From MedlinePlus GeneticsLactose intolerance is a condition that makes it difficult to digest lactose, a sugar found in milk and several other dairy products. Lactose is normally broken down by an enzyme called lactase, which is produced by cells in the lining of the small intestine. Lack or loss of lactase has both genetic and non-genetic causes.
Congenital lactase deficiency, also called congenital alactasia, is a disorder in which infants are unable to break down the lactose in breast milk or formula. This form of lactose intolerance results in very severe diarrhea. If affected infants are not given a lactose-free infant formula, they may experience severe dehydration and weight loss.
Lactose intolerance in adulthood can be caused by the reduced production of lactase after infancy (lactase nonpersistence). If individuals with lactose intolerance consume lactose-containing dairy products, they may experience abdominal pain, bloating, flatulence, nausea, and diarrhea beginning 30 minutes to 2 hours later.
Most people with lactase nonpersistence retain some lactase activity and can include varying amounts of lactose in their diets without experiencing symptoms. Often, affected individuals have difficulty digesting fresh milk but can eat certain dairy products such as cheese or yogurt without discomfort. These foods are made using fermentation processes that break down much of the lactose in milk.
https://medlineplus.gov/genetics/condition/lactose-intolerance