Here’s the problem about advice regarding proper hydration during a marathon: there’s no one correct and accepted method.
Instead, there are multiple schools of thought on how to properly stay hydrated without being overhydrated. In a recent New York Times piece, there’s a focus on avoiding drinking too much water, which could lead to the condition called hyponatremia. Ingesting more fluid than you lose through sweating or urination could dilute your blood’s sodium levels, which could lead to serious, even fatal conditions. Instead, the article cites recent guidelines from the International Marathon Medical Directors Association to recommend drinking only when you’re thirsty.
This advice probably goes against everything marathoners of all levels have heard. Indeed, a HealthDay article from this weekend offers differing thoughts from a registered dietitian. The recommendation was to stay properly hydrated and consume 16 to 20 ounces of water about 2 hours before the race, drink another 7 to 10 ounces of water 10 to 20 minutes before exercise, and take in about 6 to 8 ounces of water every 15 to 20 minutes during exercise. For activity of longer than an hour, there also should be consumption of a sports drink with 4 to 8 percent carbohydrates.
In the end, it matters what works for you. For me personally, I’ve tried drinking only when thirsty as well as drinking on a regular schedule and gotten dehydrated through both methods. If anyone has a fail-safe method, I’m all ears.
The cases of illness or death from drinking too much water, generally involve people who are on the lower end of the athletic scale. Generally, it is hard to ingest fluids fast enough to replenish the losses from sweat and metabolism in a competitive athlete. To truly know, one would need to run a sweat loss analysis under varying conditions.
The one thing I know from study and experience, is that thirst is a lagging indicator, i.e. you feel thirsty once you are already dehydrated. Couple that with slower uptake than loss rates in tough conditions, I do not wait until I am thirsty to hydrate.