There are three main enzymes found in a human body — digestive enzymes, metabolic enzymes, and food enzymes. Each human enzyme speeds up a different chemical reaction inside the body. Sometimes a natural enzyme is needed, and these can be found in enzyme supplements and many raw foods.

Some of the most important types of human enzymes are digestive enzymes, which are secreted by the mouth, stomach, pancreas, and intestines. These are necessary for breaking down and digesting foods. After food has been broken down, vital nutrients are then delivered throughout the body via the bloodstream. Each digestive enzyme is named and classified based on what complex food it breaks down.

Protease, for example, helps break down complex proteins into much simpler amino acids. Lipase breaks down fats and other lipids, and converts them to fatty acids and glycerol. Amylase helps convert complex carbohydrates to simple sugars, like glucose.

Metabolic enzymes are predominately produced in and secreted by the pancreas. These serve a few important purposes. Along with purifying the blood, they also help the body absorb oxygen from the blood. They also aid in the production of energy.

One of the other main functions of a metabolic enzyme is maintaining the body's various tissues. They do this by converting proteins, fats, and carbohydrates into necessary cells that produce the tissues in the body. As humans get older, their bodies produce a smaller amount of enzymes, and many scientists believe this is a major part of the aging process.

Natural food enzymes are the last type of human enzymes. These are found naturally in some foods and aid in digestion of the particular foods they are in. Raw, unprocessed foods are generally thought to be the best source of these, since cooking foods can lower enzyme levels. Bananas, papayas, mangoes, and avocados are considered to have high levels of these.

Several enzyme supplements on the market claim to make up for a low enzyme level in a person's body. Companies that manufacture these products claim that their enzyme supplements can improve a person's health in a number of ways, including slowing the aging process. Unfortunately, there has been no clinical evidence that supports this. Some enzyme supplements, like lactase, may help digestion for a short period of time, but they are typically destroyed long before they reach any other parts of the body.