Lactose is a type of sugar found in milk and other dairy foods. Normally, when we eat something containing lactose, an enzyme in the small intestine called lactase breaks it down into simpler sugar forms called glucose and galactose. These simple sugars are then absorbed into the bloodstream and turned into energy.

In lactose intolerance, the body doesn’t make enough lactase to break down lactose. Instead, undigested lactose sits in the gut and gets broken down by bacteria, causing gas, bloating, stomach cramps, and diarrhea.

Lactose intolerance is fairly common. Kids and teens are less likely to have it, but it is not uncommon for people to become lactose intolerant in adulthood. It is rarely dangerous.