How much should you drink during a marathon or ultra? Debra Wein, MS, RD, LDN summarizes the latest findings in this article published in National Strength and Conditioning Association Journal. Wein contends that the current American College of Sports Medicine position statement on fluid intake during exercise exceeding an hour (600-1200 ml per hour) may actually be too high for ultra athletes performing at a low intensity or smaller athletes with lower metabolic and sweat rates.
Recommendations for the Ironman distance:
500-800 ml per hour during the bike portion (500 ml is slighly more than 16 ounces)
300-500 ml per hour during the run portion (300 ml is about 10 ounces)
(lightweight athletes even less)
.3-.7 grams of sodium per liter (.5 grams equals .017 ounces)
The obvious problem with this article is failure to take into account the environmental factors that influence sweat rates, namely heat, humidity, wind, and direct sunlight.
It's a good practice for runners to always weigh themselves before and after long runs, and keep track of how much fluid is consumed- the goal is to always try to keep weight loss less than 2% of total body weight.