Digestion is the breakdown of complex food materials into simple compounds by chemical and physical means. The digestion of food supplies body cells with materials needed to carry on various activities, such as the release of energy, repair, and growth. In man, digestion starts in the mouth and is completed in the small intestine, where the simple end products are absorbed and carried by the bloodstream to all cells of the body.
The three major kinds of food are carbohydrates, which are broken down into simple six-carbon sugars; proteins, which are digested to amino acids; and fats, which are split into fatty acids and glycerol. The digestion of these complex compounds involves many separate chemical reactions. Each of these reactions requires a special enzyme, a chemical substance that speeds up a chemical reaction. Other substances, such as water, hormones, and hydrochloric acid, are also essential for the process of digestion.