[Venom]

One of the most practiced—and accurate—ways to calculate how many calories you metabolize is to use an indirect calorimetry (spirometry). You would relax, and have your oxygen content measured over a certain period of time. However many liters of oxygen you consumed, just times that by the constant 5.047, and you have how many calories you metabolized. 5.047 can be more properly adjusted if you also have your respiratory exchange ratio (RER). You could compare the amount of oxygen consumed before and after the meal.