The study of this thesis concerns the dietary influence on lipid metabolism in horses. The main issue is the effect of dietary medium chain triacylglycerols (MCT) on triacylglycerol metabolism. In certain conditions high-fat diets can be beneficial for horses. Diets rich in long-chain polyunsaturated fatty acids in the form of soybean oil decrease plasma triacylglycerol concentrations and increase the activity of heparin-released lipoprotein lipase activity in horses. The metabolic adaptation to fat supplementation in the horse appears to proceed in a manner similar to that seen in response to training. The feeding of MCT versus soybean oil raised the plasma level of triacylglycerols in horses. There was no significant effect of carbohydrate source and no fat-carbohydrate interaction. However, when diets contained soybean oil, cellulose versus glucose produced significantly greater increase in plasma triacylglycerols. A major product of cellulose fermentation is acetic acid. In the liver acetic acid can be converted into Acetyl-CoA in the cytosol and be use for in de novo fatty acid synthesis. But when horses are fed isoenergetic amounts of MCT or cellulose, cellulose did not increase the triacylglycerol concentrations in plasma. Feeding of MCT instead of soybean oil would increase the production of plasma triacylglycerols together with an increase in the clearance rate of triacylglycerols. The rate of triacylglycerol clearance was on average about two-fold higher than the calculated rate of triacylglycerol production. The LPL activity was lower when the ponies were fed MCT. It was concluded that MCT versus soybean oil causes an increase in the triacylglycerol production, leading to an elevation of plasma triacylglycerol concentration and eliciting an increase in plasma triacylglycerol clearance as a secondary feature. When horses were fed high-fat rations with either soybean oil or palm oil, there was no difference in plasma triacylglycerol levels between the two diets. Thus, dietary soybean oil probably did not affect the hepatic secretion of very-low-density-lipoproteins with triacylglycerols. Apparently, plasma triacylglycerol concentrations in horses are not sensitive to the degree of saturation of dietary fatty acids. In a cross-over study with 6 adult horses, the effect of isoenergetic replacement of dietary glucose by beetpulp on the concentration of plasma triacylglycerol was studied. The test ration contained 25% beetpulp, with 5% pectine in the dietary dry matter. The feeding of beetpulp lowered the plasma concentration of triacylglycerol in fasting state and raised the activity of heparin-released LPL. It is suggested that beetpulp ingestion activates lipoprotein lipase followed by a drop in plasma triacylglycerols. Hyperlipaemia is a condition in ponies associated with a dysfunction in lipidmetabolism. A fat-free liquid diet was formulated for the treatment of hyperlipaemia. As sole source of nutrition the liquid diet fully counteracted fasting-induced hyperlipaemia in two healthy ponies. The liquid diet has potential in the treatment of hyperlipaemia -it provides energy and nutrients to counteract the negative energy balance and low nutritional status of patients. This thesis shows that MCT-rich rations increase the plasma level of triacylglycerols in horses by increasing de novo fatty acid synthesis in the liver. The effect on triacylglycerol metabolism is different than when diets rich in soybean oil are fed to horses and indicates that MCT is not suitable for performance horses. Another result from this thesis is that the intake of beetpulp has similar effects on lipid metabolism than soybean oil
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