When your small intestine doesn’t produce enough of an enzyme (lactase) to digest the main natural sugar in milk (lactose), it results in a condition called lactose intolerance or lactose malabsorption.


Ordinarily, lactase enzyme breaks down lactose into simple sugars which are absorbed into your blood through your intestinal lining. In lactase-deficient persons, the unbroken lactose sugar in the food moves into the large intestine instead of being processed and absorbed into the blood. Normal gut bacteria in the large intestine then interact with this undigested lactose, producing hydrogen gas and causing the signs and symptoms of lactose intolerance.


There are essentially three types of lactose intolerance, and the underlying cause of lactase enzyme deficiency is different with each type.




This is by far the most common type of lactose intolerance. People with primary lactose intolerance produce plenty of lactase enzyme early in life. This enzyme is essential for infants as they get all their nutrition from milk. As we grow older and replace milk with other foods, our lactase enzyme production normally decreases, but still remains sufficient enough to digest the amount of dairy present in a typical adult diet. In primary lactose intolerance, lactase enzyme production drops off sharply, making dairy difficult to digest by the time the person reaches adulthood. Primary lactose intolerance is genetically determined and tends to run in families. It is noted to occur in a large proportion of people with African, Asian, or Hispanic ancestry. The condition is also more common among those of Mediterranean or Southern European descent than Northern European descent. Lactose intolerance is found to affect more than 90% of people in some ethnic groups and less than 5% in other ethnic groups.




This form of lactose intolerance occurs when your small intestine decreases lactase enzyme production following an illness, injury, or surgery involving your small intestine, which is the usual site of production of this enzyme. Diseases that destroy the lining of the small intestine lead to decreased production of lactase. Examples of diseases associated with secondary lactose intolerance are celiac sprue, bacterial overgrowth, and Crohn’s disease. Secondary lactose intolerance can be either permanent or temporary. Treatment of the causative underlying disorder may restore lactase levels and ease symptoms, though this can sometimes take time.




In rare instances, babies are born with lactose intolerance caused by a complete absence of lactase enzyme. This disorder is hereditary or familial, passed from generation to generation, occurring if both the mother and the father of a person pass on the same gene of lactase absence to a child. Congenital absence of lactase is a very rare cause of lactose intolerance, and if present, symptoms manifest very shortly following birth. Premature babies may have temporary lactose intolerance from inability to make lactase, but this is typically self limited.




There are several risk factors that increase your susceptibility to developing lactose intolerance.


  • Age: Lactose intolerance usually appears in adulthood, most commonly between the ages of 20 and 40. The condition is uncommon in babies and young children.
  • Ethnicity: Lactose intolerance is more common in people of African, Asian, Hispanic, and Native American descent as compared to Europeans.
  • Prematurity at birth: Infants born prematurely may have insufficient levels of lactase because the baby was born before the small intestine developed enough lactase-producing cells.
  • Diseases of the small intestine: Bacterial overgrowth, celiac disease, and Crohn’s can all lead to secondary lactose intolerance.
  • Certain cancer treatments: The risk of lactose intolerance increases with radiation therapy for cancers in the abdomen or with intestinal complications from chemotherapy.