherlands Johanna Maria Hallebeek Utrecht: Universiteit Utrecht, Faculteit Diergeneeskunde Proefschrift Universiteit Utrecht.- Met samenvatting in het Nederlands. ISBN 9039329826 Copyright J.M.Hallebeek 2002 All rights reserved Printed by ADDIX, Wijk bij Duurstede, The Netherlands Aan mijn vader en mijn moeder Contents Chapter 1 General Introduction Chapter 2 Dietary fats and lipid metabolism in relation to equine health, performance and disease Chapter 3 Effect of dietary medium chain triacylglycerols on plasma triacylglycerol levels in horses. Arch. Anim. Nutr. 2001, 54: 159-171. Chapter 4 The concentration of plasma triacylglycerols in horses fed diets containing either medium chain triacylglycerols or an isoenergetic amount of starch or cellulose. Arch. Anim. Nutr. 2002: in press Chapter 5 Production and clearance of plasma triacylglycerols in ponies fed diets containing either medium-chain triacylglycerols or soybean oil. Submitted Chapter 6 The plasma level of triacylglycerols in horses fed high-fat diets containing either soybean oil or palm oil. J. Anim. Nutr. a. Anim. Physiol.: in press Chapter 7 Influence of dietary beetpulp on the plasma level of triacylglycerols in horses. Submitted Chapter 8 A preliminary report on a fat-free diet formula for nasogastric enteral administration as treatment for hyperlipaemia in ponies. Vet. Quart. 2001, 23: 201-205 Chapter 9 General conclusions. Summary Samenvatting Curriculum vitae Dankwoord Chapter 1 Chapter 1 GENERAL INTRODUCTION 1 General Introduction 2 Chapter 1 Introduction Intensively exercised horses need to consume energy-rich concentrates to meet their energy requirement. To increase the energy density of the ration, fat can be added as such or at expense of nonstructural carbohydrates. High-fat diets can diminish the risk of digestion disorders and founder as imposed by carbohydrate-rich rations (Clarke et al., 1990; Coenen, 1990 ; Garner et al., 1978 ; Caroll et al., 1987 ; Rowe et al., 1994 ; Kronfeld, 2001). High-fat diets may also be used for horses with recurrent rhabdomyolisis (Beech, 1997; Valentine et al., 1998; De La Corte et al., 1999; MacLeay et al., 1999), for rapidly growing horses (Ott and Kivipelto, 1998; Davidson et al., 1991; Hoffman et al., 1996) and for horses that suffer from hypo-alimentation due to ilnesses or old age (Lewis, 1995; Meyer, 1992). Another example of the use high-fat diets is replacing dietary protein by fat, e.g. for patients with chronic interstitial nephritis (Lewis, 1995). It would appear that under specified conditions the feeding of extra fat to horses can be advantageous. High-fat diets have gained considerable interest in relation to exercise. Diets rich in long-chain polyunsaturated fatty acids in the form of soybean oil decrease plasma triacylgycerol concentrations and increase the activity of heparin-released lipoprotein lipase activity in horses (Geelen et al., 1999; 2001; Orme et al., 1997). Fat addition to the ration increases the relative contribution of fatty acids to the energy intake. Metabolic adaptation to fat supplementation in the form of long-chain polyunsaturated fatty acids appears to proceed in a manner similar to that seen in response to training (Geelen et al., 2001). In certain studies (Harkins et al., 1992; Eaton et al., 1995; Oldham et al., 1990) there was an improved performance in horses fed high-fat diets, which was attributed to fat-induced changes in the metabolism. Clearly, there can be beneficial effects when fats in equine nutrition are substituted fot non-structural carbohydrates and/or protein. However, high fat intakes in the form of soybean oil have been shown to decrease the apparent digestibility of fiber, protein and non-structural carbohydrates (Jansen et al., 2001). There is evidence that high fat intakes in the form of medium-chain triacylglycerols have no negative effect on the digestibility of macronutrients other than fat (Jansen et al., 2002). Thus, it was considered of interest to gain knowledge about control of equine lipid metabolism by MCTs in the diet. MCTs are laboratory-made and contain saturated fatty acids with 8 or 10 carbon atoms. Unlike long-chain fatty acids, which require the formation of biliary 3 General Introduction micelles for absorption and are transported as triacylglycerols in chylomicrons from enterocytes via the lymphatics to the blood, dietary MCTs are rapidly absorbed and carried through the portal vein directly to the liver, where they are preferentially oxidized (Bach and Babayan, 1982; Foufelle, 1992). The acetyl- CoA generated by rapid oxidation of medium-chain fatty acids may not be completely converted into CO2 and ketone bodies, but a portion may serve as substrate for fatty acid synthesis (Bach and Babayan, 1982 ; Geelen, 1994 ; Geelen et al., 1995). MCTs are used in human patients with a variety of pancreatic and gastrointestinal disorders (Sucher 1986; Velazquez et al., 1996; Bell et al., 1991). MCT-rich diets increase plasma levels of triacylglycerols and/or ketone bodies in species such as man (Swift et al., 1992), rat (Wiley and Leveille, 1973; Geelen et al., 1995), pig (Odle et al., 1991), and dog (Van Dongen et al., 2000). The effect of dietary MCTs on equine lipid metabolism had not been described. Aime and scope of this study As mentioned above, this thesis focusses on the effect of dietary MCTs on equine triacylglycerol metabolism. In the course of the studies, in order to explain the effects observed, it became opportune to also determine the effects of cellulose and starch, palm oil versus soybean oil, and pectin on triacylglycerol metabolism in horses. It was anticipated that the results obtained would provide fundamental insight into the dietary control of equine lipid metabolism. In addition, the data could be helpful to clarify the pathogenesis of disorders in equine lipid metabolism and could support the use of special diets for preventive or therapeutic purposes. In the light of the latter application, an attempt was made to develop a dietary formula for the tube-feeding of ponies with clinical hyperlipoproteinaemia. Chapter 2 presents a review on dietary fats and equine lipid metabolism. The digestibility of fat, the requirement of essential fatty acids and the metabolic responses to high-fat diets or diets with different fat sources are discussed. In addition, practical information on the formulation and the use of high-fat diets is given. In the study described in Chapter 3 the hypothesis tested was that glucose when fed to horses together with MCT would increase the plasma level 4 Chapter 1 of triacylglycerol more than the combination of MCT and cellulose would do. MCT instead of soybean oil increased the triacylgycerol level in plasma. The hypertriacylglycerolaemic effect of MCT was explained by stimulation of hepatic fatty acid and triacylglycerol synthesis. However, there was no interaction between fat and carbohydrate source with regard to plasma triacylglycerol concentrations. This lack of interaction was explained by reasoning that acetate, as produced by fermentation of cellulose in caecum and colon is as lipogenic as glucose. In horses, both glucose and acetate may be used for de novo fatty acid synthesis, their contribution as substrate depending on the composition of the ration. Isoenergetic amounts of MCT and cellulose, when compared with either starch or glucose, should then stimulate the synthesis of triacylglycerols to the same extent and thus produce similar concentrations of plasma triacylglycerols. The aim of the study described in Chapter 4 was to compare the effect of dietary cellulose on plasma triacylglycerols with that of either MCT or starch. It was hypothesized that cellulose would be as lipogenic as MCT in horses and thus would produce similar levels of plasma triacylglycerols. MCT versus an isoenergetic amount of either cellulose or starch raised plasma triacylglycerols. Unlike the hypothesis, plasma triacylglycerols showed no difference for the starch- and cellulose-rich diets. It was speculated that on both low and high-fat diets with MCT, glucose can be used for de novo fatty acids synthesis, resulting in a similar effect on triacylglycerol concentrations. However, when feeding high- fat diets with soybean oil, de novo fatty acid synthesis will be depressed as well as the citrate carrier so that cellulose, but not glucose will raise plasma triacylglycerols. In Chapter 5, the hypothesis tested was that feeding of MCT instead of soybean oil causes an increase in the production of plasma triacylglycerols, which, under steady-state conditions, is associated with an increased clearance of triacylglycerols. Ponies were used and the production of plasma triacylglycerols was indirectly measured after administration of the non-ionic detergent, Triton WR1339. The clearance of