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Gene Tied to Lactose Intolerance

Could lead to easier diagnosis of dairy problem

MONDAY, Jan. 14, 2002 (HealthDayNews) -- Researchers say they've identified a genetic variant that causes lactose intolerance, a condition that turns dairy foods into a digestive disaster.

In the January issue of the journal Nature Genetics, an international team of researchers report that a DNA variant called C/T-13910 was common to lactose intolerant people from many ethnic backgrounds.

Dallas Swallow, a professor of biology at University College London who is familiar with the findings, says that the results -- while preliminary -- are the strongest link found to date. "It gives the potential for testing, in a very simple way, whether or not milk can be digested by a person or not," she says.

Lactose intolerance is the inability to break down lactose, which is the main sugar in milk. In people who have this condition, the cells lining the small intestine don't make sufficient amounts of the enzyme lactase-phlorizin hydrolase, which breaks down lactose and allows the body to metabolize it.

Although not everyone who is lactose intolerant develops symptoms, those who do usually notice gastrointestinal discomfort between 30 minutes and two hours after consuming food or drinks containing lactose. Symptoms include nausea, cramps, bloating, gas and diarrhea.

Most people are born with the ability to break down lactose, but lactase production slows down around the age of 2, after a child is weaned.

About 30 to 50 million Americans are lactose intolerant. The prevalence of lactose intolerance varies across different ethnic groups. Only about 20 percent of northern Europeans can't digest lactose, compared to roughly 75 percent of African Americans and Native Americans and 90 percent of Asian-Americans.

Previous studies of the gene for lactase-phlorizin hydrolase had never found a mutation associated directly with the gene.

In this study, geneticists at the University of California at Los Angeles, working with colleagues in Finland, zeroed in on a DNA variant located just outside the gene.

The team collected blood samples from 236 lactose-intolerant men and women of African, Caucasian, South Korean, Finnish, German and Italian descent. All of them carried a DNA variant called C/T-13910.

The findings add weight to theories that, in the evolutionary history of humans, being lactose intolerant was normal. Dr. Leena Peltonen, the chair of human genetics at UCLA who led the study, says the variant may actually be the original form of the gene, and people who are tolerant of lactose carry a newer mutation that developed when early humans adopted dairy farming.

The fact that the variant exists in people from so many different populations suggests that it's very old, in an evolutionary sense, she adds.

Currently, doctors have several ways of testing whether a patient is lactose intolerant. The first is a lactose challenge test -- essentially, the patient consumes something containing lactose and any effects are monitored. Doctors can also test for specific gases in the breath of patients with suspected lactose intolerance. But Peltonen says the tests "are all either somewhat unreliable or very tedious."

Another, more conclusive test requires a biopsy to remove a sample of the lining of the small intestine. "That's kind of inconvenient," says Peltonen.

If this new variant proves to be conclusively linked to lactose intolerance, then the condition could be diagnosed from a blood sample, she says. "In the future, I think that this finding provides the basis for a highly reliable DNA diagnosis."

Swallow adds that this test could rule out other conditions with similar symptoms, such as irritable bowel syndrome. "People are notoriously bad at dissecting the relationship between diet and symptoms," she says. "It has been observed that quite a few people with irritable bowel [syndrome] actually are lactose intolerant."

Peltonen says the findings must now be confirmed in larger studies involving thousands of blood samples from people in multiple ethnic groups.

What to Do: To find out more about lactose intolerance, visit the National Institute of Diabetes and Digestive and Kidney Diseases or the National Dairy Council. You can also read this article from the National Digestive Diseases Information Clearinghouse.

SOURCES: Interviews with Leena Peltonen, M.D., Ph.D., professor and chairwoman, Department of Human Genetics, University of California, Los Angeles School of Medicine, Los Angeles; Dallas M. Swallow, Ph.D., professor, Department of Biology, University College London; January 2002 Nature Genetics
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