We've all heard that line, but to me, at least, it always seemed like the scale must be so small it couldn't possibly be perceptible. Then I was reading a description of the phenomenon today that said if you drank all your water ice-cold, it could be equivalent to adding a workout a week.
So I got interested, and calculated it.
It's actually fairly easy to calculate. The specific heat of water is 1 calorie per gram per degree Celsius (little calorie, not food kcal), and its density is 1 gram per milliliter. (A lot of things are defined in reference to water.) If you drink ice-cold water, it is at 0 degrees Celsius, and the temperature your water is probably usually at is around 20 degrees Celsius.
What this means is that to warm up water from 0 degrees Celsius to 20 degrees, you have to put in 20 little calories for every milliliter of water you have. When it goes into your body, it actually has to warm up further to around 37 degrees, but your body is already doing that when you drink room temp water, although that also means if you just drink more room temperature water in a day it will burn more calories too. Anyway, if you drink 2 liters of iced water a day, that's 2000 x 20 little calories = 40,000 little calories = 40 kcals, or food calories. 40 calories a day! In a week, that's 280 calories - a light workout, an extra pound every 3 months or so. Not so negligible.
What's neat about this too is that this is non-negotiable calorie burn. It's just heat transfer. It's not something your body can do more efficiently, or kind of do a crap job on to save energy. By the laws of thermodynamics the water will suck heat out of your body, and your body can either compensate or tolerate hypothermia. Granted, anorexics who try to practice this trick probably will reap more hypothermia than calorie burn from it, but for the rest of us, I doubt our bodies are desperate enough to refuse to heat up ice water. Cheers!
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