The food we eat is broken down, or metabolized, in stages by a group of complex biological molecules called enzymes. Most of the energy released at each stage is captured for function and growth. One interesting aspect of metabolism is that the overall change in energy is the same as it is in combustion. For example, the total enthalpy change for the conversion of glucose (C6H12O6) to carbon dioxide and water is the same whether we burn the substance in air or digest it in the body: C6H12O6 (s) + 6CO2 (g) -> 6CO2 (g) + 6H2O (1) AH=-2801 kJ/mol The energy content of food is generally measured in calories. The calorie (cal) is a non-SI unit of energy that is equivalent to 4.184 J: 1 cal = 4.184 J In the context of nutrition, however, the calorie we speak of (sometimes called a "big calorie") is actually equal to a kilocalorie: that is, Fuel Values of Foods Substance Apple Beef Beer Bread Butter Cheese Eggs Milk Potatoes 1 Cal 1000 cal = 4184 J AHcombustion -8 -1.5 -11 -34 -18