Metabolic processes are constantly taking place in the body. Metabolism is the sum of all of the chemical reactions that are involved in catabolism and anabolism. The reactions governing the breakdown of food to obtain energy are called catabolic reactions. Metabolic reactions provide energy in multiple-step processes in which metabolites are oxidized. The three main purposes of metabolism are: the conversion of food to energy to run cellular processes; the conversion of food / fuel to building blocks for proteins, lipids, nucleic acids, and some carbohydrates; and the elimination of nitrogenous wastes (Wikipedia).