Lactose intolerance -- an inability to tolerate dairy products -- is one of the most common inherited conditions in the world, although the exact prevalence is unknown and probably overestimated, according to the "Best Practice Journal." While uncommon in infants, lactose intolerance that develops gradually, called late-onset or adult-onset lactose intolerance, is common in adults. Gastrointestinal disorders can also cause temporary or permanent lactose intolerance. Symptoms of lactose intolerance include gas, diarrhea and abdominal pain 30 minutes to 2 hours after eating dairy products.
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Late-Onset Lactose Intolerance
The ability to digest lactose, the milk sugar in dairy products, requires the presence of an enzyme called lactase. Produced by cells in the lining of the small intestine, lactase breaks lactose down into the sugars that can be absorbed into the bloodstream.
Lactase production decreases, starting after age 2 and continuing through adolescence. The decrease is more profound in some ethnic groups than others, the "Handbook of Pediatric Nutrition" reports. Lactose intolerance affects people of African-American, Asian, Native American and Hispanic descent more often than those of European descent. Lactose intolerance is relative, with small amounts of lactose usually not causing symptoms. Most people can tolerate up to 12 grams of lactose -- about the amount found in a 1-cup serving of milk -- at a time without symptoms.
Secondary Lactose Intolerance
Acquired or genetic conditions, such as irritable bowel syndrome, Crohn disease and HIV infection, can cause secondary lactose intolerance in adults. Destruction of the cells that produce lactase because of damage to the lining of the small intestine cause lactose intolerance with these conditions. Certain drugs, such as the antibiotic tetracycline, can also decrease lactase production by adversely affecting the lining of the small intestine, the October 2007 issue of "Best Practice Journal" reports. Alcohol can also inhibit lactase production and cause secondary lactose intolerance in adults.
Acute Gastrointestinal Conditions
Lactose intolerance can cause temporary problems if you've had a viral or bacterial infection of the gastrointestinal tract. In these cases, the infection temporarily disrupts the function of the lactase-producing cells. Lactase is the first enzyme affected and the last to return to normal production, according to a 2010 "Clinical Reviews" report. You might notice that you can't eat dairy without experiencing symptoms of lactose intolerance for a short time.
Radiation and chemotherapy destroy fast-growing cells in an effort to eliminate rapidly-growing cancer cells. If you have radiation to the intestinal tract or undergo chemotherapy, you might develop a temporary lactose intolerance due to damage the fast-growing cells of the intestine. In cases of temporary lactose intolerance, gradually increasing the amount of dairy foods in the diet helps prevent symptoms.
For many older adults, the production of lactase may decrease with age, causing an increase in lactose intolerance even in people who haven't previously had a problem digesting lactose. Lactose intolerance is more common in individuals age 74 and older compared to those under age 65, the textbook "Geriatric Gastroenterology" reports. However, the National Institutes of Health points out that little evidence suggests a decrease in lactose production as people age. More research on this topic is needed.