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Summary A study to evaluate the long term effects of feeding a fat supplemented diet during medium intensity, aerobic training was conducted over 7 months using 12, 2-year-old Thoroughbreds. The control group (n=6) was fed grass hay and a fortified sweet feed (CON) and the other group (FAT)(n=6) received hay, sweet feed, a supplement pellet and 400 ml of soybean oil that supplied about 12% the DE intake of the FAT group. After 2,4 and 7 months, the horses performed 2 standardised exercise tests (SETS) on a high speed treadmill. The first test (STE:P) consisted of sequential steps of 800 m at speeds of about 4,8,9,10 and 11 ds. V,, and V,,, increased with training, but were not different between treatment groups. During the last STEP, insulin was higher post feeding in the CON horses. Blood glucose was lower at the end of the STEP in CON horses. The second test (SET3O) consisted of 30 min of trotting at about 4 ds. Cortisol was elevated during exercise in the CON horses during the last SET30. T, was unaffected by diet

The effect of dietary starch and fat content on serum creatine kinase (CK) activity and substrate availability was evaluated in 4 mares of Quarter Horse-related breeds with polysaccharide storage myopathy (PSSM). Four isocaloric diets ranging in digestible energy (DE) from 21.2% (diet A), 14.8% (B), 8.4% (C), to 3.9% (D) for starch, and 7.2% DE (diet A), 9.9% (B), to 12.7% DE (diet C and D) for fat were fed for 6-week periods (4 weeks with exercise) using a 4 X 4 Latin square design. Postprandial glucose and insulin responses were measured, and 4 hours postexercise, serum CK activity, glucose, insulin, free fatty acids (FFA), and beta-hydroxybutyrate (beta-HBA) were analyzed. Glycogen, glucose-6-phosphate, citrate synthase, 3-hydroxy-acyl-CoA dehydrogenase, lactate dehydrogenase as well as abnormal polysaccharide and lipid content were measured in middle gluteal muscle samples. Postprandial insulin and glucose response was higher for diet A versus D. Log CK activity was higher with diets A, B, and C versus D. Daily insulin was higher and FFA lower on diet A versus B, C, and D, whereas glucose varied only slightly with diet. Muscle oxidative capacity and lipid stores were low in PSSM horses and muscle glycogen and abnormal polysaccharide content high on both diets A and D. Individual variation occurred in the response of PSSM horses to diets differing in starch and fat content. However, for those horses with clinical manifestations of PSSM, a diet with5% DE starch and12% DE fat can reduce exertional rhabdomyolysis, potentially by increasing availability of FFA for muscle metabolism.