Blood-based assessment of oxidative stress, inflammation, endocrine and metabolic adaptations in eventing horses accounting for plasma volume shift after exercise

BackgroundAfter submaximal exercise, blood values of eventing horses show physiological reactions.ObjectivesThis prospective longitudinal study investigated blood parameters in 20 elite eventing horses before and after two-four-star cross-country rides.MethodsUsing a mixed model adjusting for plasma volume shift, we assessed exercise-dependent parameters and compared blood values with reference ranges for healthy horses at rest.ResultsFollowing exercise, cortisol, triiodothyronine (T3) and thyroxine (T4) showed short-term increases, and superoxide-dismutase showed a small short-term increase. Hepatic values showed short-term (haemoglobin [HGB], globulins) or sustained increases (bilirubin, glutamate dehydrogenase, alanine aminotransferase). Digestion-related parameters showed small short-term increases (alpha-amylase, triglycerides) or decreases (cholesterol, DGGR-lipase), apparent through plasma shift adjustment. Zinc decreased in the short term, and iron showed a delayed decrease. White blood cell count increased persistently after training, whereas serum amyloid A remained unchanged.ConclusionsExercised eventing horses had consistently elevated HGB and cortisol levels 10 and 30 min after submaximal exercise, exceeding the reference ranges for healthy horses at rest. Exercise activates the hypothalamic-pituitary-adrenocortical and hypothalamic-pituitary-thyroid axes. Antioxidant activity was observed. Increased energy requirements led to the mobilization of energy reserves, and a sustained increase in liver enzymes indicated hepatocellular injury. Mild haemolysis suggested increased muscle metabolism, whereas signs of inflammation were subtle. Further research is needed to identify which horses deviate from mean values. Graphical Abstract: After submaximal exercise (two-four-star cross-country rides), trained eventing horses showed consistently elevated HGB and cortisol levels, above the reference ranges for healthy horses at rest. However, physiological changes also occur within the reference ranges. Hormonal axes are activated, and there is antioxidant activity. A sustained increase in liver enzymes suggests hepatocellular injury. Signs of haemolysis reflect increased muscle metabolism. image