April 13, 2005 -- One in three marathon runners drinks more fluids than she or he needs, a study of Boston Marathon runners shows.
In the 2002 Boston Marathon, one female runner died because her body lost too much salt, a condition known as hyponatremia. Many of her race mates risked the same fate, find Christopher S.D. Almond, MD, MPH, and colleagues.
Almond's team got blood samples and other data from 488 women and men who ran the 2002 Boston Marathon. They found that 13% of the runners had low sodium levels. And three of the 488 runners analyzed had critically low sodium levels -- putting them at very high risk of headache, confusion, seizures, and death.
Since 15,000 people ran the race, this means that nearly 1,900 of the runners had too-low sodium levels at the end of the race. And some 90 runners, Almond and colleagues estimate, had critically low sodium levels. The main cause of low sodium levels: drinking too many fluids during the race, diluting the body's salt.
"These observations suggest that hyponatremia -- and particularly severe hyponatremia -- may be a greater problem than previously recognized," Almond and colleagues report in the April 14 issue of The New England Journal of Medicine.
Sport Drinks Not a Solution for Salt Loss
In terms of salt loss, it didn't seem to matter whether runners drank pure water or sports drinks. That's becausesports drinks may contain a lot more water than salt.
"Our findings suggest that the contribution of the type of fluid is small as compared with the volume of fluid ingested," Almond and colleagues write.
An editorial accompanying the Almond study underscores this point.
"It is important to recognize that currently available 'sports drinks' are not protective: Most ... provide far more water than salt," write Benjamin D. Levine, MD, director of the Institute for Exercise and Environmental Medicine at the University of Texas Southwestern Medical Center in Dallas; and Paul D. Thompson, MD, director of preventive cardiology at Connecticut's Hartford Hospital.
Levine and Thompson note that the problem isn't limited to marathon runners. All kinds of athletes tend to drink too many fluids. According to the American College of Sports Medicine, water and sports drinks aren't dangerous to athletes when used as recommended -- in amounts that approximate sweat loss.
A problem for athletes has been recent advice to drink as much fluid as tolerated while exercising and not to wait until one is thirsty. Dehydration can indeed be a terrible problem, resulting in muscle damage and death.
So how can you strike a balance between dangerous salt loss from overhydration and dehydration? Almond and colleagues note that individuals vary widely in their need for water and in the rate at which they lose water. The researchers suggest that runners weigh themselves before and after practice races. If you weigh more after a race than before the race, you drank too much. Adjust your fluid intake accordingly, preferably with sodium-containing fluids that replace salt lost from sweating.
Of course, weather conditions play a major role. It's a good idea to train and to test for appropriate fluid intake in the same weather as you expect for your race.
Levine and Thompson point to the advice of USA Track and Field: Use thirst as your guide to fluid replacement.
But the American College of Sports Medicine (ACSM) has a different point of view.
They say that thirst is often a poor indicator of our body's fluid needs.
"Water in particular quenches the sensation of thirst before body fluid replacement is achieved, so thirst should not be the only determinant of how much fluid is consumed under such conditions," says the ACSM.