On long rides: how much water is too much?

A new study of marathoners shows that a small percentage of long-distance runners stop excreting urine after a certain point in the run.

The result is water retention that reduces the sodium level in their bodies, with rather nasty--sometimes even fatal--results.

[At the Boston Marathon, the experimenters used scales], positioned in Red Cross stations every mile throughout the second half of the course . . . [the scales] are part of a new push to protect runners from hyponatremia, a rare condition that claimed the life of a healthy 28-year-old woman who collapsed during last year's race.

Only a handful of people have died from the condition, in which their blood sodium concentration falls to an abnormally low level, usually from drinking too much water. But the risk of suffering from its symptoms during a marathon is higher than most people think. . . .

Moreover, in clinical studies, he and his team of researchers discovered that almost 85 percent of the subjects had lowered blood sodium levels during or after running a marathon. Nearly 25 percent had levels below 135 milliEquivalent (mEq) per liter of blood, the point at which a diagnosis is made (136-142 is considered normal). . . .

Hyponatremia presents a particularly vexing problem for runners and race medical staff, because many of its symptoms -- throbbing headache, nausea, cramps, and dizziness -- mirror those of dehydration.

But the standard treatment for dehydration, pushing fluids by mouth or intravenously, can prompt the brain or the lungs to swell, which can produce seizure, coma and death in a person who is severely hyponatremic.

Instead such a person should receive an IV with a concentrated sodium solution, a diuretic medication to speed water loss, and an anti-convulsive medication, in case of seizure, according to Cianca.

In more moderate cases, he added, doctors should simply restrict fluids and encourage salt consumption and wait for the person to "pee off" the extra fluid. . . .

Many more details and concrete suggestions for the long-distance training rides, on the Reuters health site.