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Lactose intolerance

Medically reviewed by Last updated on Apr 7, 2023.

What is Lactose intolerance?

Harvard Health Publishing

Lactose intolerance is a common cause of abdominal cramping, bloating and loose stools. This condition occurs when the body does not have enough of the intestinal enzyme lactase. The job of lactase is to break down lactose, the main sugar in milk. Once lactose is broken down into simpler forms of sugar, these simple sugars can be absorbed into the bloodstream.

In normal digestion, lactose is digested in the small intestine without the release of gas bubbles. When lactose can't be digested well, it passes into the colon. Bacteria in the colon break down some of the lactose, producing hydrogen gas. The remaining lactose also draws water into the colon. The extra gas and water result in symptoms, such as cramping, diarrhea, bloating and flatulence (gas).

Lactose intolerance usually is genetic (inherited). In many people of African or Asian descent, the body begins making less lactase around age 5. As many as 90% of people from some areas of Eastern Asia, 80% of American Indians, 65% of Africans and African-Americans, and 50% of Hispanics have some degree of lactose intolerance. In contrast, most Caucasians (80%) have a gene that preserves the ability to produce lactase into adulthood.

A rare cause of lactose intolerance is called congenital lactase deficiency. Infants with this condition do not produce any lactase. Unable to digest lactose, the infants have diarrhea from birth. This condition was fatal before the development of lactose-free infant formulas.

Difficulty digesting lactose also can be caused by several 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 more bacteria than normal, can also cause symptoms of sensitivity to lactose in the diet. In this case, the bacteria break down lactose in the small bowel, releasing gas in the process. The gas can cause bloating, cramping and flatulence, and bacterial overgrowth can also cause diarrhea. In this case, the problem is not caused by a lack of the enzyme lactase.


Common symptoms of lactose intolerance include:

Flatulence that begins about 30 minutes to 2 hours after eating or drinking foods or drinks 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 have irritable bowel syndrome tend to have more severe symptoms from lactose intolerance.


It's possible you have lactose intolerance if your symptoms improve dramatically when you avoid lactose. A trial period of a lactose-free diet is usually all that is needed to make the diagnosis of lactose intolerance. In some cases, your doctor will want to do tests to confirm the diagnosis.

One 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 colon will break the undigested lactose into hydrogen gas. The gas is absorbed into your bloodstream and then moves through the bloodstream to your lungs and exhaled. You will be diagnosed with lactose tolerance if higher-than-normal hydrogen levels are detected during this test. Bacterial overgrowth can also cause a positive test result, so it may be considered as an alternative explanation if your test is positive.

Another test that can be used to diagnose lactose intolerance is the lactose tolerance test. This test is rarely used today. You begin this test 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 normally digested, it is broken down into glucose, and this elevates your blood sugar level. You will be diagnosed with lactose intolerance if your blood sugar levels do not change during this test, because this shows that the lactose was not digested in the normal way.

A significant number of people who have symptoms that suggest lactose intolerance will have normal results on diagnostic tests. Similar symptoms (but normal results on the tests) can be caused by fructose, sorbitol or other sugars that are not easily digested in the small intestine. Similar symptoms can also result from irritable bowel syndrome.

Expected duration

People who develop lactose intolerance as a result of gastroenteritis or another insult to digestion can recover completely when the intestinal problem is treated. This may take several weeks to months.

When lactose intolerance is genetic, the condition is permanent. However, people can avoid symptoms by avoiding foods that contain lactose (essentially, dairy products) or by eating them in moderation. In addition, commercially prepared forms of the lactase enzyme are available (for example, Lactaid). These replacement enzymes usually do not relieve the symptoms entirely.


There is no way to prevent lactose intolerance.


There are two main ways to treat lactose intolerance:

People with lactose intolerance need to read labels of all prepared foods to see if they contain lactose. The highest concentrations are found in milk. Cheeses and Greek yogurt typically have lower amounts of lactose. 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 might not be correct. 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 tolerated better than other foods that contain lactose because of its high fat content. As you gradually increase your lactose levels, 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 get rid of symptoms completely, and the results vary among people and with different product formulations.

Many people with lactose intolerance may have a difficult time getting enough calcium in their diet. Also they may have low levels of vitamin D. This increases the risk of osteoporosis, a condition in which bones become thin and fragile.

Therefore, be sure to consume other calcium rich foods. Most people with lactose intolerance are able to tolerate live culture yogurt, one good source of calcium. Vegetables such as broccoli, Chinese cabbage, collard greens and kale are also excellent sources of calcium. There are fewer good natural food sources of vitamin D. You may need a daily vitamin D supplement.

Treatment options

The following list of medications are related to or used in the treatment of this condition.

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When to call a professional

Call your doctor to discuss the possibility that you might have lactose intolerance if you develop symptoms 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 outlook for people with lactose intolerance is excellent. Symptoms can be alleviated if dairy products are limited or avoided, or if they are eaten along with a dose of commercially prepared lactase enzyme.

Additional info

National Institute of Diabetes and Digestive and Kidney Disorders

Learn more about Lactose intolerance

Treatment options

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Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.