I agree with the DK reference, but I disagree that we have the "calories in" nailed down (and of course, calories out is completely out of the question). For e

I agree with the DK reference, but I disagree that we have the "calories in" nailed down (and of course, calories out is completely out of the question). For example, food temperature must (calorically) make some difference - e.g., if I drink a liter of 3 deg(celsius) water every day, and remain at 36 dec(celsius) myself, vs. if I drink a 43 deg(celsius) tea (and remain at same temp), the "in" is significantly difference (to the tune of 40kcal/liter between the extremes), whether you count it on in or out. Furthermore, we don't even have carbs properly - depending on microbiome, undigestable fiber to you might be digestable to me, e.g. [0], and whether you eat digest papaya or carbon at the same time as other things can significantly change how much "calories in" the represent. [0] http://www.sciencemag.org/news/2010/04/japanese-guts-are-made-sushi http://www.sciencemag.org/news/2010/04/japanese-guts-are-mad...