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Dehydration is a numbers game

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“It’s complicated”

By Dave McEvoy, Aerie Backcountry Medicine

To understand dehydration, read about the financial turmoil in the world. Being in massive debt hurts. It makes you irritable, weak and crampy. It gives you a headache and makes you susceptible to infections. Worst of all, it causes you to make bad decisions, jeopardizing yourself and those with you.

Dehydration from sweat is a numbers game. Sweat is 99 percent water. This water comes directly from your blood, where you have roughly five liters in circulation. Yet only half of your blood is water, meaning you only have 2-3 liters of water to sweat.

For obvious reasons, like death, having no water in your blood would be very bad. To avoid this, your body begins running a tab on water, borrowing it from the tissues, where you have over 30 liters in reserve. This is a normal part of a healthy body process. Rest, drink and eat, and your body replenishes the loss. Trouble comes when you ignore the deficit and continue running the tab.

If dehydration were in a relationship on Facebook, it would have the descriptor “it’s complicated.” Because we borrow water from all body tissues, dehydration can be felt in isolated muscles or lead to a body-wide crisis. You might feel like you have anything from a pulled muscle to the flu, and not know what part dehydration is playing in any of it.

To keep it simple, focus more attention on avoiding water debt than on treating it.


It takes your body 6-10 days to acclimatize to a warm environment, after which time you sweat more efficiently, radiate heat more readily, and feel a lot better exercising in the heat. To best acclimatize, you need to break a sweat every day for about an hour during this process.

Moderate activity
Short, frequent breaks in the shade are essential to prolonged activities, particularly with high temperatures and humidity. Don’t mistake this as wimpy advice for pampered people. Everyone, from soldiers in combat to extreme athletes, needs to periodically reduce activity when working hard in a hot environment.

Drink enough to pee clear and often

This usually means 2 – 6 liters of water over the course of each day of strenuous outdoor activity. That’s more than we’re accustomed to drinking but is critical to avoid going into water debt with your tissues. It is possible to drink too much water. This condition, called hyponatremia, typically happens during endurance events with participants pushing themselves at high levels for many hours and significantly exceeding the above fluid recommendations.

Wear proper clothing
The key is wearing materials with little or no insulating value that allow airflow over the skin.

Consider adding electrolytes to your water
Endless debate surrounds the efficacy and need to supplement drinking water with electrolytes. While electrolyte solutions with less than 8 percent carbohydrates can help prevent and treat some forms of heat illness (particularly when a well-balanced diet isn’t possible) they aren’t a substitute for eating well and resting. By the way, recent evidence refutes an old theory that electrolytes help prevent hyponatremia.


Don’t rely on pounding water
It doesn’t work. You won’t absorb most of it and instead will feel sick or vomit. Instead, drink about a half-liter per hour of activity, as well as a liter before and after activity.

Don’t rely on adding electrolytes to water
Electrolytes are essential for basic function, and adding electrolytes to water can be very helpful, but we get most of what we need from food. Don’t believe the hype about expensive drinks at the expense of thinking about drinking water and eating well.

Don’t rely only on thirst to guide fluid consumption

Thirst is triggered only when you’re significantly dehydrated, meaning by the time you feel crappy and thirsty, you’re already well behind and hours from recovery.

Don’t think your body will get accustomed to dehydration
It won’t, and it will punish you for treating it that way.


Stop activity and slowly hydrate
If a dehydrated person is conscious but weak, tired, nauseated, has a headache and/or muscle cramps, stop the activity for at least a few hours, take off restrictive clothing, move him into the shade, lay him down, get some cool water onto him, and slowly hydrate him. Nausea will seriously limit the amount he can drink, which is usually about a quarter to a half-liter per hour. Adding electrolytes and massaging cramping muscles can be beneficial.

Cool off aggressively and plan for immediate evacuation
If a dehydrated person is hot, hallucinating, unresponsive and unable to walk, cool him as quickly as possible; the fastest means of cooling him often includes spraying with water and fanning air. He also needs an immediate trip to a hospital. Until proven otherwise with tests in that hospital, this is heat stroke, which is life-threatening emergency with a very high mortality rate. It can come on quickly, without much warning.


Don’t think recovery will be fast
Remember, it can take hours to fully replenish water and electrolyte losses.

Don’t underestimate how many ways dehydration can affect you
Dehydration affects all body systems, making a person feel tired, crampy, nauseated and irritable. As such, it’s a contributing factor to many of the most common and serious backcountry illnesses and injuries. Preventing it is fairly simple. Treating it can be long and challenging. It’s an easy choice!

David McEvoy is a paramedic and the Director of Aerie, a wilderness medicine training organization based in Missoula. For outstanding swiftwater rescue classes in the Bozeman area, he suggests the Whitewater Rescue Institute.

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