First, let me declare that I'm not obsessive! I'm just curious! I'm reading a physics book by Feynman for fun right now. Yesterday, I finished Freakonomics.
In the winter, I tend to drink my tap water at room temperature, switching to colder water as it gets warmer. It occurred to me that all the water that leaves my body, no matter the route, is me-temperature (37º C). So, my body used calories to warm it up along the way. So, I did some math.
The water to fill my 32 ounce water bottle weighs about two pounds or 907 grams. It takes one calorie to raise one gram of water one degree Celcius. My fridge water dispenser is about 4ºC, so my body must raise the water by 33ºC. That's 907 x 33 = 29,931. Sounds fantastic (except that we'd all probably starve to death!) until you take into account the fact that food calories are actually kilocalories (kCal). So, you have to divide that number by 1,000 to arrive at the "calories" you see on the nutrition label. That means just about 30 calories are burned to get my fridge-cold 32 ounces of water to body temperature. To warm up my room temperature water is about half that.
So, I'll burn an extra 15 calories if I use fridge water instead of warmish tap water. As I probably drink this thing three times per day, that's 45 calories extra - or one pound in three months. I think I'll just keep drinking it at the temperature I feel like!