What is the name of the enzyme that breaks down lactose?

The enzyme that breaks down lactose is lactase. It is found mainly on the brush border of cells lining the small intestine, where it hydrolyzes lactose into the simpler sugars glucose and galactose. Those sugars can then be absorbed through the intestinal wall and used for energy.

Lactose is the naturally occurring sugar in milk. Babies generally produce abundant lactase because milk is their primary food, but in most humans, lactase production declines after weaning. People who continue producing significant amounts into adulthood have lactase persistence, a trait that evolved independently in several populations with long histories of dairying.

When the small intestine produces too little lactase, undigested lactose passes into the colon. Gut bacteria ferment it, which can cause bloating, gas, abdominal cramps, or diarrhea; this is lactose intolerance. Lactase should not be confused with lipase, which digests fats, pepsin, which begins protein digestion in the stomach, or amylase, which breaks down starch. Commercial lactase is also added to lactose-free dairy products.