Training- and competition-related changes on circulating adrenocorticotropic hormone and cortisol response in Thoroughbred Racehorses.

Training and competitive racing sessions cause different changes in stress hormone levels in Thoroughbred racehorses, with racing triggering stronger hormonal responses before and after exercise compared to training. This study explored how exercise type, age, and sex affect the horses’ adrenocorticotropic hormone (ACTH) and cortisol levels.

Background and Purpose

• Exercise challenges the body and activates the hypothalamic-pituitary-adrenal (HPA) axis, which regulates stress hormones.
• The HPA axis releases adrenocorticotropic hormone (ACTH) and cortisol, key indicators of physiological stress.
• This study aimed to see if different exercise modes—training versus competitive racing—cause distinct HPA axis activations in Thoroughbred horses.
• Researchers also investigated if age (2 vs. 3 years old) and sex (male vs. female) influenced these hormonal responses.

Methods

• Subjects: 10 healthy Thoroughbred racehorses (6 females, 4 males); 4 were 2 years old, 6 were 3 years old.
• Exercise sessions: Each horse participated in two conditions:
  • Training session
  • Competitive race session
• Sampling times: Blood samples were collected at three time points:
  • At rest (baseline)
  • 5 minutes post-exercise
  • 30 minutes post-exercise
• Analytes measured: Circulating levels of ACTH and cortisol.
• Data analysis: One-way ANOVA was used to compare hormonal changes relative to baseline, assessing the effects of exercise type, age, and sex.

Results

• Both training and racing sessions significantly increased ACTH and cortisol levels at 5 and 30 minutes post-exercise compared to baseline (P < .001).
• Racing induced significantly larger increases than training:
  • ACTH increases were higher at baseline by 64%, at 5 minutes by 110%, and at 30 minutes by 283% after racing compared to training.
  • Cortisol increased more after racing by 89% at 5 minutes and 128% at 30 minutes post-exercise compared to training.
• These findings suggest an anticipatory hormonal response before racing, indicated by elevated ACTH levels at baseline.
• No specific effects of age or sex were reported, implying the main differences were driven by the exercise type.

Conclusions and Implications

• Competitive racing triggers a stronger HPA axis activation than training, evident both before and after exercise.
• The heightened baseline ACTH before racing indicates a preparatory or anticipatory stress response, possibly due to factors like excitement or psychological arousal.
• Understanding these hormonal dynamics helps in managing horse welfare and performance, as excessive stress responses may affect health and racing outcomes.
• Further research could explore how managing pre-race stress might optimize horse well-being and performance.