What is in milk that makes people lactose and tolerant?

Milk contains lactose, which is a sugar made up of glucose and galactose. People who are lactose intolerant lack sufficient amounts of lactase, the enzyme needed to break down lactose into these simpler sugars for digestion. As a result, when lactose is consumed, it can lead to gastrointestinal symptoms such as bloating, gas, and diarrhea. In contrast, those who are lactose tolerant produce enough lactase to effectively digest lactose without discomfort.