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Health A-Z

Medical Content Created by the Faculty of the
Harvard Medical School

Lactose Intolerance

What Is It?

Lactose intolerance is a common cause of abdominal cramping, bloating and diarrhea. 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.

Lactose intolerance is a common cause of abdominal cramping, bloating and diarrhea. 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.

From Health A-Z, Harvard Health Publications. Copyright 2007 by the President and Fellows of Harvard College. All rights reserved. Written permission is required to reproduce, in any manner, in whole or in part, the material contained herein. To make a reprint request, contact Harvard Health Publications. Used with permission of StayWell.

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