Enzymes, or energized protein molecules, are catalysts, substances that accelerate and precipitate the hundreds of thousands of biochemical reactions in the body that control life's processes. They are essential for digesting food, stimulating the brain, providing cellular energy, and for repairing all tissues, organs, and cells. Each enzyme has a specific function in the body that no other enzyme can fulfill. The substance on which an enzyme acts is called the substrate. Because there must be a different enzyme for every substrate, the body must produce a great number of different enzymes. Enzymes are often divided into two groups: digestive enzymes and metabolic enzymes. Digestive enzymes are secreted along the gastrointestinal tract and break down foods, enabling the nutrients to be absorbed into the bloodstream and then used by the various bodily functions. There are three main categories of digestive enzymes: amylase, protease, and lipase: Amylase, found in saliva and in pancreatic and intestinal juices, break down carbohydrates. Different types of amylase break down different sugars. Lactose breaks down lactose (milk sugar), maltase breaks down maltose (malt sugar), and sucrase breaks down sucrose (cane and beet sugar). Protease, found in the stomach juices and the pancreatic and intestinal juices, helps digest protein. Lipase, found in the stomach and pancreatic juices, and also present in fats in foods, aids in fat digestion. Hydrochloric acid (HCl), although not technically an enzyme, interacts with digestive enzymes as they perform their functions. HCl comes in many different forms, including lysine HCl and betaine HCl. Betain HCl is derived from sugar beets. Supplemental HCl is not sold in powder or liquid form because contact with the teeth can damage tooth enamel.