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, 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, more than 90% 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 United States' 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.
Cow's milk allergy is different from lactose intolerance. In contrast to lactose intolerance, people with cow's milk allergy can digest milk properly, but proteins in the milk trigger a response by the immune system (see Overview of Allergic Reactions). Cow's milk allergy usually affects children.
People with lactose intolerance usually cannot tolerate milk and other dairy products, all of which contain lactose. Adults usually develop symptoms only after they consume more than 8 to 12 ounces (250 to 375 milliliters) of milk. Some people recognize early in life that milk and other dairy products cause gastrointestinal problems 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.
Children with milk allergy also develop symptoms after consuming milk or milk products. However, these symptoms usually resemble other allergic reactions with itching, rash, and/or wheezing. Sometimes children have digestive tract symptoms, such as vomiting, abdominal pain, and rarely diarrhea.
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, but in some people, doctors confirm the diagnosis with a hydrogen breath test. In this test, people consume a small, measured amount of lactose. Before and after consuming the lactose, doctors measure the amount of hydrogen gas in the person's breath. They measure hydrogen because intestinal bacteria produce hydrogen when they digest unabsorbed lactose. If the amount of hydrogen in the breath rises significantly, then the person is lactose intolerant. The lactose tolerance test is an alternative, less sensitive test that is now rarely done. After people consume lactose, doctors monitor their symptoms and measure their blood glucose levels several times. People who can digest lactose develop no symptoms and their blood sugar rises.
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. Yogurt is often tolerated because it naturally contains lactase produced by Lactobacilli. 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 January 2013 by Atenodoro R. Ruiz, Jr., MD