During exercise, nobody disputes the fact that significant dehydration impairs performance and can lead to life threatening heat stroke. Knowing this, it is common practice among endurance athletes to keep their tanks full by drinking plenty of water, especially in the hours just prior to and during exercise. That still makes perfectly good sense. The question is, how much water should he or she drink throughout the day routinely? Should the athlete keep their body's water supply topped off despite making frequent trips to the bathroom? There are some who think this practice actually may be working against them.
The body regulates fluid by producing vasopressin, an anti-diuretic hormone which signals the kidneys to preserve water. When the body becomes dehydrated, the concentration of sodium in the blood increases, triggering the production of vasopressin, which in turn signals the kidneys to conserve more water. If you drink more water than your body actually needs, blood sodium levels drop. Less vasopressin is produced, and rather than conserve water, the kidneys increase production of urine. It is a constant balancing act, with vasopressin playing a critical role. You can see the potential problem for somebody that pushes water all day long, even when they don't apparently need it or aren't thirsty. No vasopressin when you might need it, as in during a long, hot race.
According to an article published in the December 2007 issue of Running Times, vasopressin is required by the kidneys to produce aquaporin-2, responsible for transporting water molecules from the kidneys back into the bloodstream. When you drink too much water and vasopressin levels drop, aquaporin-2 disappears. Overhydrating on a daily basis may cause your body to essentially become less effective at conserving water, not something a runner needs going into an important race.
To avoid this problem, the suggestion in Running Times is for runners to regularly perform sweat producing workouts, making sure not to OVER hydrate between workouts and races. The theory is that athletes who undergo periods of moderate dehydrate condition their kidneys to conserve water more efficiently, decreasing their requirement for fluid replacement during competition.
So what is an athlete supposed to do? Weigh yourself in the morning and before and after each workout. Monitor weight to make sure fluids are being replaced after a workout, but once weight is back up to normal and urine is being produced, there is no need to continue drinking excessively, unless you are thirsty.