the food we eat will broke into small tiny pieces until they were absorbed by the small intestine. then these molecules were send to the cells, lipid molecules, amino acids etc through the blood cappilleries to gain energy. the cells use carbohydrates to gain energy, in that process lot of energy will be released. these carbohydrates is absorbed as glucose. then these are metabolize into water and carbon dioxide and the ATP also forms here which is called as energy currency. here some heat is released during this process, such that ADP produces. thus heat is produced. generally we call this reactions as exothermic reations.