With the Honolulu Marathon barely a month away, this is a good time to review some common myths. One belief is that in long runs, one gets dehydrated well before one feels thirst, so one needs to drink fluids often, even if you're not thirsty.

But folks who overdrink may develop a dangerous over-dilution of their blood with low sodium levels, called hyponatremia.

In the American Journal of Medicine of May of this year, Dr. Arthur Siegel and others did blood tests on various runners in the 2001 to 2004 Boston and Marine Corps marathons, including two women, ages 24 and 32, who collapsed and died after running more than four hours. These women had consumed fluids at aid stations at every mile — one of them drank only Gatorade, rather than water.

Both had blood overdilution from overdrinking, leading to brain swelling. Blood tests on them and others who developed overdilution found that prolonged running may cause muscles to leak proteins that travel to the pituitary, causing it to produce too much ADH, a hormone that forces the kidneys to make a concentrated urine.

With overdrinking, the kidneys can't get rid of the extra water, causing dangerous over-dilution of the blood. In 2002, 13 percent of runners in the Boston Marathon had overdilution, and drinking sports drinks with electrolytes instead of plain water did not matter.

But don't we need extra salt while running to replace losses from sweat? Not really. In 2006, Dr. Tamara Hew-Butler and others reported that when they had half of 413 triathlon participants take 1 to 2 extra sodium tablets an hour during the 12-hour event, those taking extra salt did no better in finishing times, blood dilution or weight loss.

If you run longer than 2 hours, it's actually normal to lose between 2 percent and 4 percent of your weight from sweating.

Losing less than 2 percent is due to overdrinking and can cause overdilution — and further tires you because you're heavier than you need to be.

Losing more than 4 percent can cause overconcentration and likewise worsens performance.

The best way to stay within this sweet zone? A July 2006 article "Updated Fluid Recommendation: Position Statement From the International Marathon Medical Directors Association" in the Clinical Journal of Sport Medicine says to drink when thirsty and to not force yourself to drink when not.

Thirst is actually very accurate in keeping your blood volume and concentration exactly where it should be during marathons.

Your normal diet will replenish your electrolyte losses in the hours and days after the run.

Dr. Landis Lum is a familypractice physician for Kaiser Permanente and an associate clinical professor at the University of Hawai'i's John A. Burns School of Medicine. Send your questions to: Prescriptions, Island Life, The Advertiser, P.O. Box 3110, Honolulu, HI 96802; islandlife@honoluluadvertiser.com; or fax 535-8170. This column is not intended to provide medical advice.