The three groups, by time, and their weight loss:
Those who finished under three hours averaged a 3.1% body weight loss.
Those who finished between three and four hours lost 2.5%.
Those who took more than four hours to finish lost 1.8%.-Weight Loss During Marathon Improves Running Time
MY COMMENT: Ok, this is exactly the opposite of you would expect, but Tim Noakes, an expert on hydration and heat related problems, has been on the trail of optimal hydration during marathon and ultra events for several years and knows his data.
What I'm wondering about is what the runners weighed just prior to the start of this marathon compared to their normal weight (I assume when you are optimally hydrated and carb. loaded you are a 2-3 pounds heavy, which makes this finding even more amazing! I'd also like to know the average body weight of runners in each group- my guess is the faster runners were lighter, yet they lost the highest percentage of total weight. (note: I'm too cheap to pay for access to the orginal article published in the British J of Sports Med)
Noakes is clear on this: "Drinking either more or less than to thirst impairs exercise performance".