How much hydration is enough, and how much is excessive? Researchers at the University of Illinois at Urbana-Champaign recently found that hyperhydrating (giving extra fluids to) equine athletes before a simulated cross country day of a three-day event did not cause arterial hypoxemia (low blood oxygen levels) in the horses. These findings were contrary to another study that showed that pre-exercise hyperhydration caused “arterial hypoxemia (deficient oxygenation of blood) in horses performing moderate intensity exercise simulating the second day of an equestrian 3-day event competition, which may adversely affect performance,” said the authors of the current study in their abstract, which appeared in the European Journal of Applied Physiology.

“Dehydration and the associated impairment of cardiovascular and thermoregulatory function comprise major veterinary problems in horses performing prolonged exercise, particularly under hot and humid conditions,” said the researchers. “For these reasons, there is considerable interest in using pre-exercise hyperhydration to help maintain blood volume in the face of the excessive fluid loss associated with sweat production during prolonged exertion.”

In 2002, Australian researchers published a study that suggested hyperhydration could have adverse effects by causing arterial hypoxemia. “These findings are contrary to data from horses performing short-term maximal exertion, wherein hyperhydration did not affect arterial oxygen tension/saturation,” said the University of Illinois researchers in their current study.

The Illinois researchers carried out control and hyperhydration studies on seven healthy Thoroughbreds in random order, seven days apart. During the control study, horses received no medications, while they received nasogastric NaCl (a saline solution) during the hyperhydration trials. Arterial blood CO2 tension, pH, and blood lactate concentrations were not affected by pre-exercise hyperhydration. They concluded that “pre-exercise hyperhydration of horses…is not detrimental to arterial oxygenation of horses performing an exercise test simulating the second day” of a three day event