What You Need to Know About Runner Hydration

The Over and Under of Hydration

A serious shortage of fluids can cause problems. Towards the end of every marathon or ultra- marathon, there are always runners staggering into the medical tents, suffering from nausea, diarrhea, and weakness caused by dehydration.

"It's pretty common for athletes to hit at least one or two percent dehydration during endurance events," says Craig Horswill, Ph.D., senior research fellow at the Gatorade Sports Science Institute. "The body's temperature-regulating mechanism is affected even at one percent dehydration."

Even small amounts of dehydration should be avoided because it will affect performance. It is recommended to avoid more than 2% dehydrated. For a 130-pound woman, that would mean losing no more than 2.6 pounds of fluid during any run.


At the other end of the spectrum, overhydrating can be even more dangerous than not drinking enough. Hyponatremia occurs when your fluid intake exceeds your rate of fluid loss from sweating, which results in low blood-sodium levels.

Symptoms—nausea, disorientation, muscle weakness—can be similar to dehydration. Giving additional liquids to hyponatremic runners only exacerbates the problem by diluting their blood-salt levels even more, which can lead to coma and, in the worst cases, death.

Experts hypothesize that hyponatremia has become more of an issue in the past few years because so many beginning runners are attempting marathon and ultramarathon distances. This means people are on the course for five or six hours, or even more. "Slower, back of the pack runners who aren't sweating as much don't need to replace so much fluid," says William O. Roberts, M.D., past president of ACSM and long-time medical director of the Twin Cities Marathon.

Women, smaller runners, slower runners, and those who are not as well trained face the greatest risk of hyponatremia.

The idea that every runner needs to down a cup at every aid station can be a dangerous one, warns Dr. Maharam, because such a rigid fluid-replacement strategy doesn't account for differences in body size, running pace, terrain, climate, metabolic rate, and sweat rate.

With the Halo Edge you can make sure you stay properly hydrated.