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Medical Author:

Jay W. Marks, MD

Jay W. Marks, MD

Jay W. Marks, MD, is a board-certified internist and gastroenterologist. He graduated from Yale University School of Medicine and trained in internal medicine and gastroenterology at UCLA/Cedars-Sinai Medical Center in Los Angeles.

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Medical Editor:

Bhupinder Anand, MD

In this Article

• 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
• Intestinal biopsy
• 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?
• Lactose Intolerance At A Glance
• Patient Discussions: Lactose Intolerance - Causes
• Patient Discussions: Lactose Intolerance - Symptoms at Onset of Disease
• Lactose Intolerance Glossary
• Lactose Intolerance Index
• Find a local Gastroenterologist in your town

How are lactase deficiency and lactose intolerance diagnosed?

Although there are several good ways to diagnose lactose intolerance, most people who consider themselves lactose intolerant have never been formally tested for intolerance. This is unfortunate because at least 20% of people who think they are lactose intolerant are not, and 20% of people who think they are not intolerant, in fact, are intolerant.

Why should so many people believe that they are lactose intolerant when they are not? This erroneous belief may be common for several reasons. People with unexplained (undiagnosed) gastrointestinal symptoms are looking for an explanation for their symptoms. Since lactose intolerance is a well-known condition, it provides these people with a ready (and welcome) explanation for their symptoms. Confirmation that lactose intolerance is present often is made subjectively and without careful correlation between ingestion of milk or milk products and symptoms. Extrapolating from data on the effect of placebo, it can be hypothesized that between 20 and 40 percent of people who think they feel better on stopping milk, in fact, are no better. Feeling better for them is analogous to a beneficial (positive) response to placebo.

Formal testing for lactose intolerance is valuable. Not only can testing confirm lactose intolerance and prompt the institution of a reduced or lactose-free diet, but it also can exclude lactose intolerance and direct attention to diagnosing other conditions and diseases that are responsible for the symptoms.

Elimination diet

Probably the most common way that people self-diagnose lactose intolerance is by an elimination diet, a diet that eliminates milk and milk products. There are several problems with this type of "testing."

1. Milk products are so common in prepared foods from the supermarket or restaurant that it is likely that an elimination diet that is not rigorous (i.e., does not eliminate all milk-containing products) will still include substantial amounts of milk. Thus, persons with severe lactase deficiency attempting an elimination diet may be ingesting enough lactose to have symptoms and erroneously conclude that lactose intolerance is not responsible for the symptoms.
   
   
2. People often make the assumption that they are lactose intolerant based on a short trial of elimination. A short trial may be adequate if symptoms are severe and occurring daily, but not if the symptoms are subtle and/or variable. In the latter case, an elimination diet may need to be continued for several weeks.
   
   
3. Because symptoms of lactose intolerance are subjective and variable, there always is the possibility of a "placebo effect" in which people think they feel better eliminating milk when, in fact, they are no better. As discussed previously, with subjective symptoms such as those of lactose intolerance, a placebo effect might be expected to occur 20%-40% of the time.

If an elimination diet is to be used for diagnosing lactose intolerance, it should be a rigorous diet. A rigorous diet requires counseling by a dietician or reading a guide to a lactose-elimination diet. The diet also needs to be continued long enough to clearly evaluate whether or not symptoms are better. If there is doubt about improvement on the diet, particularly if symptoms normally fluctuate in intensity over weeks or months, repeated periods of lactose elimination should be tried until a firm conclusion can be drawn. Elimination of all milk products should eliminate symptoms completely if lactose intolerance alone is the cause of the symptoms.