Nearly half of recreational runners may be drinking too much fluid during races, according to a survey of runners by Loyola University Health System researchers.

Expert guidelines recommend runners drink only when thirsty. But the Loyola survey found that 36.5 percent of runners drink according to a preset schedule or to maintain a certain body weight and 8.9 percent drink as much as possible.

Nearly a third of runners (29.6 percent) incorrectly believe they need to ingest extra salt while running. And more than half (57.6 percent) say they drink sports drinks because the drinks have electrolytes that prevent low blood sodium. In fact, the main cause of low sodium in runners is drinking too much water or sports drinks. "Many athletes hold unscientific views regarding the benefits of different hydration practices," researchers concluded. The study was published in the June, 2011, issue of the British Journal of Sports Medicine.

Drinking too much fluid while running can cause a potentially fatal condition called exercise-associated hyponatremia. It occurs when runners drink even when they are not thirsty. Drinking too much during exercise can dilute the sodium content of blood to abnormally low levels.

Drinking only when thirsty will prevent overconsumption of fluids. "It's the safest known way to hydrate during endurance exercise," said Loyola sports medicine physician Dr. James Winger, first author of the study. Symptoms of hyponatremia can include nausea, vomiting, headache, confusion, loss of energy, muscle weakness, spasms or cramps. In extreme cases, the condition can lead to seizures, unconsciousness and coma.

In recent years, there have been 12 documented and 8 suspected runners' deaths from hyponatremia, said Loyola exercise physiologist Lara Dugas, PhD, a co-author of the study.

The International Marathon Medical Directors Association recommends that runners drink only when thirsty. The Loyola researchers surveyed 197 runners who competed in the 2009 Westchester, Il. Veterans Day 10K and 5K runs and two other runs on Chicago's lakefront.

The 91 male runners, on average, had been running for 13 years and had run an average of 1.9 10K races and 0.9 marathons. The 106 women, on average, had been running 8.3 years and had run an average of 1.3 10K races and 0.7 marathons.

In the survey, the runners generally said advertising by sports drink manufacturers had little or no influence on their beliefs. But the behaviors of many of the runners indicate otherwise.

During the 1980s and 1990s, sports drinks ads warned about the supposed dangers of dehydration, and recommend that runners drink as much as 1.2 liters (five cups) per hour. Sports drink manufacturers generally have stopped promoting overdrinking. But the unscientific beliefs persist that runners should drink as much as they can or according to a preset schedule.

"We have been trained to believe that dehydration is a complication of endurance exercise," Dugas said. "But in fact, the normal physiological response to exercise is to lose a small amount of fluid. Runners should expect to lose several pounds during runs, and not be alarmed."

Winger is an assistant professor in the Department of Family Medicine and Dugas is a research assistant professor in the Department of Preventive Medicine and Epidemiology at Loyola University Chicago Stritch School of Medicine. The third co-author is Jonathan Dugas, PhD, director of clinical development at The Vitality Group.