Question

Why does lactase deficiency lead to the retention of lactose within the small intestine and prevent transport across the intestinal lumen?

Answer 1

Lactose is a disaccharide composed of glucose and galactose. Milk is a rich source of lactose. Lactose has to be hydrolyzed to glucose and galactose before it can be absorbed through the intestinal epithelium.

This hydrolysis reaction is catalyzed by the lactase enzyme. Lactose intolerance is due to loss of function mutations in the lactase enzyme.

Lactose is a polar molecule. So, it requires membrane carriers for its protein. The transport proteins in the intestinal epithelium are highly specific to their respective sugars.

In the absence of the carriers, lactose cannot be metabolized.