The day of the marathon, your goal is to prevent dehydration so you stay out of the medical tent! Be sure to drink extra fluids (water, juice, sports drink) the day pre-marathon and then on Marathon Morning, drink up until 2.5 hours before the start. This gives you time to urinate the excess before getting into the corral. (Your kidneys need about 90 minutes to process fluid.) Then, tank up again 10 to 15 minutes before the start of the marathon.
During the marathon, prevent dehydration and hypoglycemia (low blood sugar) by drinking 6 to 8 ounces of sports drink every 15 to 20 minutes, depending on the weather, your body size, your sweat rate, thirst, and pace. Because sports drinks contain carbohydrates (sugar), they can fuel both your muscles and your mind. They invest in greater stamina and endurance.
If sports drinks upset your stomach, drink plain water and nibble on energy bars, hard candies, jellybeans, gels, chomps, or other carbs that you can tuck into a pocket.
Whatever you do, do not OVER-hydrate. If your stomach is sloshing, stop drinking!
Resources: Nancy Clark’s Sports Nutrition Guidebook
Food Guide for Marathoners: Tips for Everyday Champions