All calories are not created equal, and even the word itself has different definitions. A calorie in physics is the energy required to raise the temperature of 1 gram of water 1 °C. The calorie you see on a food package, however, is actually a kilocalorie, or 1,000 calories, the amount of energy needed to raise the temperature of 1 kilogram of water 1 degree Celsius.
The original method used to determine calories in a given food directly measured the energy it produced. The food was placed in a bomb calorimeter, a sealed container surrounded by water. The food was completely burned and the resulting rise in water temperature was measured.
Calories in food and calories as measured in physics are not the same. Nutritional calories exclude non-digestible components, such as dietary fiber and artificial sweeteners, both of which could be burned in a bomb calorimeter to generate heat. Many of these human non-digestible components, like cellulose, are the primary energy source for other species.
These human nutritional calories are the energy available in food, which our body can use for chemical reactions or store in reserve as fat or glycogen; we count the calorie value that the body can extract from the food consumed, ignoring the thermic energy consumed in breaking the food down for consumption, the non-digestible components, and the interaction of that food as a signaling molecule
It’s what your body does with the information encoded in your food that is important, not just that there is energy contained in the chemistry. Knowing the alphabet doesn’t make you a poet; it’s what you do with the individual words. Some are drunken sailors and others are poets, though they use the same letters.