Ice water, calories, and kisses

This New Year's Eve, 3steak and I went to New York City for the annual Midnight Run in Central Park. We convinced our friends to come along for the adventure, so we spent the day/night with two other couples.

While we were sitting in our hotel room waiting for dinner, we started talking about New Year's resolutions and how many people try to lose weight each year. Someone brought up the common dieting suggestion to drink iced water instead of room temperature water because your body supposedly burns more calories warming the water up to room temperature. Although I don't contest the physical reality of this observation, I wondered just how significant this effect really is.

Now, remember that I'm an engineer, so don't be surprised that I worked up a quick calculation of how many calories you burn heating up your water. Actually, I thought it would be more fun to put the answer in units we all care about, such as numbers of Hershey's kisses. (I chose this unit because one of our friends recently took a new job as a chocolate engineer with Hershey's.)

Okay, let's get started:

1 calorie is the energy required to raise 1 g of water by 1 C.

1 kcal = 1000 cal (this is what we call a food calorie)

Volume of a glass of water: Vgw = 8 US fl oz = 236.6 cm3

Density of water: rhoH20 = 1 g/cm3

Mass of a glass of water: mgw = rhoH20 * Vgw = 236.6 g

Temperature difference to bring ice water to body temperature:

T0 = 10 C (assume average of ice temp, 0C, and room temp, 20C)

Tf = 37 C (body temp)

DeltaT = 37-10 = 27 C

Energy required to bring ice water to body temperature:

Egw = (1cal/g*C)*mgw*DeltaT = 1*236.6*27 = 6388 cal = 6.4 kcal (food calories)

So, if you drink 8 8-oz glasses of ice water, you'll burn 6.4*8 = 51.2 kcal. Now, each Hershey's Special Dark Chocolate Kiss has 20 kcal. Therefore, drinking ice water burns the equivalent of ~2.5 dark chocolate kisses.

But don't get too excited...you were going to drink those 8 glasses of water anyway, either at room temperature or iced temperature. If you drank the water at room temp, the energy required to heat it to body temperature would have been:

DeltaT = 37 - 20 = 17 C

Egw = 1*236.6*17 = 4022 cal = 4 kcal

So the difference in energy expenditure between room temp and iced water is:

DeltaE = 6.4 - 4 = 2.4 kcal per glass

For 8 8-oz glasses, you'll burn an extra 2.4*8 = 19.2 kcal if you ice your water versus drinking it at room temp. That's only 1 dark chocolate kiss. Bon appetit!