What Is A “Calorie”, Anyway?
The concept of the “calorie”, as applied to nutrition, is an oversimplification so extreme as to be untrue in practice. The dietary calorie is defined as “the amount of energy required to increase the temperature of 1 kilogram of water by 1 degree Kelvin.”
The dietary calorie is actually a “kilocalorie” = 1000 calories, which is why you’ll occasionally see it abbreviated as “kcal”. It’s an obsolete unit: the “joule” is the modern unit of energy. There are 4.184 joules in a calorie, and 4184 in a dietary calorie (kilocalorie).
Problem: Our Bodies Don’t Use “Calories”
You may already see the problem here: a “calorie” is a unit of energy transfer. We determine the number of “calories” in a food by, quite literally, burning it and measuring how much heat it generates. Unfortunately, our bodies are not steam engines! They do not burn the food we eat in a fire and convert the heat into mechanical work. Thus: There is no biochemical system in our bodies whose input is a “calorie”.