'All serious distance athletes should find out what their rate of fluid loss is and individualize their fluid intake prior to a distance event. It's also good to accept some mild dehydration during a long race. There are plenty of Web sites available now that show how to customize your fluid intake.'

Dr. Benjamin Levine, professor of internal medicine at UT Southwestern Medical Center and author of the article, said competitive runners are less likely to suffer from hyponatremia.

"Those who are running to finish the race very fast don't have time to drink a lot of water along the way," Dr. Levine said. "Those who are not running the race competitively tend to stop at every water station and take a drink. Over the course of a long race, they can dilute themselves."

Individualized Fluid Replacement Recommended

Popular sports drinks don't always include enough sodium to offset the body's loss of the mineral during exercise. The drinks often carry more water with smaller concentrations of salts than are normally found in the human body; therefore, they do not replace salts adequately, according to Dr. Levine, who also is medical director of the Institute for Exercise and Environmental Medicine, a collaboration between UT Southwestern and Presbyterian Hospital of Dallas.

Dr. Levine's article accompanies a study in the same journal that evaluates the blood concentration of sodium in runners both before and after a long race and examines their risk factors for developing hyponatremia. It recommends individualized fluid-replacement consumption by all competing athletes.

The researchers at Children's Hospital in Boston and Harvard Medical School who conducted the study "found a surprisingly large number of runners had actually gained weight during the race and their sodium concentrations were very low -- some were dangerously low," Dr. Levine said. "The recommendations listed in the study that fluid-replacement schedules be individualized for all athletes competing in long-distance events should be taken seriously by all competitors."

Salty Snacks May Help

People lose water and salts from their bodies at different rates during exercise, explained Dr. Levine. Heat and humidity also play a role in the rate of loss. Calculating fluid loss is as simple as weighing yourself before and after exercise and comparing that number to the amount of fluid you consumed throughout.

"All serious distance athletes should find out what their rate of fluid loss is and individualize their fluid intake prior to a distance event," Dr. Levine said. "It's also good to accept some mild dehydration during a long race. There are plenty of Web sites available now that show how to customize your fluid intake."

Taking along salty snacks to eat during the race is a good way of combating hyponatremia, he added. Generally, athletes of all types are instructed prior to activities that water consumption is necessary to prevent illness from heat and to maintain performance levels.

It is also clear, however, that fixed global recommendations for fluid replacement may not be optimal for individual athletes of different body types and with varying degrees of training and heat acclimatization.