Water intake and fluid shifts in horses: effects of hydration status during two exercise tests.

The research explores factors that influence how much water horses drink post-exercise. The study, which involved four Standardbred horses, considered three scenarios: normal hydration, dehydration for 24 hours before exercise, and hyperhydration prior to exercise. The study found that horses drank water immediately after exercise when they had been both dehydrated and had an increased plasma sodium concentration.

Study Design and Methodology

• The study involved four Standardbred horses, each having a mean body weight of around 500kg. The horses were subject to two exercise trials: an incremental exercise test (INCR) and a 40-minute constant velocity exercise test (CONST), both carried out on a treadmill.
• Three hydration scenarios were considered before the exercise tests: normal hydration (N), dehydration for 24 hours before exercise (DEH), and hyperhydration where horses were given 12L of water 30 minutes before the workout (HH).

Results and Findings

• Dehydration led to a bodyweight loss of 3%, and signs of fluid uptake were observed before exercise in both HH trials.
• By the end of the INCR test, the calculated change in plasma volume (PVcalc) was -13, -21, and -11% in the N, DEH, and HH trials, respectively.
• During the highest exercise velocities, a hypotonic shift of fluid was seen in all INCR trials, indicating a movement of fluids from within cells to outside due to changes in sodium concentration.
• Hyperhydration led to a greater build-up of plasma lactate probably due to the extra weight to be carried.
• All trials led to similar fluid loss (3%) during the CONST test, but the decrease in PVcalc was significantly smaller in hyperhydrated horses (-7) than in normally hydrated ones (-14) and dehydrated ones (-19).
• In both the DEH-INCR and DEH-CONST trials, plasma sodium concentration (pNa) was higher than in N until drinking water was offered one hour post-exercise.
• The study observed that horses drank water immediately when offered post-exercise when they were dehydrated and had higher plasma sodium concentration.
• Rehydration was slower in the N-CONST trial, where there was a significant decrease in calculated PVcalc but no increase in plasma sodium concentration when water was provided.
• Plasma aldosterone concentration (a hormone that helps regulate sodium and potassium balance in the body) increased to the same level in all trials after around 10 minutes, irrespective of the type of exercise or hydration status.

Conclusion

• The researchers concluded that the presence of both an osmotic and hypovolemic thirst stimulus led to more rapid rehydration post-exercise. This means that when horses experience dehydration-related thirst (hypovolemic) and thirst due to high sodium concentration in blood (osmotic), they tend to drink water immediately when offered after exercise, rehydrating more quickly.