People with lactose intolerance or lactase deficiency cannot metabolize lactose properly; they lack lactase, an enzyme required in the digestive system to break down lactose. Patients typically experience bloating, flatulence, and diarrhea after drinking milk.Lactose is a sugar that is found most notably in milk. Lactose intolerance is characterized by abdominal cramps and diarrhea after consumption of food that contains lactose (milk, ice cream).eople with adequate amounts of lactase can break down lactose into glucose and galactose (a type of sugar) which are then absorbed into the bloodstream. In people with low levels of lactase, the lactose is not broken down and does not absorb into the bloodstream, instead it moves into the large intestine (colon).