Lactose intolerance is due to a deficiency of lactase in the small bowel mucosa. The causes of lactase deficiency include congenital lactase deficiency, primary lactase deficiency with delayed onset (adult lactase deficiency) and secondary lactase deficiency. Congenital lactase deficiency is extremely rare. "Adult" lactase deficiency (or delayed onset primary lactase deficiency) may commence from 7 years of age and is extremely common in non-Caucasians. It probably affects over half of the world's population. Secondary lactase deficiency occurs in response to a variety of intestinal disorders including giardiasis, viral infections, coeliac disease, bacterial overgrowth and Crohn's disease.

Lactose intolerance is the symptomatic response to lactose malabsorption. Common symptoms in children and adults include abdominal pain, bloating, distension, flatulence and watery diarrhea. People with lactose intolerance generally do not need to avoid every trace of lactose. Small amounts (eg. 10 g of lactose a day) are often well tolerated because of persisting residual lactase activity, which is inadequate to break down a large lactose load. In people with lactase deficiency, symptoms may occur after consuming 4 g of lactose in one serve (1/3 glass of milk) but there is considerable individual variation in the severity and perception of symptoms in adults. In infants with diarrhea, lactase deficiency is suggested when the stool has an acidic pH and contains more than 0.5% reducing substances. Reducing substances can only be measured in liquid stool. If the stool is formed, there is no significant lactase deficiency.

The gold standard for diagnosis is the lactase level in small bowel biopsies but this test is rarely performed. In patients from high-prevalence populations with typical symptoms, it is reasonable to proceed to a trial of lactose withdrawal from the diet. If the diagnosis remains unclear, a breath test should be considered (where challenge with lactose results in a rise in breath hydrogen, methane or labeled carbon dioxide concentration +/- symptoms). Measurement of lactase levels from duodenal biopsies may be appropriate in selected patients.

A diagnosis of lactose intolerance should not mean that milk and milk products are completely withdrawn from the diet, as they are valuable sources of calcium. Management options include:
~ choosing low-lactose and lactose-free products where possible.
~ spreading the consumption of lactose-containing foods and fluids over the course of the day (rather than consuming large quantities at a time).
~ enzymatically digesting lactose prior to the ingestion of milk using lactase enzyme (available in drop and tablet form from pharmacies).

In secondary lactose intolerance, symptoms usually improve or resolve with treatment of the underlying disease. Persistence of lactose intolerance often implies failure to resolve the underlying condition.

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