Water intake and fluid shifts in horses: effects of hydration status during two exercise tests.
Authors: Nyman S, Jansson A, Lindholm A, Dahlborn K
Journal: Equine veterinary journal
Summary
# Editorial Summary Nyman and colleagues investigated how pre-exercise hydration status influences both physiological responses during work and voluntary drinking behaviour in the recovery period, using four Standardbred horses exercised on a treadmill under three conditions: normal hydration, 24-hour dehydration (resulting in 3% bodyweight loss), and hyperhydration via 12 litres administered 30 minutes pre-exercise. During incremental exercise, plasma volume contracted most severely in dehydrated horses (−21%), whilst hyperhydrated horses experienced less plasma volume reduction but elevated blood lactate accumulation—likely attributable to the additional weight burden. The critical finding emerged in recovery: horses rehydrated rapidly and completely only when *both* plasma sodium elevation and plasma volume deficit were present simultaneously, as occurred in the dehydrated trials; conversely, horses that were normally hydrated before constant-velocity exercise showed smaller plasma volume changes without osmotic stimulus and drank considerably less in the subsequent hour, despite objective fluid losses. These results carry important implications for practical management: fluid supplementation strategies before competition should account for timing and volume, as excessive pre-exercise hydration may impair lactate clearance without enhancing rehydration drive when it matters most. More significantly, passive rehydration protocols—offering water alone—may prove insufficient unless dehydration has genuinely triggered osmotic thirst mechanisms, suggesting that electrolyte provision, not merely water access, warrants consideration in recovery feeding regimens following demanding work.
Read the full abstract on PubMed
Practical Takeaways
- •Dehydrated horses will drink more eagerly postexercise if both thirst mechanisms are triggered; ensure access to water immediately after work in dehydrated animals
- •Pre-exercise hyperhydration does not prevent plasma volume loss during high-intensity work and increases lactate accumulation; standard hydration before exercise is preferable
- •Monitor both sodium status and plasma volume indicators in dehydrated working horses, as both must be elevated to drive strong postexercise rehydration responses
Key Findings
- •Dehydration (3% bodyweight loss) resulted in greater plasma volume reduction (-21%) during incremental exercise compared to normohydration (-13%) and hyperhydration (-11%)
- •Combined osmotic (increased plasma sodium) and hypovolemic (decreased plasma volume) thirst stimuli triggered rapid postexercise rehydration, while either stimulus alone resulted in slower fluid intake
- •Constant velocity exercise induced similar absolute fluid loss (3%) across all hydration states, but plasma volume changes varied significantly (HH: -7% vs DEH: -19%)
- •Plasma aldosterone concentration increased uniformly within 10 minutes of exercise onset regardless of hydration status or exercise type