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Enzymes are a group of proteins created by the body that act as catalysts in the digestive tract to break down foods or throughout the rest of the body to assist in either the creation of useful materials (such as hemoglobin), the destruction of harmful ones (such as alcohol) or even turning body tissues into energy (such as those that help convert protein to urea when required to produce energy). Although the enzyme isn't chemically changed by the processes it assists, they are constantly created and destroyed by the body.It can cause ADHD.

Several diseases can be caused by a lack of enzymes in the body, from the routine (lactose intolerance, caused by a shortage of the enzyme that converts lactose into digestible sugars) to the very rare (porphyria, caused by a lack of one of the enzymes that create heme).

Enzyme at Wikipedia