Can you lose weight (diet) by drinking cold water?

Chris: I was having this discussion when I was in Australia recently. I was at dinner with a wonderful guy. His name is Leslie Burnett and he's actually president for the Australasian Society for Clinical Biochemists. We were having this dinner and talking about this question and we were trying to work out the answer but, because we'd drunk too much of another kind of water - the water of life - we kept getting the sums wrong on the back of the napkin.

Nevertheless, this has obviously piqued his curiosity, because he's written to me. He says:

"Dear Chris, I hope you've now returned to Cambridge safely and haven't got too much of an accent following your stay in Australia and New Zealand. Regarding the dinner conversation we had about weight loss and drinking lots of water, I happened to go camping with a colleague of mine from Sydney University of Technology at the weekend. We did the necessary calculations on thermal capacity. If you recall, if most diets recommend you drink five to six glasses of tap water a day. I think you lose a lot of weight just by the act of drinking cold water because you're warming it up to body temperature and then passing it out as urine at 37 degrees. Why bother with the diet if drinking cold water is all you need to do?

Ambient temperature of the water is 17 degrees centigrade, and body temperature is 37 degrees Celsius. If you take the glass of water to be 200ml (1/5 of a litre), if you drink five of those a day that's a litre of water in a day, let's say.

It takes one calorie of energy to heat one ml of water by one degree Celsius. In other words, heating the water from your ambient 17 degrees Celsius to body temperature, 37 degrees Celsius, is a 20-degree increase. This will mean that you have to use 20 calories per ml of water, times a 1000. That's 20,000 calories to get the water up to body temperature."

Here's the problem. The definition of a calorie that dietetics use (what you see on the back of a packet) isn't a calorie, it's a kilocalorie - a thousand calories. Although it gives you the impression it's twenty thousand calories to heat the water up it's actually 20 Calories. As Leslie points out:

"Yes you will lose energy by drinking a litre of water and then passing it out as warm urine but the energy you use is only equivalent to eating a small apple. Definitely less energy than you would gain by eating a block of chocolate or a packet of potato crisps."

A packet of crisps, for example, contains about 200 calories so that's about ten times the energy to warm up the water. So you'd have to gulp down half of the Atlantic before you managed to burn off half of a chocolate bar. There's no easy answer; I'm afraid you have to go with the exercise and reduce your calories. There's no quick fix!