Johanna C EscherMD PhD and Hans A BullerMD PhD

Lactose intolerance is a clinical diagnosis and consists of symptoms such as abdominal pain, cramps, nausea, bloating, acidic diarrhea and flatulence after the ingestion of lactose (Suarez et al, 1995). The symptoms can begin 30 minutes to 2 hours after eating or drinking foods containing lactose, primarily dairy products. The severity of symptoms varies depending on the amount of lactose each individual can tolerate. Lactose intake varies with age. Lactose is the primary carbohydrate in milk, accounting for almost 35 to 55% of the daily caloric intake in infants. As weaning foods are introduced, lactose intake falls and gradually approaches the levels ingested by adults. The carbohydrate intake of adults on a typical western diet is approximately 300 g, with a lactose content of 5% (Chitkara et al, 2003). Intolerance to lactose-containing foods is a common problem worldwide except in northern Europe. The prevalence is high in the population from eastern Asia (90% or more), among Native Americans (80 to 95%), and Blacks or African Americans (65 to 75 %) (Huang and Bayless, 1968; Bayless and Rosensweig, 1966).

Lactose malabsorption is a diagnosis that is made in patients with typical symptoms in whom the intestinal malabsorption of lactose has been confirmed by a test (such as the lactose breath hydrogen test). To be absorbed, lactose needs to be hydrolyzed by a p-galactosidase, lactase-phlorizin-hydrolase, generally called lactase. The enzyme lactase hydrolyzes lactose to glucose and galactose. Lactase is found most abundantly in the jejunum at the tip of intestinal villi.

When lactose is not absorbed in the small intestine, it arrives in the colon, where bacterial fermentation will occur. Lactose is converted to short chain fatty acids (SCFA) and hydrogen gas by the bacterial flora, producing acetate, butyrate, and propionate. This increased osmotic load will attract water, and diarrhea may develop. The production of hydrogen serves as the basis for the lactose breath hydrogen test used to diagnose lactose malabsorption. In this test, an oral dose of lactose (2 g/kg, maximum dose 25 g) is given in the fasting state, and breath hydrogen is tested before ingestion and at 30-minute intervals for 3 hours.

Lactose intolerance is not always the result of lactose malabsorption. A significant proportion of patients with suggestive symptoms have normal breath hydrogen tests. Besides a false-negative test (in subjects who are nonhy-drogen producers, or after recent use of antibiotics), symptoms may be caused by psychological factors, coexistent irritable bowel syndrome, intolerance to other components in milk, or by maldigestion of other carbohydrates (Johnson et al, 1993).

Lactase deficiency exists when the enzyme lactase is absent or (more often) present in low levels in the intestine, as shown by enzymatic assay in intestinal biopsies. Symptoms of lactose intolerance may be caused by lactose malabsorption, which is usually attributed to lactase deficiency (or insufficiency). Lactase deficiency can be primary (genetically determined) or secondary (as a result of damage to the small intestinal mucosa). A genetic determinator is now available commercially.

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