I have talked before about hydration in a run, particularly long runs of an hour or more. I also have blogged about the dangers of hyponatremia, or water intoxication, which can occur because of consumption of too much water without replacement electrolytes during a run. So how much fluid should you drink during a run?
USA Track & Field urges runners to consume 30 ounces of fluid before a race, 17-20 of those ounces taken 2-3 hours before the race, and the remaining 10-12 just before its start. However, in truth, what you need to take in depends upon you....not some general guideline. Everyone's body functions differently. (Is that a no-brainer statement or what?) A good suggestion is the following.
Dr. John Cianca, the medical director in 2008 for the Chevron Houston Marathon, (and an accomplished distance runner himself) states that individuals should determine their own “sweat rate,” as it is the liquids lost due to sweat that need to be replaced. Dr. Cianca's test involves running for an hour—without drinking anything—under temperature conditions similar to those in which you will be running, and weighing yourself before and after the run. For every pound you lost in sweat, plan to drink 16 ounces of fluid, “preferably with sodium,” during every hour of your marathon run. The sodium is the electrolyte essential to maintain appropriate hydration balance.