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Some Grand Canyon hikers who fall ill to apparent heat exhaustion are actually suffering from a malady brought on in part by drinking too much water and not eating enough food, a researcher says.

First diagnosed here in 1988, the illness struck in 20 of last year's 125 heat-related hiker emergencies, says Dr. Howard Backer, a California physician who's heading a research project on the illness here this summer.He's talking about hypo-natremia - water intoxication, which in its early stages mimics the fatigue and nausea of heat exhaustion and sometimes confuses National Park Service rangers called to hikers' aid. In later stages its victims may behave strangely and go into a coma.

"It's become the most common cause of serious heat illness in the Canyon," Backer said. Heat exhaustion is more common but doesn't usually lead to unconsciousness and confusion. It can, however, progress to potentially deadly heat stroke.

None of the 20 recorded canyon cases of hyponatremia resulted in permanent damage, officials say.

"People treat these hikes like a spa retreat and bring a banana, apple and some raisins," says Sherrie Collins, who heads emergency services in the canyon. "They tell themselves they'll get in shape and lose some weight. Also, in drinking a lot of water, they deplete their bodies of electrolytes, particularly sodium."

Besides sodium, electrolytes include potassium, phosphorous, magnesium and other chemicals and compounds essential to transmission of the body's electrical impulses.

In the 1988 case, a woman plodding back up a canyon trail with her husband was complaining of fatigue. They summoned a ranger, who gave her a water-electrolyte mixture and escorted her to a rest house two miles away.

Half an hour later, she apparently tried to eat her flashlight and then became unconscious. Hospitalized in Flagstaff, she emerged from her coma four days later, apparently with no permanent brain damage.