Medical Definition of Lactose

  • Medical Author:
    William C. Shiel Jr., MD, FACP, FACR

    Dr. Shiel received a Bachelor of Science degree with honors from the University of Notre Dame. There he was involved in research in radiation biology and received the Huisking Scholarship. After graduating from St. Louis University School of Medicine, he completed his Internal Medicine residency and Rheumatology fellowship at the University of California, Irvine. He is board-certified in Internal Medicine and Rheumatology.

Lactose: The sugar found in milk. Lactose is a large sugar molecule that is made up of two smaller sugar molecules, glucose and galactose. In order for lactose to be absorbed from the intestine and into the body, it must first be split into glucose and galactose. The glucose and galactose are then absorbed by the cells lining the small intestine. The enzyme that splits lactose into glucose and galactose is called lactase, and it is located on the surface of the cells lining the small intestine.

Lactose intolerance is a common medical condition that results in diarrhea, abdominal pain, and gas (flatulence) and is caused by reduced or absent activity of enzyme lactase. When there is a deficiency of lactase, the lactose in the intestine cannot be split for digestion.

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Reviewed on 12/21/2018

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