Diarrhea occurs due to undigested lactose in your small intestine, which causes water to move into your digestive tract. Once it reaches your colon, the lactose is fermented by the bacteria in your gut, forming short-chain fatty acids and gas. This causes the bloating, flatulence and pain.

Why does lactose intolerance cause gastrointestinal problems?

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.

What happens when lactose is not digested?

People with lactose intolerance are unable to fully digest the sugar (lactose) in milk. As a result, they have diarrhea, gas and bloating after eating or drinking dairy products. The condition, which is also called lactose malabsorption, is usually harmless, but its symptoms can be uncomfortable.