A calorie is a calorie only in the tautological sense. Paper and wood pulp are carbohydrates, but you cannot get any energy from eating those. Petrol is 12kca

A calorie is a calorie only in the tautological sense. Paper and wood pulp are carbohydrates, but you cannot get any energy from eating those. Petrol is 12kcal/g, more calorie dense than fat at 9kcal/g, but you can't get any energy from eating it. "Ah, I mean the digestible things" I hear you say; well then - we've already established that "a calorie is not a calorie" if you need to add exceptions. But, with this exception: Different kinds of foods have a different calorie extraction/absorption profile. paper/petrol have 0; pure sugar has 100%, most things defined as "food" are high on that scale, but usually not 100%; It also takes nontrivial energy to digest some foods. This is often lumped up into "atwater factors", the values of which seem to vary by 20-80% among different individuals and the same individual at different times. The statement that "a calorie is just a calorie" is incredibly useless in the context of human metabolism.