Does Training Impact Performance? Unveiling the Link Between Exercise and Inflammation in Horses
"Explore how training affects inflammation and fitness in Thoroughbred racehorses, and what it could mean for equine athletes."
For years, exercise has been known to trigger a cascade of changes within the equine body, particularly influencing oxidant and antioxidant balances. Of increasing interest is the role of plasma myeloperoxidase (MPO), a key indicator of neutrophil activity and potential disruption of the delicate oxidant/antioxidant equilibrium. Neutrophils, a type of white blood cell, increase after exercise and subsequently release MPO, making it a marker worth investigating.
Increased plasma MPO concentrations have been observed following exercise in various equine disciplines, including eventing, endurance riding, and even in Standardbred trotters. These observations have led researchers to wonder whether training impacts the magnitude of the MPO response or if the timing of sample collection in relation to exercise influences the results.
To address these knowledge gaps, a recent study delved into the dynamics of plasma MPO concentrations in Thoroughbred racehorses. The goal was to evaluate MPO levels at different time points before and after high-intensity exercise, assess the impact of training on these levels, and explore the relationships between MPO concentrations, physiological measurements, and racing performance.
Training and Plasma MPO
Exercise in horses induces neutrophil degranulation and subsequent increases in plasma myeloperoxidase concentrations. The effect of training and the timing of sampling in relation to exercise were not known. A study published in 2016 evaluated plasma MPO at different time points in Thoroughbreds before and after high-intensity training.
Measuring Equine MPO
Myeloperoxidase is a protein of interest because of its involvement in equine pathologies. Earlier equine diagnostic research focused on detecting extracellular MPO, while cellular MPO content in neutrophils offered additional information. Researchers developed SIEFED, or specific immunological extraction followed by enzymatic detection, to measure active equine MPO while removing proteins and interfering molecules.
MPO in Peroxidase Research
Peroxidases are enzymes that catalyze the oxidation of various electron donors using hydrogen peroxide. The historical overview discusses myeloperoxidase alongside eosinophil, lactoperoxidase, and thyroid peroxidases, which share important structural and functional similarities. It also describes how products of MPO and hydrogen peroxide can react with a wide range of soluble molecules.
Key Findings: MPO, Training, and Thoroughbreds
The study tracked 26 Thoroughbred racehorses in active training. Over nine months, the horses underwent exercise tests on a high-speed treadmill at the beginning, middle, or end of the training period. Researchers carefully monitored heart rate, speed, and distance, and collected blood samples at specific times: before exercise (To), at maximal speed (TVmax), 5 minutes post-exercise (T5min), and 4 hours post-exercise (T4h).
- White Blood Cell Count (WBC): WBC increased above baseline (To) at all sampling points, with no discernible training effect.
- Neutrophils (Neut) and %Neut: Both Neut and %Neut increased above baseline at T4h. Notably, values at TVmax, T5min, and T4h increased as training duration progressed.
- Plasma MPO Concentration: MPO levels increased above baseline at all sampling points in response to exercise. All values increased as training duration increased.
- Training Intensity: Training intensity (number of fast work sessions) did not significantly affect any of the measurements.
- Correlations: The study did not identify significant correlations between biological markers (including MPO) and physiological/training measurements. Also, no significant differences were found between elite and non-elite horses.
Exercise, Inflammation, and Laminitis
Recent work emphasizes that the key question is not simply whether training or competition triggers an acute inflammatory response, but when that response benefits or harms the horse. Research on endocrinopathic laminitis found increased myeloperoxidase, especially in the dermal lamellae, in histologic sections from pEHC-treated horses. The findings suggest that neutrophil MPO release may contribute to the pathophysiology of endocrinopathic laminitis.
Limits of Whole-Blood Models
An in vitro whole-blood model was developed to test how different stimulus conditions affect the release of myeloperoxidase and elastase by equine neutrophils. Because the model examines responses in vitro, its results concern controlled blood-based stimulation rather than the full physiological experience of a trained or competing horse. This limits how directly such findings can be used to explain inflammation during exercise.
Training Responses Across Studies
Racehorse research collected blood before exercise and 30 minutes afterward to assess baseline and exercise-induced physiological responses. In moderately exercised horses, TBARS generally remained at the pre-exercise baseline of 2.7 µM malondialdehyde, although it increased by 0.24 to 0.41 µM on day 2 of week 2 and stayed elevated through day 3. A separate competition study reported mean blood lactate concentrations of 15.8 +/- 5.8 mmol/l after the CIC* and 2.7 +/- 0.2 mmol/l after the 160 km course, illustrating differences between exercise settings.
The Bottom Line: MPO as a Potential Fitness Indicator
The study concludes that plasma MPO concentration is influenced by exercise and training duration, paving the way for future investigations into MPO as a potential tool for assessing fitness and readiness for competition in horses. Further research could explore the factors that influence MPO responses, helping trainers and veterinarians optimize training programs and monitor the well-being of equine athletes.
MPO as an Exercise Marker
Human exercise studies have shown neutrophil degranulation, respiratory burst activity, and increased plasma myeloperoxidase. The cited equine research asked whether the same response occurs in horses. This makes plasma MPO a relevant measure for investigating exercise-related neutrophil activity, while the horse-specific response requires direct study rather than assumption from human findings.
Future Directions in Equine Research
The 2025 Horse Species Symposium brought together experts from universities, government agencies, and industry. Discussions addressed the current state and future direction of equine research and practice. The symposium also focused on challenges and opportunities for promoting a sustainable field of equine industry research.
Fitness, Readiness, and Inflammation
The broader discussion examines how exercise intensity and training duration influence inflammation markers in horses. It highlights myeloperoxidase as a marker relevant to assessing fitness and readiness for competition. This framing connects training decisions with the interpretation of inflammatory responses in equine athletes.