Water is obviously an essential nutrient, and when exercising, water requirements will obviously be increased. For instance, dogs competing in a 300-mile race had a measured water turnover rate of approx. 250mL/kg/day: this represents 5L per day for an average size dog. In comparison, when not racing, water turnover is “only” 0.9L/day.
Exercising dogs must therefore be provided with fresh water frequently during racing to offset the losses and prevent dehydration. The National Research Council suggests that dogs “drink in anticipation of a need when exercising, but not at rest”. Dogs which ran and were offered water immediately after a run drank, but those offered water five minutes after a run did not drink unless they were greater than 0.5% dehydrated. Because of the risk of gastric torsion, it is recommended to wait for the dogs to calm down after exercising before offering them water.
In humans, exercise usually leads to electrolyte (sodium, potassium) loss through sweating, and that is why human athletes often take rehydrating drinks that are enriched in these electrolytes. Dogs are however different: dogs maintain core body temperature via panting rather than by sweating and as such, have less exercise-induced losses of electrolytes. There is no evidence that electrolyte containing products or fluids offer any advantage over rehydrating dogs with water. Most maintenance diets should be sufficient to provide adequate electrolytes for training and performance.