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Friday, December 23, 2005

Lactose Intolerant? Blame Your Ancestors

I've said this before, mainly in my book - Milk Is Not for Every Body - but whether you are or aren't lactose intolerant depends mostly on whether your ancestors were.
And your ancestors' decision whether or not to domesticate milkable animals for dairy products led to the spread of the mutation that allows for lactose tolerance all through adulthood.

More confirmation of this comes from the work of evolutionary biologist Paul Sherman, a professor of neurobiology and behavior at Cornell University. His work appeared in a recent issue of Evolution and Human Behavior, but I'm going to spare you and just quote from an article about it, written by Michael Kanellos at CNET News.

Researchers at Cornell University have shown in a recent study that lactose intolerance is largely a factor of cultural evolution. That is, members of ethnic groups that emerged in regions where raising cattle was common often are more genetically predisposed to digest milk products. Meanwhile, people whose ancestors came from regions where extreme temperatures, short growing seasons and dangerous animal-borne diseases made animal husbandry expensive and difficult often feel cramped and nauseous after eating dairy products.

In cheese-happy Denmark, for instance, only 2 percent of the population studied was lactose intolerant. In Zambia, near the equator, 100 percent of the individuals studied were lactose intolerant.

"The implication is that harsh climates and dangerous diseases negatively impact dairy herding and geographically restrict the availability of milk, and that humans have physiologically adapted to that," Sherman said.

The key to lactose intolerance is the enzyme lactase, which is required to digest milk. Although infants around the world produce the enzyme, more than half the people in the world, particularly those of Asian and African descent, stop producing it as they mature. People of northern European descent tend to continue to produce the enzyme because of a genetic mutation, according to Sherman. Thus, they can drink milk throughout life.

The full journal article can be found at Bloom, G. and P.W. Sherman. 2005. Dairying barriers and the distribution of lactose malabsorption. Evolution and Human Behavior 26:301-312.

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