Why do people with lactose intolerance get sick from drinking milk?

People with lactose intolerance get sick from drinking milk because their bodies lack sufficient amounts of an enzyme called lactase, which is needed to digest lactose, the sugar found in milk and other dairy products. Lactose intolerance is not the same as a milk allergy, which involves the immune system; it is a digestive disorder.

When a person with lactose intolerance consumes milk, the lactose in the milk travels to the colon instead of being processed and absorbed in the small intestine. In the colon, bacteria interact with the undigested lactose and cause symptoms such as bloating, gas, diarrhea, and abdominal pain.

The severity of symptoms can vary from person to person and can depend on the amount of lactose ingested and how much lactase the individual's body produces. Some people can handle small amounts of lactose, while others cannot tolerate any dairy products without experiencing discomfort.