M. Christine Zink, D.V.M., Ph.D.
Q: What is the best way to keep my dog hydrated when he is competing in agility?
A: There is a lot of misunderstanding about fluid balance in active dogs, particularly among people who participate in canine athletic competitions. Some people claim that exercising dogs should be given electrolytes. Others insist that they just need water, but that something tasty like chicken bullion should be added to ensure that the dogs drink. Who’s right?
I think that a lot of our beliefs about dog hydration come from applying our personal experiences to dogs. When we exercise in hot weather, we become hot and dehydrated. If there is one piece of advice people give regarding preventing heatstroke in humans, it is to drink plenty of water. But our physiology is markedly different from dogs.
When people cool themselves by sweating, they lose both fluids and electrolytes (ions such as chloride and sodium) through pores in the skin. Unless they replace the substances that are lost through sweating by drinking water (or if the exercise is extreme, water with electrolytes), the result is dehydration.
But what happens when dogs exercise? A study was performed in which dogs were exercised by trotting on a treadmill continuously until they were exhausted, which took about 90 minutes (Kruk, B. et al. “Hypothalamic, rectal, and muscle temperatures in exercising dogs: effect of cooling.” J. Appl. Physiol. M(5): 1444-1448, 1985). Blood tests taken before and after the exercise showed no difference in blood osmolality, a measure of dehydration. In other words, even maximally exercising dogs doesn’t lead to dehydration. They certainly don't lose enough water via evaporation. This makes sense because the only way that dogs might lose significant amounts of water when exercising is through drool when they pant. If you give an active dog electrolytes, it might actually become more dehydrated because the excess electrolytes in the gastrointestinal tract can draw water from the body into the intestine.
So how should you care for actively exercising dogs? Just offer them cool, fresh water. If they don’t want to drink, they don’t need to drink. It can’t get much simpler than that.