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Causes
By Mayo Clinic staff
The cells that line your small intestine produce an enzyme called lactase. Lactase breaks down lactose into two simple sugars — glucose and galactose — which can be absorbed into your bloodstream. Without lactase, the unprocessed lactose moves on to the colon, where the normal intestinal bacteria contend with it. This causes the hallmarks of lactose intolerance — gas, bloating and diarrhea.
There are three types of lactose intolerance.
Normal result of aging for some people (primary lactose intolerance)
Normally, your body produces large amounts of lactase at birth and during early childhood, when milk is the primary source of nutrition. Usually your lactase production decreases as your diet becomes more varied and less reliant on milk. This gradual decline may cause symptoms of lactose intolerance.
Result of illness or injury (secondary lactose intolerance)
This form of lactose intolerance occurs when your small intestine decreases lactase production after an illness, surgery or injury to your small intestine. It can occur as a result of intestinal diseases, such as celiac disease, gastroenteritis or an inflammatory bowel disease like Crohn's disease. This type of lactose intolerance may last only a few weeks and be completely reversible. However, if it's caused by a long-term illness, it may be permanent.
Condition you're born with (congenital lactose intolerance)
It's possible for babies to be born with lactose intolerance. This rare disorder is passed from generation to generation in a pattern of inheritance called autosomal recessive. This means that both the mother and the father must pass on the defective form of the gene for a child to be affected. Infants with congenital lactose intolerance are intolerant of the lactose in their mothers' breast milk and have diarrhea from birth. These babies require lactose-free infant formulas.
