Reasons for performing study: Recurrent exertional rhabdomyolysis (RER) occurs in fit, nervous Thoroughbreds fed high nonstructural carbohydrate (NSC) diets. Clinical signs are diminished by feeding low NSC, high fat diets; however, the mechanism is unclear.Objective: To determine if the glucose, insulin and cortisol response to isocaloric diets varying in fat and NSC availability differ in fit vs. unfit Thoroughbreds with RER.Materials and methods: Four fit (10 weeks treadmill training) RER Thoroughbred mares were exercised and fed 3 isocaloric (121 MJ/day) diets in a 5 day/diet block design. Two high NSC concentrates, sweet feed (SF) and a processed pelleted feed (PL) and a low starch high fat feed (FAT) were used. After 24 h of rest and a 12 h fast, horses ate half their daily concentrate. Blood sampled for [glucose], [insulin] and [cortisol] was obtained before, immediately after and at 30-60 min intervals for 420 min. After 3-6 months detraining period, the block design was repeated. Results: Results for SF and PL were similar. Regardless of diet, cortisol was higher in fit vs. unfit horses. Fit horses on SF/PL had higher post prandial [insulin] and insulin:glucose ratio than unfit horses. FAT resulted in lower post prandial [glucose] and [insulin] vs. SF/PL. Higher [insulin] in fit vs. unfit horses was not seen on the FAT diet.Conclusions: Increased post prandial [glucose], [insulin] and [cortisol] induced by high NSC, but not high fat, feeds are enhanced by fitness in RER horses. This combination may trigger rhabdomyolysis through increased excitability in RER Thoroughbreds.