The disaccharide lactose is an important dietary carbohydrate that is present in the milk of all mammals. Lactose digestion occurs in the small intestine, where the enzyme lactase (β-galactosidase) breaks the molecule apart to form galactose and glucose. Surprisingly, the vast majority of the world’s population lacks this enzyme by the age of 4. These people often suffer from bloating, flatulence, abdominal pain, and diarrhea when they drink milk. The diarrhea results from fluids moving into the small intestine in response to the osmotic pressure from undigested lactose. Such an inability to metabolize lactose is called lactose intolerance. People who suffer from lactose intolerance are able to eat cheese, yogurt, and other cooked foods that contain milk because the lactose is hydrolyzed when milk is fermented or cooked.