The Effect of Dietary Energy Source on Blood Metabolites in Standardbred Horses During Exercise

The basic driving force behind all types of equine exercise is the conversion of chemically bound energy into mechanical energy for muscular movement. The only source of energy that can be used directly to produce mechanical work in muscular activity is the dephosphorylation of adenosine triphosphate (ATP). In order to sustain high rates of energy generation (and work), ATP must be resynthesized at a rate equal to its hydrolysis in muscle. Two fundamental reactions can be employed for this resynthesis: 1) Glycolysis, which is the anaerobic metabolism of glucose or glycogen to lactate. 2 ) Oxidative phosphorylation, which involves the reduction of substrates (carbohydrates, fats, and proteins) and their oxidation via the tricarboxylic acid cycle and electron transport chain within mitochondria. The choice of substrate for energy generation during exercise is dependent on a number of factors including degree of fitness, intensity of exercise, duration of exercise and diet. Several studies with humans (Christensen and Hansen, 1939; Martin et al., 1978; and Jansson, 1980) have shown that the relative proportions of fat and carbohydrates in the diet influences the relative oxidation of these substrates in working muscle. The following study was conducted to determine whether changes in dietary energy source (carbohydrate, protein and fat) influence substrate selection by equine muscle during different exercise intensities. The effect of these dietary manipulations on cardiovascular response and muscle and liver glycogen storage and utilization have already been reported (Pagan et al., 1986) This paper will concentrate on the effect of dietary energy source on blood metabolites in horses during exercise.