Lactose intolerance is the inability to digest the sugar lactose (which is present in all dairy products) because of a deficiency of the digestive enzyme lactase, leading to diarrhea and abdominal cramping.

• Lactose intolerance is caused by a lack of the enzyme lactase.
• Children have diarrhea and may not gain weight, whereas adults have abdominal bloating, cramps, diarrhea, flatulence, nausea, audible bowel sounds (borborygmi), and an urgent need to have a bowel movement.
• The diagnosis is based on recognizing that symptoms occur after a person has consumed dairy products.
• Treatment involves taking supplemental lactase enzymes and avoiding lactose, particularly in dairy products.

Lactose, the predominant sugar found in milk and other dairy products, is broken down by the enzyme lactase, which is produced by the cells in the inner lining of the small intestine. Lactase breaks down lactose, a complex sugar, into its two components, glucose and galactose. These simple sugars are then absorbed into the bloodstream through the intestinal wall. If lactase is lacking, lactose cannot be digested and absorbed. The resulting high concentration of lactose draws fluid into the small intestine, causing diarrhea. The unabsorbed lactose then passes into the large intestine, where it is fermented by bacteria, resulting in flatulence and acidic stool.

Lactase levels are high in infants, permitting them to digest milk. However, in most ethnic groups (80% of blacks and Hispanics, almost 100% of Asians), lactase levels decrease after weaning. These decreased levels mean that older children and adults in these ethnic groups are unable to digest much lactose. However, 80 to 85% of whites of Northwest European descent produce lactase throughout life and are thus able to digest milk and milk products as adults. Therefore, because of the ethnic composition of the U.S. population, it is likely that between 30 million and 50 million people in the United States are lactose intolerant. It is interesting to note that this "intolerance" is really the normal state for more than 75% of the world's population.

Intolerances to other sugars can also occur but are relatively rare. For example, a lack of the enzyme sucrase prevents the sugar sucrose from being absorbed into the bloodstream, and a lack of the enzymes maltase and isomaltase prevents the sugar maltose from being absorbed into the bloodstream.

Symptoms

People with lactose intolerance usually cannot tolerate milk and other dairy products, all of which contain lactose. Some people recognize this early in life and consciously or unconsciously avoid dairy products.

A child who is lactose intolerant has diarrhea and may not gain weight when milk is part of the diet. An adult may have abdominal bloating, cramps, diarrhea, flatulence, nausea, audible bowel sounds (borborygmi), and an urgent need to have a bowel movement between 30 minutes and 2 hours after eating a meal containing lactose. For some people, severe diarrhea may prevent proper absorption of nutrients because they are expelled from the body too quickly. However, the symptoms that result from lactose intolerance are usually mild. In contrast, symptoms that result from malabsorption in such conditions as celiac disease, tropical sprue, and infections of the intestine are more severe.

Diagnosis and Treatment

A doctor suspects lactose intolerance when a person has symptoms after consuming dairy products. If a 3- to 4-week trial period of a diet free of dairy products eliminates the symptoms, the diagnosis is confirmed. Specific tests are rarely necessary.

Lactose intolerance can be controlled through diet by avoiding foods containing lactose, primarily dairy products. Lactase enzymes are available in liquid and tablet forms without a prescription and can be added to milk. Lactose-reduced milk and other products are available at many supermarkets. People who must avoid dairy products should take calcium supplements to prevent calcium deficiency.

Last full review/revision December 2007 by Atenodoro R. Ruiz, Jr., MD