Lactose Intolerance

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Lactose intolerance refers to the clinical syndrome in which symptoms of bloating, flatulence, abdominal discomfort, and diarrhea arise following the consumption of lactose-containing foods. Lactose is a sugar found in dairy products. It is a disaccharide, meaning it is made of two monosaccharides (simple sugars), namely glucose and galactose. As an aside, the third monosaccharide is fructose, or fruit sugar. Fructose, when combined with glucose, forms the disaccharide sucrose, known as table sugar. In order for it to be absorbed into the body, lactose must be broken down in the gut by the enzyme lactase into glucose and galactose. Failure to break down lactose in the gut creates the symptoms of lactose intolerance. Simple sugars are absorbed at the tips of the microscopic ridges and valleys of the small intestine. These tiny ridges and valleys look like hairs under a microscope and are called villi. In addition, bacteria found in the gut also help to breakdown lactose. Altering the bacterial makeup of the intestines, known as the gut microbiome, may decrease the ability to break down lactose.

Most babies are born with enough lactase to break down milk they are fed. Congenital lactose intolerance, which is rare, causes the baby to begin to have abdominal pain and diarrhea several days after birth. After about three months of age, the levels of lactase in some infants begins to drop, causing symptoms of lactose intolerance. Lactase levels are determined by a person’s genetic background. A minority of humans retain persistently high lactase levels into adulthood. These people usually have a northern European or Scandinavian heritage. The current theory for this is that their ancestors domesticated cattle centuries before other peoples did and therefore consumed diets rich in dairy products. Those of southern European heritage have lower levels of lactase, while those of Asian or African heritage have the lowest levels.

While primary lactose intolerance is genetically determined, secondary lactose intolerance may occur. Diseases that destroy the villi in the intestines, like Crohn’s disease and celiac disease, may cause secondary lactose intolerance, as may diseases or conditions that alter the gut microbiome, for example intestinal infections, radiation/chemotherapy enteritis, and prolonged antibiotic treatment. Lactose intolerance can usually be diagnosed by symptoms and dietary history alone, but there are confirmatory tests. There is the lactose tolerance test, in which the patient ingests a 50-gram dose of lactose and serial fingerstick glucose samples are taken. If the glucose level rises 20 mg/dl or more as compared to baseline, the patient has sufficient lactase to breakdown the lactose into glucose, therefore the patient is not lactose intolerant. There is also a lactose breath test, which measures the level of hydrogen in the breath following a 25-gram lactose challenge. Lactase deficient people will have the lactose they consumed remain in the gut unabsorbed where it is fermented by gut bacteria, creating hydrogen gas.

Treatment is simple—avoidance of lactose containing foods. However, because most dietary calcium and vitamin D come from dairy products, those avoiding dairy must get these nutrients from other foods. Lactose intolerant people can consume up to about one cup of whole milk without causing an increase in symptoms. Alternatively, some people take a pill containing lactase before a meal containing dairy. This helps to prevent symptoms as well.

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 By Peter Galvin, MD

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