The unabsorbed sugar is then fermented by bacteria in the large intestine, producing acidic stool and flatulence. Enzyme deficiency occurs in celiac disease, tropical sprue, and infections of the intestine. Enzyme deficiency also can be congenital, or it can be caused by antibiotics, especially neomycin. Some degree of lactose intolerance occurs in about 75 percent of adults. It affects fewer than 20 percent of adults of northwestern European origin but 90 percent of Asians.
Lactose intolerance is common among people from the Mediterranean area.
About 75 percent of nonwhite North Americans gradually develop lactose intolerance between ages 10 and 20. Symptoms People with lactose intolerance usually can't tolerate milk and other dairy products, which contain lactose. Some people recognize this early in life and consciously or unconsciously avoid dairy products. A child who can't tolerate lactose has diarrhea and doesn't gain weight when milk is part of the diet. Severe diarrhea may prevent proper absorption of nutrients because they're expelled from the body too quickly.