lactose intolerance

inability or difficulty digesting milk sugar (characterized by symptoms such as cramping, diarrhea, etc. following the consumption of dairy products)

Lactose intolerance (or hypolactasia) is the term used to describe a decline in the level of lactase, an enzyme needed for proper metabolization of lactose (a sugar that is a constituent of milk and other dairy products), in human beings. Although an estimated 70% of adult humans are considered lactose intolerant, it is uncommon in healthy northern westerners and a few others groups.
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Lactose intolerance refers to the inability of the body to digest lactose. [more]Lactose intolerance - Community and Resources

Noun
1. congenital disorder consisting of an inability to digest milk and milk products; absence or deficiency of lactase results in an inability to hydrolyze lactose
(synonym) lactase deficiency, milk intolerance
(hypernym) genetic disease, genetic disorder, genetic abnormality, genetic defect, congenital disease, inherited disease, inherited disorder, hereditary disease, hereditary condition

inability to digest the milk sugar lactose.