Lactose intolerance is a common malabsorption disease caused by insufficient lactase enzyme in the small intestine resulting in flatulence, diarrhea, bloating, and cramping abdominal pain after ingesting lactose-containing foods. The incidence of lactose intolerance varies among ethnicities, with patients of African, Asian, Native American, and Hispanic descent most affected.

Lactase deficiency can be primary, secondary, or congenital.

• Primary – Low levels of lactase enzyme. Symptom onset may be in childhood or adulthood.
• Secondary – Reversible lactase deficiency caused by mucosal injury, often in the setting of viral illnesses.
• Congenital – A rare autosomal recessive absence of lactase activity at birth leading to diarrhea and hypercalcemia in infancy, predisposing patients to nephrocalcinosis.