A common concern during a marathon is knowing how much to drink. A general recommendation during exercise is to drink 8-12 ounces every 20-30 minutes. However, the fear of dehydration during prolonged exercise, such as a marathon, has led some to recommend marathon runners to drink as much as they can tolerate. Add to this the notion that runners shouldn't wait until they are thirsty to drink because the brain's thirst mechanism lags behind the body's need to water and you have some marathon runners drinking to the point they become hypontremic (low blood sodium) which can cause all sorts or problems including death (see New England Journal of Medicine, April 2005.)
The increase in the number of hyponatremia in marathon runners over the past years have caused an reevaluation of the drinking recommendations for marathon runners. Dr. Tim Noakes was one of the first to recommend marathon runners to drink fluids ad libitum or as they felt the need to drink. But, wouldn't this led to dangerously high body temperatures? Noakes argues that other factors such as running speed, body size, and environmental factors play a larger role in causing hyperthermia. (see Clinical Journal of Sports Medicine, September, 2003.)
(My heart rates during the 2002 Wash. D.C. marathon. The increase at the end was not due to me running faster but a compensatory response to being dehydrated and having less blood volume to pump. Yes, the miles are off. It wasn't a 28 mile marathon.)
The American College of Sports Medicine agrees. They recommend drinking enough during a marathon to prevent losing more than 2% body weight through dehydration and this can likely be accomplished by drinking, according to thirst, 400 ml (13.5 oz) to 800 ml (17 oz) every hour. (see Medicine and Science in Sport and Exercise, Feb. 2007).
Just like the recommendation to drink 8 glasses of water a day is a myth (see British Medical Journal, December, 2007), so is drinking more than your thirst dictates during a marathon.