Originally Posted by
Dan The Man
I'm not saying don't drink water. I am saying that given the option of sweating a cup of water or evaporating a cup of water off your skin, the second is the better option if it is really hot out. You will be evaporating that cup of water one way or another, doesn't matter if it's inside you or outside. If you are working and producing heat, and the air is hotter than your body, you will need to evaporate a certain amount of water or else your temperature will rise to unhealthy levels.
In desertlike conditions where it is extremely hot and extremely dry, the mass-rate that you need to evaporate water to keep cool while riding can easily exceed the maximum mass-rate that your body can absorb water, and the salt-loss rate that goes with that sweat can also exceed the amount of salts you take in. Splashing about half of your water on your skin will keep you cool without dehydrating you. You will still sweat as well, and you will still need to drink water. But you won't sweat as much and you won't need to drink as much.
Basically, riding in the desert you need to evaporate a certain amount of water each hour no matter what. Throwing a portion of that water on your skin saves you from dehydration.
The problem is that you don't know, usually, what half your water is. I've been on any number of tours where using half of my water to wet my body outside would have meant going without water for drinking for a significant amount of time. That's deadly in hot conditions. It's not great in cool conditions either.
The other issue is that water that goes in you will come out somewhere. If you drink too much, your kidneys will eliminate it if the sweat glands don't utilize it. You may have some problem with electrolyte loss but that's pretty easy to deal with.
Water on the outside is nice and, if you have adequate water and ready access to more, you can have a dandy wet t-shirt contest. But if you don't know where the next water source is, the wise choice is to keep it inside. Putting it on the outside doesn't keep you from dehydrating. Only drinking water will keep you from dehydrating.