Friday, June 3, 2005

Can’t drink milk? Blame the cows, the genes and the ancestors rather than the gut.

After studying data from 270 African and Eurasian populations in 39 countries, Cornell University researchers have concluded that the ability to digest milk is hereditary, developing only among those whose distant relatives once tended dairy herds.

In America, the idea translates into numbers. The condition plays ethnic favorites: Up to 75 percent of blacks and American Indians, plus 90 percent of Asian-Americans are unable to digest lactose, a sugar in milk, according to the National Institutes of Health. Overall, 50 million Americans are considered lactose intolerant.

“This is a spectacular case of how cultural evolution — in this case, the domestication of cattle — has guided our biological evolution,” said Paul Sherman, a professor of neurobiology and behavior at Cornell. The findings will be published in upcoming issue of the journal Evolution and Human Behavior.

Those whose ancestors lived in areas with weather extremes in Africa and Asia “do not have the ability to digest milk after infancy.”

Without dairy herds — and milk — around to prompt them, the majority of those populations stopped producing lactase, a human enzyme made in the small intestine that digests the sugar in milk.

Adults who trace their roots to the cow-friendly climes of Europe developed the ability to digest milk — they literally “passed on gene mutations that maintain lactase into adulthood,” the research notes.

A milk-free life is actually more the norm, at least from a biological standpoint. Humans are the only mammals on the planet to persist in drinking milk past infancy. Overall, Mr. Sherman and his research team say that the milk-challenged are actually in the majority worldwide: 61 percent of the populations studied were lactose intolerant.

There are marked geographical divides, though. Just 2 percent of the residents of Denmark have the condition, but it is present in 100 percent of those living in Zambia. The research says that the presence of the condition rises with temperature. The hotter the country, the more lactose intolerance.

The researchers are mystified, though, by a dozen small groups in Africa and the Middle East who happily drank milk, though their neighbors could not.

“The most likely explanation is nomadism,” Mr. Sherman theorizes, noting that historically, the groups often kept small cattle herds and moved before some hidden disease pathogen could pose a threat to their cows.

The trends are part of something called “Darwinian medicine,” Mr. Sherman added — a field of study that emerged around 1980, citing evolutionary explanations for human vulnerabilities to diseases or medical conditions.

Lactose intolerance, which can cause a spectrum of annoying digestive symptoms is a market force, however. According to industry figures, Americans annually buy more than $100 million worth of lactose-free medications and products.

“Hidden” lactose can be a challenge as well. It is found as a binder in 20 percent of our prescription drugs and six percent of over-the-counter medications, and a huge variety of commercially prepared foods.

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