Lactose Intolerance

  • Medical Author:
    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.

  • Medical Editor: Bhupinder Anand, MD

Quick GuideThe Most Common Food Allergies for Kids and Adults

The Most Common Food Allergies for Kids and Adults

Is there test for lactose deficiency?

Although there are several good ways to diagnose lactose intolerance, most people who consider themselves lactose intolerant have never been formally tested for intolerance; approximately 20% of people who think they are lactose intolerant are not lactose intolerant.

Why do 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 and common 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. Also, there are placebo responses, that is, people think they are better when they are not.

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.

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.

Milk challenge

A milk challenge is a simpler way of diagnosing lactose intolerance than an elimination diet. A person fasts overnight and then drinks a glass of milk in the morning. Nothing further is eaten or drunk for 3-5 hours. If a person is lactose intolerant, the milk should produce symptoms within several hours of ingestion. If there are no symptoms or symptoms are substantially milder than the usual symptoms, it is unlikely that lactose intolerance is the cause of the symptoms. It is important that the milk that is used is fat-free in order to eliminate the possibility that fat in the milk is the cause of symptoms. It is not possible to eliminate the possibility that symptoms are due to milk allergy, a very different condition than lactose intolerance; however, this usually is not confusing since allergy to milk is rare and primarily occurs in infants and young children. (If milk allergy is a consideration, pure lactose can be used instead of milk for the challenge.)

An important issue is the amount of milk required for the milk challenge.

  • If a person drinks several glasses of milk or ingests large amounts of milk-containing products in their normal diet, then a larger amount of milk should be used in the challenge, 8-16 ounces in an adult, equivalent to one or two large glasses of milk.
  • If the person being tested usually does not drink several glasses of milk or ingest larger quantities of milk-containing products, there may be a problem with using 8-16 ounces of milk for testing. These larger quantities of milk used for testing may cause symptoms, but the smaller amounts of milk or milk products that these persons ingest in their normal diet may not be enough to cause symptoms. Technically, they may be lactose intolerant when they are tested with larger amounts of milk, but lactose in their normal diet cannot be responsible for their usual symptoms.

Recognition of this issue is important in interpreting the results of a milk challenge. Also, it is important to remember that lactose intolerance does not mean that there is a genetic lactase deficiency.

Breath test

The hydrogen breath test is the most convenient and reliable test for 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 lactose intolerant. The amount of hydrogen or methane excreted in the breath is roughly proportional to the degree of lactose intolerance (and potential lactase deficiency), that is, the larger the amount of hydrogen and/or methane produced, the greater the intolerance or 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 lactose intolerance and potentially lactase deficiency, 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 genetic lactase deficiency; the abnormal test is due to an intestinal condition. 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.

Medically Reviewed by a Doctor on 10/28/2016

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