Lactase is an enzyme needed to convert milk sugar (lactose) into
simple sugars that the body can use (glucose and galactose). Lactase is
produced in the small intestine.
When a person becomes lactase-deficient (also called
lactose-intolerant), he or she has difficulty digesting milk and milk products.
The undigested lactose is used as food by bacteria that are normally found in
the intestine. This can cause diarrhea, gas (flatulence), and cramps.
Production of lactase is highest at birth in full-term infants. It
begins to decrease at about age 3. Lactose intolerance is common in adults. As
their small intestines produce less lactase, most people naturally decrease the
amount of lactose they eat or drink. Lactase production may also decline
temporarily in response to damage of the intestinal lining, which can occur
from a gastrointestinal illness.