On a thread somewhere around here in GQ, it was mentioned that soda and caffeine don’t cause net losses of fluid, despite popular conception that they only make you more thirsty in the long run. However, are there any commonly consumed drinks that would cause a net loss of water? Stuff like desiccant gel is out - I’m looking for something that an average human being might drink.
How about alcoholic beverages…espeically liquor. Keep drinking vodka, and it’s gonna dry you out. I suspect beer is somewhere around the break even point. Neither terribly dehydrating nor good for getting water back into your system.
I don’t know for sure that this is true, so I’ll await confirmation from more knowledgeable Dopers.
Mmmm, beer…
Fry
If St. John Ambulance know what they’re talking about (or at least my Standard First Aid instructor), aerodave is correct. We were taught that alcohol is the only normally ingested liquid that causes a net loss of hydration. Of course, it was hardly medical school… 
A dry white wine will leave you badly dehydrated the next day if you drink enough of it in one sitting. And I didn’t need to go to medical school to learn that either. 
Caffeine and alcohol both dehydrate you – as in they increase the rate at which you lose water. Depending on how much water they are mixed with your beverage is either dehydrating or re-hydrating or neutral and a lot will depend on your personal physiology and other factors. It’s fairly easy to see that sodas are not dehydrating – there are people who drink only sodas for prolonged periods of time without short-term ill effects.
Espresso is probably dehydrating. I only drink when I’m thirsty and with my caffeine intake I tend to drink 2-3 liters of water a day on top of coffee (and no, I’m not diabetic). If I don’t drink coffee I generally don’t get thirsty – whatever liquid is in my food is sufficient.
My Dad used to tell a story of a time when he was a deckhand on a Portugese fishing boat in CA. They started with several Thermos of Coffee. On board was a large keg. It turned out to have wine. There wasn’t a drop of drinking water on the boat, just coffee in the AM and then wine the rest of the day. Out in the sun, hard physical labor for up to 48 hours. The CW would say those men were all dead or dehydrated, but my Dad said he was “just a little drunk most of the time”. The “pros” weren’t tipsy at all.
I used to go fishing and hunting with a guy who drank coffee in the AM, then beer all the rest of the day. He was fine. I used to drink diet soda all day, every day, with nary a drop of water.
This is becuase Beer, wine, and soda all hydrate more than they dehydrate, even if they are not as hydrating as water or “sports drinks”.
Sure, Expresso, as **groman ** mentions, or hard liquor- like vodka- would likely dehydrate more than hydrate.
Ipecac
Salt water.