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Lactose Intolerance


Zubair Malik

, MD, Lewis Katz School of Medicine at Temple University

Reviewed/Revised Mar 2023
Topic Resources

Lactose intolerance is the inability to digest the sugar lactose because of a lack of the digestive enzyme lactase, leading to diarrhea and abdominal cramping.

  • Lactose intolerance is caused by a lack of the enzyme lactase.

  • Symptoms in children include diarrhea and poor weight gain, whereas symptoms in adults include abdominal bloating and cramps, diarrhea, flatulence, and nausea.

  • The diagnosis is based on recognizing that symptoms occur after a person has consumed dairy products and can be confirmed with a hydrogen breath test.

  • 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 watery diarrhea. The lactose then passes into the large intestine, where it is fermented by bacteria, producing gases that cause flatulence, bloating, and abdominal cramps.

The Small Intestine
Overview of Lactose Intolerance

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 cow's milk trigger a response by the immune system (see Overview of Allergic Reactions Overview of Allergic Reactions Allergic reactions (hypersensitivity reactions) are inappropriate responses of the immune system to a normally harmless substance. Usually, allergies cause sneezing, watery and itchy eyes, a... read more Overview of Allergic Reactions ). Cow's milk allergy usually affects children.

Did You Know...

  • Except for people of Northern European descent, most healthy adults cannot digest significant amounts of lactose and are thus normally "lactose intolerant."

Causes of Lactose Intolerance

Lactase levels are high in infants, permitting them to digest milk. However, in most ethnic groups (80% of Black people and Hispanic people, more than 90% of Asian people), 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 White people 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 that this "intolerance" is really the normal state for more than 75% of the world’s population.

Temporary lactose intolerance can develop when a disorder, such as an intestinal infection (see Overview of Gastroenteritis Overview of Gastroenteritis Gastroenteritis is inflammation of the lining of the stomach and small and large intestines. It is usually caused by infection with a microorganism but can also be caused by ingestion of chemical... read more Overview of Gastroenteritis ), damages the lining of the small intestine. Once people recover from these disorders, they are able to digest lactose again.

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 broken down and absorbed into the bloodstream, and a lack of the enzymes maltase and isomaltase prevents the sugar maltose from being broken down and absorbed into the bloodstream.

Symptoms of Lactose Intolerance

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 and cramps, watery diarrhea, flatulence, nausea, rumbling or gurgling sounds in the bowel (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 Overview of Malabsorption Malabsorption syndrome refers to a number of disorders in which nutrients from food are not absorbed properly in the small intestine. Certain disorders, infections, and surgical procedures can... read more in such conditions as celiac disease Celiac Disease Celiac disease is a hereditary intolerance to gluten (a protein found in wheat, barley, and rye) that causes characteristic changes in the lining of the small intestine, resulting in malabsorption... read more Celiac Disease , tropical sprue Tropical Sprue Tropical sprue is a rare disorder of unknown cause affecting people living in tropical and subtropical areas who develop abnormalities of the lining of the small intestine, leading to malabsorption... read more , and infections of the intestine are more severe.

Cow's milk allergy

Children with cow's milk allergy also develop symptoms after consuming cow's milk or cow's milk products. However, these symptoms, such as itching, rash, and/or wheezing, resemble other allergic reactions. Sometimes children have digestive tract symptoms, such as vomiting, abdominal pain, and rarely diarrhea.

Cow's milk allergy is rare in adults and also may cause vomiting and symptoms of esophageal reflux.

Diagnosis of Lactose Intolerance

  • A doctor's evaluation of symptoms that occur after consuming lactose

  • Sometimes a hydrogen breath test

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, and symptoms then return when the person consumes dairy products, the diagnosis is confirmed.

Specific tests are rarely necessary, but in some people, doctors confirm the diagnosis with a breath test.

The hydrogen breath test (also called a lactose breath test) is a 4-hour test. For 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 at 1-hour intervals. They measure hydrogen because intestinal bacteria produce hydrogen when they digest unabsorbed lactose. If the amount of hydrogen in the breath rises significantly after consuming lactose, then the person is lactose intolerant.

The lactose tolerance test is an alternative, less sensitive test that is now rarely done. After people consume a measured amount of lactose, doctors monitor their symptoms and measure their blood sugar (glucose) levels several times. People who can digest lactose develop no symptoms and their blood glucose level rises. People who cannot digest lactose develop diarrhea, abdominal bloating, and discomfort within 20 to 30 minutes and their blood glucose level does not rise.

Treatment of Lactose Intolerance

  • Avoiding lactose

  • Taking lactase supplements

  • Sometimes calcium supplements

Lactose intolerance can be controlled through diet by avoiding foods containing lactose, primarily dairy products. Yogurt is often tolerated because it naturally contains lactase produced by lactobacilli. Cheese contains lower amounts of lactose than milk and is often tolerated, depending on the amount eaten. 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.

Lactase enzyme supplements are available without a prescription and can be taken when eating or drinking products that contain lactose.

Drugs Mentioned In This Article

Generic Name Select Brand Names
Colief, Lac-Dose , Lactaid, Lactaid Fast Act, Lactrase
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