Food energy is expressed in food calories (labeling: EU kcal, U.S. calories) or kilojoules (kJ). Food calories, or the "calorie" units used often in nutritional contexts, measure amounts of energy 1000 times greater than the units in scientific contexts known also as calories, or gram calories (cal). Food calories are thereby referred to less ambiguously in some formal contexts as kilocalories (kcal). One food calorie is equal to 4.184 kilojoules. Within the European Union, both the kilocalorie (kcal) and kilojoule (kJ) appear on nutrition labels. In many countries, only one of the units is displayed.
Carbohydrates, fiber, fats, proteins, organic acids, polyols, and ethanol all release energy during respiration — this is often called 'food energy'. When nutrients react with oxygen in the cells of living things energy is released. A small amount of energy is available through anaerobic respiration. Fats and ethanol have the greatest amount of food energy per mass, 9 and 7 kcal/g (38 and 30 kJ/g) respectively. Proteins and most carbohydrates have about 4 kcal/g (17 kJ/g). Carbohydrates that are not easily absorbed, such as fiber or lactose in lactose-intolerant individuals, contribute less food energy. Polyols (including sugar alcohols) and organic acids have less than 4 kcal/g.
I don't agree. learn some chemistry and about ghrelin here.