Title: Lactose intolerance - Symptoms and causes - Mayo Clinic
Headings: Lactose intolerance
Lactose intolerance
Overview
Products & Services
Symptoms
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Causes
Primary lactose intolerance
Secondary lactose intolerance
Congenital or developmental lactose intolerance
Risk factors
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Lactose intolerance

Content: Primary lactose intolerance
People who develop primary lactose intolerance — the most common type — start life producing enough lactase. Infants, who get all their nutrition from milk, need lactase. As children replace milk with other foods, the amount of lactase they produce normally drops, but usually remains high enough to digest the amount of dairy in a typical adult diet. In primary lactose intolerance, lactase production falls off sharply by adulthood, making milk products difficult to digest. Secondary lactose intolerance
This form of lactose intolerance occurs when your small intestine decreases lactase production after an illness, injury or surgery involving your small intestine. Diseases associated with secondary lactose intolerance include intestinal infection, celiac disease, bacterial overgrowth and Crohn's disease. Treatment of the underlying disorder might restore lactase levels and improve signs and symptoms, though it can take time. Congenital or developmental lactose intolerance
It's possible, but rare, for babies to be born with lactose intolerance caused by a lack of lactase. This disorder is passed from generation to generation in a pattern of inheritance called autosomal recessive, meaning that both the mother and the father must pass on the same gene variant for a child to be affected. Premature infants can also have lactose intolerance because of an insufficient lactase level.