Lactose intolerance is characterised by reduced lactase concentration in the mucosal brush border of the small intestine (also known as hypolactasia).
It exists in four distinct forms: primary, secondary, congenital, and developmental.
Symptoms can be gastrointestinal and/or systemic.
Dietary elimination and challenge are generally diagnostic.
Treatment includes reduction or elimination of dietary lactose.
If elimination of lactose is necessary, alternative calcium sources are recommended.
Lactose intolerance refers to the development of gastrointestinal and/or systemic symptoms secondary to malabsorption of lactose caused by hypolactasia. It is characterised by reduced lactase (lactase-phlorizin hydrolase enzyme) concentration in the mucosal brush border (microvilli) of the small intestine, usually at the age of weaning.    Malabsorption of lactose, however, does not always manifest as gastrointestinal symptoms; development of symptoms and symptom severity depend on the amount and rate of lactose reaching the colon, as well as the type and amount of colonic flora. 
University College London
University College Hospital and the National Hospital for Neurology and Neurosurgery (Queen Square)
AE declares that he has no competing interests.
Dr Anton Emmanuel would like to gratefully acknowledge Dr Mohammad Azam and Dr Richard J. Farrell, previous contributors to this monograph. MA and RJF declare that they have no competing interests.
Assistant Professor of Medicine
Harvard Medical School
Beth Israel Deaconess Medical Center
AL declares that he has no competing interests.
Coordinator of the Gastrointestinal Motility Laboratory
University of Pavia
1st Division of Medicine
IRCCS S. Matteo Hospital Foundation
MDS is the author of references cited in this topic.
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