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What is Lactose Intolerance?

Lactose Intolerance Is Very Common And Treatment Includes Avoiding Milk Products


Updated June 05, 2014

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.


A glass of milk can be a source of stomach upset, diarrhea, and even pain for people who have lactose intolerance.

Photo © Steve Woods

What is Lactose Intolerance?

Lactose intolerance is caused by the body's inability to digest milk sugar or lactose. In order to break lactose down into simple sugars, or monosaccharides, the body must produce the enzyme called "lactase," which is produced in the small intestine. Without lactase, the lactose from milk products is unable to be digested. This causes the symptoms of gas, cramps and diarrhea that many people experience after eating or drinking dairy products.

Lactose intolerance is a condition that normally develops over time. After about 2 years of age, the body begins to produce less of the enzyme lactase. The reasons for this are not understood. It is rare for an infant to be born lactose intolerant, which can cause vomiting and a "failure to thrive." Symptoms of lactose intolerance can appear years after childhood.

Who Gets Lactose Intolerance, And Why?

Experts estimate that as many as 50 million American adults are lactose intolerant. Lactose intolerance primarily affects people of Chinese, Korean, Japanese, Jewish and African descent. People of northern European and some Middle Eastern (Bedouins, Saudis, Yemenis) descents have little incidence of lactose intolerance. Geographical regions seem to play a role in the incidence of lactose intolerance. Descendents of people from northern Europe, for example, have been dependent on milk products as a food source in their geographic region for a few thousand years. Ethnicities that have a higher percentage of adults with lactose intolerance have not been dependent on milk products in the geographic regions of their ancestors.

What Are The Symptoms Of Lactose Intolerance?

Symptoms of lactose intolerance can include gas, diarrhea, bloating, cramps, nausea and bad breath. These symptoms can begin anywhere from 30 minutes to 2 hours after ingesting lactose and can last for up to 3 days after. The severity of symptoms varies from person to person and is dependent upon the amount of lactose that can be tolerated.

What Does Lactose Intolerance Have To Do With IBD?

Many people with inflammatory bowel disease (IBD) also suffer from lactose intolerance. The symptoms of gas, bloating and diarrhea caused by IBD are compounded by the same symptoms caused by lactose intolerance.

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