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It’s a mantra for deployed U.S. servicemembers during hot weather: “Drink your water. If you’re thirsty, it’s already too late — you’re dehydrated.”

It’s not uncommon for troops in high summer in Iraq and Afghanistan, who are deployed to in the desert on long, stressful missions, or those whose jobs are on air fields or in other hot places, to drink three or more gallons of water, Gatorade, and Rip-It energy drink. Many still feel thirsty at day’s end when they finally strip off T-shirts that are stained white with body salts lost to the day’s sweat.

Good NCOs monitor their troops carefully to make sure everyone is drinking enough to replace what he is losing to sweat, and some even institute forced-fluid drinking schedules. However, there is such a thing as drinking too much water. It’s called hyponatremia, or water intoxication, and it’s the opposite of dehydration. Water intoxication may not be very well known compared to dehydration, and it isn’t as common.

But gung-ho troops, in particular, are vulnerable to the condition. Water intoxication occurs when someone who is sweating buckets drinks too much plain water to compensate. The plain water increases blood plasma, which is the liquid part of blood. As this takes place, the salt content of the blood is diluted.

Eventually, the amount of salt available to the body tissues decreases over time, to a point where the loss interferes with brain, heart and muscle function. Hyponatremia can lead to a coma and even death if not treated promptly. Unfortunately, the symptoms of water intoxication are virtually the same as those of dehydration: apathy, confusion, nausea and fatigue — although mental changes can be very subtle, and some individuals show no symptoms at all; they just go down like a pile of rocks when their bodies reach the limit. (Medics who suspect they may have a case of hyponatremia, not heatstroke, on their hands will know by checking rectal temperature — a normal range of 37 to 40 degrees Celsius excludes heatstroke — and then as promptly as possible, measuring serum sodium concentration).

According to official Army doctrine, “the body normally absorbs water at the rate of 1.2 to 1.5 quarts per hour. A reasonable upper limit for a total consumption estimate for a 12-hour work day is 12-15 quarts,” or three to four gallons. Sweat rates during work in the heat can often exceed 1 quart per hour, the doctrine notes.

But the doctrine also warns that the formula varies greatly — not only by the temperature, but by body weight; individual metabolism; how hydrated a person was to begin with (drinking alcohol 24 to 48 hours before exposure to high heat conditions greatly increases your chances of dehydration); wind and humidity; the amount of work a trooper is doing at the time; and how much armor and gear he’s wearing. The best defense against either dehydration or water intoxication is knowing your own body and what it needs, and taking care of yourself (and your buddies).

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