Lactose Intolerance


What Is It?

Lactose intolerance is a common cause of abdominal cramping, bloating and diarrhea. This condition is caused by insufficient levels of the intestinal enzyme lactase, which breaks down lactose, the predominant sugar in milk, into simpler forms that can then be absorbed into the blood stream. When there is not enough lactase in the small intestine to digest the amount of lactose consumed, the undigested lactose rapidly passes into the colon. Bacteria in the colon then break down some of the lactose, producing hydrogen gas as a byproduct. The remaining lactose also draws water into the colon, causing diarrhea, bloating and flatulence.

The most common cause of lactose intolerance is the genetically determined reduction in lactase production that occurs at about age 5 in many people of African or Asian descent. As many as 90 percent of Asian-Americans, 80 percent of Native-Americans, 65 percent of African-Americans and 50 percent of Hispanics have some degree of lactose intolerance. In contrast, most Caucasians (80 percent) have a gene that preserves the ability to produce lactase into adulthood. There is also a rare syndrome of congenital lactase deficiency in which affected infants have a complete absence of lactase. Unable to digest lactose, the infants have diarrhea from birth. This condition usually was fatal before the development of lactose free infant formulas.

Lactose malabsorption can be caused by various gastrointestinal disorders. Viral or bacterial gastroenteritis and other diseases, such as celiac sprue, can destroy the lactase producing cells that line the small bowel. A condition called bacterial overgrowth, in which the small bowel contains higher than normal levels of bacteria, can cause lactose intolerance. The bacteria break down lactose in the small bowel, causing flatulence and diarrhea.


Common symptoms of lactose intolerance include watery, bulky odd smelling stools; nausea; abdominal pain; cramps; bloating; and flatulence that begin about 30 minutes to two hours after ingesting foods containing lactose. The severity of symptoms varies, depending on the amount of lactose an individual can tolerate, the amount of lactose ingested, and the size and fat content of the meal. People who also suffer from irritable bowel syndrome tend to have more severe symptoms from lactose intolerance.


The diagnosis of lactose intolerance often is suggested when the gastrointestinal symptoms dramatically improve on a lactose free diet. In some cases, further testing is performed to establish the diagnosis.

The most commonly used test to confirm the diagnosis is the lactose breath hydrogen test. The test is painless and noninvasive. You cannot eat food for several hours beforehand.

You begin the test by drinking a liquid that contains lactose. Your breath is then sampled for hydrogen over a few hours. Normally, very little hydrogen is detected in your breath. However, if you have lactose intolerance, the bacteria in your intestine will break the undigested lactose into hydrogen, which is then transported through the bloodstream to your lungs and exhaled. Hence, lactose tolerance can be diagnosed if higher-than-normal hydrogen levels are detected during this test.

An alternative test is the lactose tolerance test. Once again, you take the test on an empty stomach, and you begin by drinking a lactose solution. This test measures blood sugar levels at selected intervals over a few hours to determine your ability to digest lactose. If the lactose is digested, it produces glucose, which elevates your blood sugar level. Unchanged blood sugar readings confirm a diagnosis of lactose intolerance.

A significant number of people with symptoms that suggest lactose intolerance will have normal diagnostic testing. Other possible causes of symptoms similar to lactose intolerance include intolerance to another substance in dairy products, poor digestion of other sugars or complex carbohydrates (which can be determined with a careful dietary history), and irritable bowel syndrome.

Expected Duration

People who develop lactose intolerance as a result of another gastrointestinal disease can expect full recovery of their lactase levels by treating the intestinal problem, but this may take several weeks to months.

Most people with lactose intolerance have genetically determined lactase insufficiency, which is a permanent condition. However, people can avoid symptoms by avoiding foods that contain lactose or by eating them in moderation. In addition, enzyme formulations are for sale, although these products usually do not alleviate the symptoms entirely.


There is no way to prevent genetically determined decreases in lactase levels, which in many people (especially those of African or Asian descent) occurs by age 5. Lactose intolerance that occurs as a result of gastrointestinal disease will go away when the intestinal disease is treated.


The cornerstone of treatment for lactose intolerance involves monitoring the amounts of lactose in foods and taking commercially available enzyme substitutes.

People with lactose intolerance need to read labels of all prepared foods to identify the presence of lactose. The highest concentrations are found in ice cream and milk. Cheeses typically have lower quantities. Some products listed as nondairy, such as powdered coffee creamer and whipped toppings, may contain lactose if they contain ingredients that are derived from milk. When you read food labels, look for words such as whey, curds, milk byproducts, dry milk solids and nonfat dry milk powder. If any of these ingredients are on the label, the product probably contains lactose.

If you completely avoid lactose, your symptoms should go away. If they do not, the diagnosis should be reconsidered. Many people will be able to tolerate a gradual increase in lactose intake if they are careful to monitor their symptoms. Doctors often recommend ice cream for this. It tends to be better tolerated than other foods that contain lactose because of its high fat content. As you gradually increase your lactose levels, review your diet with your doctor or a nutritionist to be sure that you are eating the proper proportions of fat, protein and other nutrients.

There are several commercially available enzyme formulations (tablets and liquids) that can serve as lactase replacements. You can add these to foods that contain lactose to reduce symptoms significantly. However, these products rarely alleviate symptoms completely, and the results vary among people and with different product formulations. Pretreated dairy products are an extremely effective alternative. You can add enzyme drops to milk and then refrigerate the milk for 24 hours before use or you can purchase predigested dairy products. Note that acidophilus milk is not sufficiently lactose depleted to be useful to most people.

Many people with lactose intolerance have a difficult time consuming enough calcium in their diet. This increases the risk of osteoporosis, a condition in which bones become thin and fragile. Studies have shown that people who are lactose intolerant have double the risk of fractures. Therefore, it is essential that you consume at least 1,000 milligrams of calcium each day (1,200 milligrams if you are a postmenopausal woman). Most people with lactose intolerance are able to tolerate live culture yogurt as long as it does not contain milk products. Vegetables such as broccoli, Chinese cabbage, collard greens and kale are excellent sources of calcium. If you are not able to ingest an adequate amount of calcium, take a daily calcium supplement.

When To Call A Professional

Call your doctor if you develop symptoms of lactose intolerance after eating dairy products. Although this condition is not dangerous, it can be distressing. There are effective treatments available, so there is no need to suffer.


The prognosis for people with lactose intolerance is excellent. Symptoms can be alleviated if dairy products are limited or avoided, or taken in combination with a commercially available enzyme formulation.

Johns Hopkins patient information

Last revised:

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All ArmMed Media material is provided for information only and is neither advice nor a substitute for proper medical care. Consult a qualified healthcare professional who understands your particular history for individual concerns.