Normally, the enzymes break these sugars into simple sugars, such as glucose, which are then absorbed into the blood through the intestinal wall. If the necessary enzyme is lacking, the sugars aren't digested, and they can't be absorbed. Thus, they remain in the small intestine. The resulting high concentration of sugar draws fluid into the small intestine, causing diarrhea. 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.