How many runners think its a good idea to chug extra water in the days leading up to a marathon?
According to an article in the December issue of Running Times Magazine by Calista Harbaugh and Mark Knepper, M.D., Ph.D., drinking too much water supresses the production of the hormone vasopressin. Vasopression, which acts as an antidiuretic by helping the body to preserve water, is typically produced when the body is dehydrated. When the body is in a constant state of overhydration, vasopressin does not get produced, resulting in a depletion of aquaporin-2, a "water channel" involved in the transport of water back to the blood.
In layman terms, when you drink too much water over time, your kidneys may not able to conserve water as efficiently when you begin to dehydrate.
Obviously more research needs to be conducted on this subject, but the authors suggest routine sweat-producing workouts and avoiding over-hydration between races.