Causes and symptoms of lactose intolerance

Lactose intolerance is usually due to a deficiency of the digestive enzyme lactase in the small bowel mucosa, resulting in lactose malabsorption. Incompletely absorbed lactose is subsequently fermented by bacteria in the large bowel causing abdominal pain, bloating, flatulence, watery diarrhoea, and occasionally, nausea and vomiting. Symptoms associated with lactose ingestion are not always due to lactase deficiency; they may be due to rapid transit of lactose through the small bowel.

Lactase deficiency can be primary with delayed onset (ie adult lactase deficiency), secondary or congenital. Delayed-onset primary lactase deficiency may begin from 7 years of age. It affects over half the world’s population; it is more common in people of Asian and southern European heritage, and in Aboriginal and Torres Strait Islander peoples than in people of northern European descent. Secondary lactase deficiency occurs in response to a variety of intestinal disorders (eg giardiasis, acute gastroenteritis, coeliac disease, small intestinal bacterial overgrowth, Crohn disease), and symptoms usually improve when the causative disorder resolves. Congenital lactase deficiency is rare.