Almost half of recreational runners drink too much fluid during races, showing many athletes hold unscientific views regarding the benefits of different hydration practices .

Expert guidelines recommend runners drink only when thirsty but a recent 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.

Yet runners stated advertising by sports drink manufacturers had little or no influence on their beliefs.

Drinking too much fluid while running can cause a potentially fatal condition called exercise-associated hyponatremia, which 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. In recent years, there have been 12 documented and 8 suspected runners' deaths from hyponatremia.  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.

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.

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.

The normal physiological response to exercise is to lose a small amount of fluid and runners should expect to lose several pounds during runs, and not be alarmed.

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 published in the British Journal of Sports Medicine.