Metabolic effects of training endurance horses in a field setting

Changes in serum biochemistry and skeletal muscle of horses in response to individual endurance rides of varying distance have been reported, as have the effects of submaximal treadmill training. This study evaluated the effects of a controlled field endurance training programme on energy metabolism. Five Arabian horses 6.8 ± 2.0 years (mean ± standard deviation) were trained for 16 weeks over progressively longer distances. The biceps femoris, triceps long head, and semitendinosus muscles were biopsied before and after 4-weekly runs of 10, 20 and 30 miles, respectively, and before and after 10, 20, 30 and 50 miles of a fourth run at the end of the training programme to measure 3-hydroxyacyl-CoA-dehydrogenase (HAD) and succinate dehydrogenase (SDH) activities and triglyceride (TG) and glycogen concentrations in those biopsies. Blood samples were obtained simultaneously to measure serum TG, free fatty acid (FFA), glycerol, glucagon, insulin, and cortisol concentrations. Data were analysed using RM-ANOVA for effects of training and distance. was considered significant. Training increased resting glycogen and TG concentrations in the biceps (, ), triceps (, ) and semitendinosus (, ), respectively. Both HAD and SDH activities increased in the biceps femoris (; ) and triceps (; ), respectively, but not in the semitendinosus. There was no training effect detected on serum hormone responses to exercise distance although serum glycerol (), glucagon () and cortisol () concentrations increased with increasing run distance, while insulin concentration decreased (). Glucagon and cortisol concentrations were correlated (r = 0.67; ). Serum TG decreased with exercise but at a slower rate following training, and serum FFA were higher during exercise following training when compared to the partially trained state. Overall, endurance training enhanced peripheral mobilisation of fat and its utilisation by working muscle, and resting muscle glycogen stores increased.