An old joke going around the internet is that one could actually lose weight on a diet of nothing but cold beer and ice cream, as it would take more body energy to heat up the food than what would be gained by its consumption. Although it carries the same cadence that other jokes you'd typically find around on the internet—TNR font, pseudo-joking demeanour, skimpy but correct-sounding backup information—it seems very, very true. Almost too good to be true, but still highly believable.
The logic behind the claim went as follows. One calorie is defined as the energy it takes to heat a gram of water by one degree Celsius, so if you were to eat a gram of cold food, you would spend a calorie per degree below body temperature, to balance out the heat you spend on warming up the consumed product. For example, each gram of food at 0°C is going to take about 37 calories of energy. A 6 oz (approx 168 g) serving of ice cream has about 1200 latent calories, and 1 cal g-1 °-1 × 168 g × 37 ° = 6216 cal wasted on heating up the ice cream means you've just burned maybe a net five thousand calories. And the same thing with cold beer: each ounce works out to about 1000 or so calories lost, after counting out the handful latent calories that are actually in beer. So there you have it, world, it's time to break out the waffle cones and bottle openers!
Except you shouldn't. Common sense dictates that if we, as mammals, had to have expended that much energy to warm up all the food that entered our bodies, we would not have been able to survive, since comfortable room temperature is 20°C, and the average temperature outside most of the time is lower than that. The resolution to the "paradox" is the fact that a calorie and a Calorie are two different things—the latter is the way that most food distributors denote the energy content of their food, and is also sometimes referred to as a dietetic calorie. 1 Calorie is equal to 1000 calories, or 1 kilocalorie (kcal). Taking this into account, the "1200 latent calories" is more like 1,200,000, making the total number of little-c-calories burned from consuming 6 oz of ice cream somewhere along the lines of "negative 1,195,000".
Basically, if you want a diet that works, just stick to counting calories. Leave this one for pranking and subtly sabotaging close friends.
The original joke can be found here.