Alterations in blood metabolites as biomarkers of fatigue and recovery in thoroughbred horses performing repeated bouts of high-intensity exercise

High-intensity exercise in horses resulting in fatigue requires a better understanding of biomarkers defining the condition such that protocols detailing the return to work can be established. This study examined blood metabolite profiles after multiple sessions of high-intensity exercise to define physiological exhaustion. Adult Thoroughbred geldings (n = 10) underwent a standardized exercise test (SET) on a high-speed treadmill, with BHB, BCAAs, alanine, lactate, and CK measurements at regular intervals up to 6 h post-SET. Before and 24 h post-SET gluteus medius muscle samples were taken. The SET was repeated three times at 48 h intervals. After 8 wk of moderate intensity conditioning, the horses repeated the 3-SET exhaustive exercise procedure. Muscle temperatures and treadmill performance were recorded during each workout. Training improved (P < 0.05) gallop times, energy expenditures, and muscle temperatures post-SET. Post-SET plasma lactate and CK increased (P < 0.05) and returned to baseline in 6 or 24 h. In unfit horses, muscle glycogen content decreased (P < 0.05) 24 h after SET2 and SET3 but remained unchanged after each SET after conditioning. A temporal increase (P < 0.05) in plasma BHB post-SET was noted, regardless of training status, with a cumulative effect on SET3 concentrations (P < 0.05). Post-SET recovery, alternative fuel utilization was associated with a continuous increase (P < 0.05) in plasma Ala concentrations within 1 h of intense exercise, returning to baseline by 6 h. With increasing bouts of exercise, plasma Leu, Ile, and Val concentrations increase (P < 0.05), regardless of fitness level. Plasma CK concentrations remained within normal ranges suggesting sequential bouts of strenuous exercise did not create irreparable muscle damage. During post-exercise recovery, the exhausted horse may use greater amounts of amino acids and BHB providing the foundation for tissue-level research on strenuous exercise and muscle repair.