Leena Peltonen of UCLA has discovered the genetic basis for lactose intolerance. Her study supports the theory that retaining the ability to digest milk evolved only in some peoples during the past ten thousand years, as an adaptation to dairy farming.

For the majority of people in the world, including most southern European, Asian and African populations, lactose intolerance is normal. It sets in at weaning or shortly after, when the body stops producing lactase, the enzyme it needs to digest the milk sugar lactose. Without lactase, lactose passes through the stomach undigested and reaches the bacteria in the large intestine. There bacteria feed on it, producing by-products that can make people feel gassy and nauseous.

Lactase persistence also seems to be most common among peoples with a long tradition of dairy farming, such as northern Europeans, some groups in India and the Tutsis in central Africa. The fact that the same variations occur in distantly related populations supports the theory that all humans were once lactose intolerant, and that ?lactase persistence? evolved only after people domesticated animals and began drinking their milk. ?I find it ironic that a so-called disease actually represents the original condition,? says Peltonen.

It shows how a genetic change can be caused by a cultural practice, says Kevin Laland of Cambridge University in the U.K. ?There are bound to be thousands of such changes, but there are comparatively few where the gene has been isolated.?

Peltonen?s team studied nine extended Finnish families, as well as some Germans, Italians and South Koreans. They found two variations in the human genome associated with lactose intolerance. One of these ?single nucleotide polymorphisms,? or SNPs, was present in all 236 people who were lactose intolerant, while the other was found in 229. Both SNPs are near the lactase gene, and probably affect proteins that regulate the expression of the gene.

The widespread prevalence of lactose intolerance was first recognized in the 1960s. Before that, the dislike of milk in countries such as China was thought to be due to cultural differences.

A quick and cheap genetic test will soon be able to identify people with lactose intolerance. This will help the many people who suffer from the condition without realizing it, since existing tests are time-consuming and unreliable.

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