How Much Should You Really Drink in a Marathon? Expert Breaks Down Fluids, Carbs, and Salt

​Hydration advice for marathoners has long been confusing. According to US Army physiologist and Boston Marathon consultant Samuel Cheuvront, the solution is not a universal rule but a personalized plan built around body weight, pace, and temperature.

​The primary goal is to avoid losing more than 2 to 3 percent of your body weight during the race.

Step one: weigh yourself before and after long training runs.

​If you lose about 1 kilogram of body weight, you should plan to replace roughly 1 liter of fluid over the next several hours.

​If you consistently lose more than 2 percent of your body weight, increase how much you drink per hour during long efforts.

​Before race day, arrive normally hydrated.

Drink about 500 milliliters of fluid 3 to 4 hours before the start. Around 30 minutes before the gun, add another 120 to 240 milliliters to top off your fluid levels.

​During the marathon, most runners should consume approximately 400 to 800 milliliters of fluid per hour. Increase toward the higher end in hot weather or at faster paces.

​Combine fluids with 30 to 60 grams of carbohydrates per hour using sports drinks or gels.

If your sports drink provides 30 grams of carbohydrates per 500 milliliters, adjust your intake to meet your target without exceeding what you have tested in training.

​Sodium needs are usually covered by standard sports drinks. Add extra only in hot, long races or if you notice heavy salt stains on your clothing.