ABSTRACT

When we digest and metabolize food, the oxidation of fats, carbohydrates, protein and alcohol releases chemical energy that can be used for body functions. The first law of thermodynamics states that ‘energy can neither be created nor destroyed but only changed from one form to another’. This means that all of the energy released during the oxidation of the macronutrients in food (the metabolizable energy) must be accounted for. It must be used to keep the body’s internal systems functioning (this is ultimately lost from the body as heat energy), to do external work (e.g. move something) or be stored within the body (e.g. as fat in adipose tissue).