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What causes lactose intolerance?

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Not having enough lactase in the body is the cause of lactose intolerance. The names for the three types of lactose intolerance describe why a person may not have enough lactase1:

  • Primary lactose intolerance. This type develops in people who were once able to fully digest lactose. It is the most common type. Almost all infants make enough lactase to fully digest lactose found in human milk and infant formulas. But at some point after being weaned, most children in the world begin to make less lactase. Most people with primary lactose intolerance can consume some milk products without having symptoms.2
  • Secondary lactose intolerance. This type results from damage to the intestines, such as from severe illness or disease.
  • Congenital lactose intolerance. Infants born with this rare type make no lactase at all. It is not uncommon for secondary lactose intolerance to be misdiagnosed during the newborn period as congenital lactose intolerance.

  1. Eunice Kennedy Shriver National Institute of Child Health and Human Development, NIH, DHHS. (2006). Lactose Intolerance: Information for Health Care Providers (05-5305B). Washington, DC: U.S. Government Printing Office. [top]
  2. Suchy, F. J., Brannon, P. M., Carpenter, T. O., Fernandez, J. R., Gilsanz, V., Gould, J. B., et al. (2010). NIH consensus development conference statement: Lactose intolerance and health. NIH Consensus and State-of-the-Science Statements, 27(2), 1–27. PMID 20186234 [top]

Last Reviewed: 05/06/2014
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