Why can lactose intolerance cause bloating and gas?

Lactose intolerance occurs when the body lacks sufficient amounts of lactase, an enzyme produced in the small intestine that is responsible for breaking down lactose, the primary sugar found in dairy products.21

When this enzyme is deficient, undigested lactose passes into the colon instead of being absorbed. Once in the colon, this sugar undergoes fermentation by gut bacteria. This process produces gas, which leads to symptoms such as bloating and flatulence. Additionally, the presence of undigested lactose in the colon attracts water, which can contribute to further gastrointestinal discomfort.21