I don't remember where I actually read this, but water--good ol' water--was shown to actually have negative calories. Apparently, it takes more energy than the water contains for your body to properly digest it, so you burn a small amount of fat or other energy block. I believe the figure was -4 calories for an eight ounce cup of the stuff. Neato, eh?

I suspect they just label bottled water at zero calories to prevent a mad rush of people obsessed with their weight from picking up crates of bottled water in the hopes of drinking those extra pounds away.

A european friend told me about a water diet. Apparently you can lose weight by drinking a lot of cold water and then immediately jumping into a hot jacuzzi. The theory is that your body will spend a lot of energy to bring the cold water up to body temperature. Eventually you need to get out an pee away the (now)-warmed water and you do it all over again.

It sounds like it could work, but I have better things to do in my life than spend it laying around a jacuzzi. Don't I?

The water diet thing forgets the fact that a food calorie (properly spelled Calorie, with the capital C) is actually a kilocalorie, hence the math error cited above. The energy used to heat the water to body temperature is measurable in calories, not Calories, so we're off by at least one (and maybe more, I haven't seen the math) order of magnitude.

Plain water contains zero calories. Absorbing it from the gastro-intestinal tract generally costs the body no energy. Why? Just think osmotic pressure.

Excreting any excess water costs the body a little energy, not in pushing the water out, but rather in reabsorbing the solutes in the ultrafiltrate in the kidney's nephrons so that the body does not lose valuable electrolytes together with the water.

Overall, the energy spent on getting rid of excess water is not a lot of energy. Drinking too much water^* can have serious effects on your body's electrolyte balance when your homeostatic systems decompensate. Having said this, it is better to err slightly on the side of drinking more rather than less water, especially if you live in an environment where water is not scarce. A good way to tell is by the colour of your urine - if it's concentrated (deep yellow), you should be drinking more water and if it's almost colourless, you can hold back the drinking a little.

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^* In a healthy normal body, it's actually pretty difficult to drink water to the point that the body decompensates. Most people simply cannot force themselves to drink that much water - more than 20 liters in less than 24 hours for a healthy adult male. It's much easier for someone with cardiac failure or renal failure to overdrink.

My understanding is that people who are overweight are advised to drink more water. The reason, however, is not because it consumes calories, but because water (and, for that matter, anything with bulk but low calories) fills the stomach, preventing them from eating foods that do contain calories. Since the body can process water quickly, and most people exist in a state of mild dehydration, this isn't an unhealthy practice (and may be healthy if you are dangerously overweight).