Imagine this: A runner has just begun the Badwater Ultramarathon, the hottest, toughest ultramarathon in the world, which winds 135 miles through Death Valley in the middle of July. After hours of intense racing, she feels incredibly thirsty and quickly gulps down a lot of water at an aid station. Worried about her hydration, she continues to drink water throughout the race. Later, she experiences nausea, swelling in the hands and feet, and an intense headache that cause her to drop out of the race. It turns out this runner has a condition called exercise-associated hyponatremia (EAH), otherwise known as “water intoxication.”

Sodium is an important electrolyte that maintains water balance in the body and allows our nerves and muscles to work. EAH is a condition in which overhydration causes an athlete’s blood sodium levels to become diluted. Often, EAH has no symptoms or causes mild fatigue, nausea and headaches. Severe cases, however, can have life-threatening consequences. Excess water rushes into the cells, causing them to swell and potentially leading to nervous system damage. That can cause confusion, convulsions, brain damage or death. EAH is associated with long races (marathons and other ultra-races) because there is more time and opportunity to hydrate. It’s rarely seen in shorter races.

For any given endurance race, around 20% of athletes may experience EAH. Luckily, treatment for mild cases is simple and easy—resting and eating a salty snack can completely reverse the symptoms. But in severe cases, a medical professional may need to give a saline solution through an IV to return sodium levels to normal.

Many endurance athletes still do not know how to practice proper hydration, which has led researchers to study EAH prevention strategies. Researchers have found that drinking when thirsty—rather than forcing yourself to drink water at each aid station along the race course—is the best way to stay hydrated while minimizing the risk for EAH during a long competition. However, this strategy may be difficult because water availability and the number of rest stations differ on each course. Other studies have shown that it’s not necessary to take sodium supplements while racing, and it may even increase the likelihood of EAH.

The general consensus among experts is that athletes need to be educated to promote safe hydration strategies. Do your research in advance and check your race registration bags (some event organizers provide information). And next time you find yourself racing toward the finish line, take the advice of these researchers and sip your water sparingly.

Kayley Irwin is a recent graduate of the honors physiology program at Michigan State University. She is interested in how health care providers can better tailor injury guidelines and training protocols to improve performance in female athletes.