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Lactose intolerance is the inability of the body to break down 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, 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 mucous membrane of the small intestine. Normally, lactase breaks down lactose 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 is not digested and absorbed. The resulting high concentration of lactose draws fluid into the small intestine, causing diarrhea.  

The lactose then passes into the large intestine, where it is fermented by bacteria, resulting in flatulence and acidic stool.

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 prevent the sugar maltose from being absorbed into the bloodstream.

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.

A doctor suspects lactose intolerance when a person has symptoms after consuming dairy products. If a 3- to 4-week 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 by diets avoiding foods containing lactose, primarily dairy products. Lactase is 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.

1. Patients with lactose intolerance should take calcium supplements to prevent calcium deficiency.