[This, forwarded by Don Bell, is from the Gatorade Institute]

Hydration Recommendations for Marathoners

Marathoners know that proper training, tapering, nutrition, and pacing are all part of a successful marathon. The same is true for hydration.

During training and competition, fluid lost in sweat, urine, and respiration contributes to dehydration. Fluid loss - particularly from sweating - varies widely among runners. Some runners are light sweaters and lose relatively little fluid each hour. Others sweat a lot and can dehydrate quickly. Regardless of whether you're a light or a heavy sweater, your sweat loss can change dramatically from one race to another depending on your fitness, the environmental conditions, your pace, your heat tolerance, your clothing, and your hydration status.

So what's a marathoner to do about hydration?

The simplest advice is to drink enough during a race to minimize dehydration (weight loss), but avoid over-drinking (weight gain) that can increase the risk of hyponatremia (low blood sodium). But how much is enough? That depends on how much sweat you're losing. There's no way to measure that during a race, so it's essential to gauge your fluid needs during training. That's easily done by stepping on a bathroom scale before and after long training runs. If you lose more than 2% of your body weight (e.g., 3 lb for a 150-lb runner), increase your fluid intake the next time out. If you've gained any weight at all, cut back in future sessions. It's a good idea to record your weight-change values in your training log - along with the weather conditions - so you can refer to them when needed. After some trial and error, you'll become good at gauging your hydration needs.

To make sure that your hydration plans work for you, here are some things to keep in mind …

* It's wise to stay well hydrated during the days leading up to a race, but remember that you're a human, not a camel, so don't quaff large volumes. And heed the color of your urine; if it's light yellow, like lemonade, that's usually a sign of good hydration. Crystal clear urine often indicates over-hydration and the need to cut back.
* If you are a heavy sweater or if you finish races and training sessions with your skin caked with white residue, your diet should contain enough salt to replace those losses. Salting your food to taste will usually suffice. Favoring sports drinks over water during long runs will also help.
* If you are expecting to sweat a lot during the race, it's a good idea to head to the starting line with some fluid already in your stomach. No need to overdo it though. Ingesting 4 - 20 oz of water or sports drink within 30 minutes of the start should suffice, depending on your body size (smaller runners typically need smaller volumes).
* During a race, drink small volumes at regular intervals. Marathoners who lose little sweat might only have to drink 14 oz (400 ml) each hour - roughly 3 oz (100 ml) every 15 minutes. Marathoners who sweat a lot might require four or more times as much. That wide range is why it's essential to gauge your hydration needs during training. You're unique, so don't copy what others are doing.
* You can choose to drink both water and sports drinks during the race. Sports drinks contain the water, carbohydrate, and electrolytes your body can use to make your race easier.
* Be ready to alter your fluid intake based on race conditions. If it's hotter or colder than expected, adjust your hydration needs accordingly. Same is true if you find yourself running slower or faster than expected. Slower paces generally mean less sweat loss and therefore less fluid intake.
* At the Boston Marathon, you'll have a chance to weigh yourself before, during, and after the race. Do so to determine if your pre-race weight has changed markedly (up or down). If it has and you're not feeling well, head to the med tent.
* After the race, have something to drink if you're thirsty. There's no rush to rehydrate unless you are severely dehydrated. You'll have plenty of opportunity in the hours after the race to restore your fluid balance. If you feel that you've hydrated properly during the race and you're still not feeling well (headache, nausea, upset stomach, bloating in hands or feet, wheezy breathing), do not drink until after you've urinated. If those symptoms persist, seek medical attention.