The most common cause of primary lactose malabsorption is lactase enzyme non-persistence. However, lactose malabsorption may be secondary to an underlying intestinal disease.
Primary Causes of Lactose Malabsorption.
Racial or ethnic – E.g. no
Developmental
Congenital lactase deficiency
Secondary Causes of Lactose Malabsorption.
Small intestinal bacterial overgrowth
Infectious enteritis e.g. Giardiasis
Mucosal injury – e.g. Celiac disease, IBD (esp. Crohn disease); Drug-induced enteritis, radiation-induced enteritis
Reference
Heinz F Hammer, MDChristoph Högenauer, MD. Lactose intolerance: Clinical manifestations, diagnosis, and management. In: UpToDate, Post, TW (Ed), UpToDate, Waltham, MA, 2014.