Hydration Confusion: How Much Is Too Much?

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With a recent study about over-hydrating in the New England Journal of Medicine catching so many headlines, don't feel bad if you're more confused than ever about how to hydrate during long-distance events.

After being told for years to drink up to prevent dehydration, now athletes are being warned about the dangers of drinking too much.

The NEJM study tracked 488 participants in the 2002 Boston Marathon and found 13 percent finished the race hyponatremic, when blood sodium concentration drops so low it can cause severe brain dysfunction, including seizures, coma and even death.

At greatest risk were extremely thin runners who gained a lot of weight during the race (4.5 to 11 pounds) and finished in more than four hours (longer finishing times mean more time to take in fluid).

Women at Greater Risk

Although the researchers didn't find gender, in itself, to be a significant risk factor (unlike what earlier studies had suggested), they did determine that body size matters. And since women tend to have less body mass than men, they may be more susceptible to hyponatremia than men are.

"I personally think it's purely a size effect. Women are more likely to develop a fluid overload simply because it takes less fluid for smaller people to become overloaded," says Lewis G. Maharam, M.D., the medical director for the Rock 'N' Roll marathons and the ING New York City Marathon.

Maharam also warns that taking pain medications such as ibuprofen (Advil, Motrin IB, Nuprin) and naproxen sodium (Aleve) during endurance events may increase the risk of hyponatremia. "A big part of the problem is the inability of the athlete to excrete the excess fluid because of high levels of fluid-retaining hormones ADH and AVF." NSAIDs (non-steroidal anti-inflammatory medications) decrease blood flow to the kidneys, thereby increasing ADH and AVF.

Know Your Sweat Rate

The answer to successful and safe hydration is balance. Keep in mind, dehydration is still one of the biggest risks in endurance exercise, but the key to performing at your best is drinking smart, not gulping liquids.

The best way to avoid drinking too much or too little is to take in about the same amount of fluid as you sweat out. Here's how to figure out your sweat rate:

  1. Weigh yourself without shoes or clothes, and record it in your log, noting temperature and humidity.
  2. Work out for one hour.
  3. Dry off and weigh yourself, noting the weight lost. Also note any fluids you may have taken while running.
  4. Use the following formula to determine your hydration needs: One pound lost = 16 oz. of fluid.

So, if you lost two pounds in that hour, you should replenish 32 ounces of fluid (about 8 ounces every 15 minutes) in the same temperature and humidity level.

Do this test a few times in different conditions to get a sense of how your hydration needs change in varying temperatures and humidity levels, and intensity levels.

Tips for Smart Hydration

To make sure you meet, but don't exceed, your hydration needs:

  • Set your watch alarm to remind you when to drink.
  • Aim to replenish 80 to 100 percent of fluids lost.
  • Avoid gaining weight (a sign of over hydrating).
  • Consume a sports drink with electrolytes (sodium, potassium, etc.) during your training sessions and in races longer than 60 minutes. Plain water is fine for workouts shorter than 60 minutes, but longer sessions require a sports drink to replenish electrolytes and energy.
  • Flavor your meals with salt right before long workouts and races to boost electrolyte levels.
  • Avoid drinking more than usual during race week. This will dilute your blood sodium levels, putting you at higher risk of developing hyponatremia. Your fluid needs drop during this taper week. So, drink normal amounts of fluid and use the urine test to determine if you've had enough: If it runs pale yellow, you're well hydrated and ready to race.