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Medical Author:

Siamak T. Nabili, MD, MPH

Siamak T. Nabili, MD, MPH

Dr. Nabili received his undergraduate degree from the University of California, San Diego (UCSD), majoring in chemistry and biochemistry. He then completed his graduate degree at the University of California, Los Angeles (UCLA). His graduate training included a specialized fellowship in public health where his research focused on environmental health and health-care delivery and management.

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Medical Editor:

Bhupinder Anand, MD

Lactose intolerance is due to a basic deficiency of the lactase enzyme. This enzyme is normally found on the inner lining of the intestinal wall and splits the carbohydrate lactose into smaller components, glucose and galactose. These breakdown products are then absorbed and transported from the intestinal wall for further digestion.

Lactose intolerance may occur because of a deficient or completely absent lactase enzyme level. The complete absence of lactase is a rare genetic condition where the gene responsible for producing the lactase enzyme is defective. This form of lactase deficiency results in a marked lactose intolerance from birth.

More commonly, lactose intolerance is an acquired condition, not present at birth. The intestinal wall may become damaged due to a variety of reasons, commonly from infections or certain medications. Some of the possible causes of lactase deficiency include:

• gastroenteritis by viruses, bacteria or worms,
• celiac disease,
• Crohn's disease,
• radiation therapy,
• diabetic enteropathy,
• HIV enteropathy, and
• some chemotherapeutic drugs.

The most common cause of lactose intolerance is the gradual loss of lactase after childhood. In this type of lactose intolerance, there is a genetically progressive decrease in the lactase enzyme levels. In populations with high prevalence rates of lactose intolerance, a gradual loss of lactase is more common and begins at an earlier age. In Asians and Asian Americans, the decline in lactase enzyme levels is more prevalent, followed by Native Americans, African Americans, and Hispanics.

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