Lactose is a sugar found in milk and milk products. The small intestine—the organ where most food digestion and nutrient absorption take place—produces an enzyme called lactase. Lactase breaks down lactose into two simpler forms of sugar: glucose and galactose. The body then absorbs these simpler sugars into the bloodstream.
Lactase deficiency and lactose malabsorption may lead to lactose intolerance:
- Lactase deficiency. In people who have a lactase deficiency, the small intestine produces low levels of lactase and cannot digest much lactose.
- Lactose malabsorption. Lactase deficiency may cause lactose malabsorption. In lactose malabsorption, undigested lactose passes to the colon. The colon, part of the large intestine, absorbs water from stool and changes it from a liquid to a solid form. In the colon, bacteria break down undigested lactose and create fluid and gas. Not all people with lactase deficiency and lactose malabsorption have digestive symptoms.
People have lactose intolerance when lactase deficiency and lactose malabsorption cause digestive symptoms. Most people with lactose intolerance can eat or drink some amount of lactose without having digestive symptoms. Individuals vary in the amount of lactose they can tolerate.
People sometimes confuse lactose intolerance with a milk allergy. While lactose intolerance is a digestive system disorder, a milk allergy is a reaction by the body’s immune system to one or more milk proteins. An allergic reaction to milk can be life threatening even if the person eats or drinks only a small amount of milk or milk product. A milk allergy most commonly occurs in the first year of life, while lactose intolerance occurs more often during adolescence or adulthood
Many people can manage the symptoms of lactose intolerance by changing their diet. Some people may only need to limit the amount of lactose they eat or drink. Others may need to avoid lactose altogether.
Lactose is in all milk and milk products. Manufacturers also often add milk and milk products to boxed, canned, frozen, packaged, and prepared foods. People can check the ingredients on food labels to find possible sources of lactose in food products.
Common symptoms of lactose intolerance include:
- abdominal bloating, a feeling of fullness or swelling in the abdomen
- abdominal pain
Symptoms occur 30 minutes to 2 hours after consuming milk or milk products. Symptoms range from mild to severe based on the amount of lactose the person ate or drank and the amount a person can tolerate.
A health care provider makes a diagnosis of lactose intolerance based on
- medical, family, and diet history, including a review of symptoms
- a physical exam
- medical tests
Medical, family, and diet history. A health care provider will take a medical, family, and diet history to help diagnose lactose intolerance. During this discussion, the health care provider will review a patient’s symptoms. However, basing a diagnosis on symptoms alone may be misleading because digestive symptoms can occur for many reasons other than lactose intolerance. For example, other conditions such as irritable bowel syndrome, celiac disease, inflammatory bowel disease, or small bowel bacterial overgrowth can cause digestive symptoms.
Physical exam. A physical exam may help diagnose lactose intolerance or rule out other conditions that cause digestive symptoms. During a physical exam, a health care provider usually
- checks for abdominal bloating
- uses a stethoscope to listen to sounds within the abdomen
- taps on the abdomen to check for tenderness or pain
A health care provider may recommend eliminating all milk and milk products from a person’s diet for a short time to see if the symptoms resolve. Symptoms that go away when a person eliminates lactose from his or her diet may confirm the diagnosis of lactose intolerance.
Medical tests. A health care provider may order special tests to provide more information. Health care providers commonly use two tests to measure how well a person digests lactose:
- Hydrogen breath test. This test measures the amount of hydrogen in a person’s breath. Normally, only a small amount of hydrogen is detectable in the breath when a person eats or drinks and digests lactose. However, undigested lactose produces high levels of hydrogen. For this test, the patient drinks a beverage that contains a known amount of lactose. A health care provider asks the patient to breathe into a balloon-type container that measures breath hydrogen level. In most cases, a health care provider performs this test at a hospital, on an outpatient basis. Smoking and some foods and medications may affect the accuracy of the results. A health care provider will tell the patient what foods or medications to avoid before the test.
- Stool acidity test. Undigested lactose creates lactic acid and other fatty acids that a stool acidity test can detect in a stool sample. Health care providers sometimes use this test to check acidity in the stools of infants and young children. A child may also have glucose in his or her stool as a result of undigested lactose. The health care provider will give the child’s parent or caretaker a container for collecting the stool specimen. The parent or caretaker returns the sample to the health care provider, who sends it to a lab for analysis.
Many people can manage the symptoms of lactose intolerance by changing their diet. Some people may only need to limit the amount of lactose they eat or drink. Others may need to avoid lactose altogether. Using lactase products can help some people manage their symptoms.