- Take the Tummy Trouble Quiz
- Boost Digestive Health
- Digestive System Problem Foods
- Patient Comments: Lactose Intolerance - Symptoms
- Patient Comments: Lactose Intolerance - Causes
- Patient Comments: Lactose Intolerance - Treatment
- Find a local Gastroenterologist in your town
- Lactose intolerance facts
- What is lactose intolerance?
- What causes lactose intolerance?
- What are the symptoms of lactose intolerance?
- How are lactase deficiency and lactose intolerance diagnosed?
- Elimination diet
- Milk challenge
- Breath test
- Blood glucose test
- Stool acidity test
- What are the sources of lactose in the diet?
- How is lactose intolerance treated?
- What are the long-term consequences of lactose intolerance?
- What is new in lactose intolerance?
Quick GuideAllergy List: Watch Out for the Most Common Food Allergies
The hydrogen breath test is the most convenient and reliable test for lactase deficiency and lactose intolerance. For the breath test, pure lactose, usually 25 grams (the equivalent of 16 oz of milk), is ingested with water after an overnight fast. In persons who are lactose intolerant, the lactose that is not digested and absorbed in the small intestine reaches the colon where the bacteria split the lactose into glucose and galactose and produce hydrogen (and/or methane) gas. Small amounts of the hydrogen and methane are absorbed from the colon into the blood and then travel to the lungs where they are excreted in the breath. Samples of breath are collected every 10 or 15 minutes for 3-5 hours after ingestion of the lactose, and the samples are analyzed for hydrogen and/or methane. If hydrogen and/or methane are found in the breath, it means that the small intestine of the person was unable to digest and absorb all of the lactose. He or she is lactase deficient. The amount of hydrogen or methane excreted in the breath is roughly proportional to the degree of lactase deficiency, that is, the larger the amount of hydrogen and/or methane produced, the greater the deficiency. The amount of hydrogen and/or methane in the breath, however, is not proportional to the severity of the symptoms. In other words, a person who produces little hydrogen and/or methane may have more severe symptoms than a person who produces a large amount hydrogen and/or methane.
The breath test is the best test for determining lactase deficiency and lactose intolerance, but it has several weaknesses. The first is that it is a long, boring test. The second is that it suffers from the same issue as the milk challenge test with respect to the quantity of lactose that should be used. (See previous discussion.) Lastly, the breath test can be falsely abnormal when there is spread of bacteria from the colon into the small intestine, a condition called bacterial overgrowth of the small bowel. When overgrowth occurs, the bacteria that have moved up into the small intestine get to the lactose in the intestine before there has been enough time for the lactose to be digested and absorbed normally, and these bacteria produce hydrogen and/or methane. This may lead erroneously to a diagnosis of lactose intolerance. Other conditions also interfere with the breath test. Thus, diseases that markedly speed up transit of lactose through the small intestine prevent lactose from being fully digested and absorbed, leading to a misdiagnosis of lactose intolerance. Recent treatment with antibiotics can suppress colonic bacteria and their production of hydrogen or methane and lead to a misdiagnosis of lactose tolerance. Fortunately, these latter conditions are uncommon and usually can be anticipated on the basis of a person's history or symptoms.