Enzymes are proteins that act as biological catalysts. They decrease the amount of energy needed (called activation energy) to start a metabolic reaction. Without enzymes, you would not be able to obtain energy and nutrients from your food.

For example, when a person has lactose intolerance, their system cannot produce lactase, the enzyme that breaks down milk sugar (lactose) in most dairy products. Because of this, if any dairy is eaten, the milk sugar affects digestion, resulting in bloating, gas pains, and, if severe, vomiting and/or diarrhea. While this condition is not life-threatening for most adults, it can have severe consequences for infants, children, and the elderly. (For more about lactose intolerance, see the chapter “Biology and You.”)