A USA Today article on the study (“Sixty percent of adults can’t digest milk”) explains the phenomena that is at work: “First off, most people who have bad reactions to milk aren't actually allergic to it, in that it's not their immune system that's reponding [sic] to the milk. Instead, people who are lactose intolerant can't digest the main sugar — lactose — found in milk. In normal humans, the enzyme that does so — lactase — stops being produced when the person is between two and five years old. The undigested sugars end up in the colon, where they begin to ferment, producing gas that can cause cramping, bloating, nausea, flatulence and diarrhea.”
Retaining the ability to digest lactose is genetically heritable and is found primarily in particular ethnic groups. Thus, in the estimation of the USA Today: “It's not normal. Somewhat less than 40% of people in the world retain the ability to digest lactose after childhood. The numbers are often given as close to 0% of Native Americans, 5% of Asians, 25% of African and Caribbean peoples, 50% of Mediterranean peoples and 90% of northern Europeans. Sweden has one of the world's highest percentages of lactase tolerant people. Being able to digest milk is so strange that scientists say we shouldn't really call lactose intolerance a disease, because that presumes it's abnormal. Instead, they call it lactase persistence, indicating what's really weird is the ability to continue to drink milk.”
Presumably, it is the high correlation between Northern European ancestry and the ability of an adult to digest lactose that permits USA Today to label the phenomena as “not normal,” “strange,” and “really weird” — such reckless abandonment of the usual politically correct hand-ringing associated with using such pejorative language in a discussion of any genetic phenomena which correlates to particular ethnicities is almost refreshing.
According to the published study, “Lactase persistence (LP) is common among people of European ancestry, but with the exception of some African, Middle Eastern and southern Asian groups, is rare or absent elsewhere in the world.... It is unlikely that LP would provide a selective advantage without a supply of fresh milk, and this has lead to a gene-culture coevolutionary model where lactase persistence is only favoured in cultures practicing dairying, and dairying is more favoured in lactase persistent populations.”
Essentially, what this means is that a latent capacity for digesting lactose was already present; conditions in certain areas of the world were favorable to passing along this propensity to the next generation and thus it became more common within certain population groups.
According the USA Today article, “The researchers used a computer to model the spread of lactase persistence, dairy farming, other food gathering practices and genes in Europe. Today, the highest proportion of people with lactase persistence live in Northwest Europe, especially the Netherlands, Ireland and Scandinavia. But the computer model suggests that dairy farmers carrying this gene variant probably originated in central Europe and then spread more widely and rapidly than non-dairying groups.... The European mutation is different from several lactase persistence genes associated with small populations of African peoples who historically have been cattle herders.”
Nevertheless, USA Today was able to identify lactase persistence as one more occasion for American and European ‘insensitivity’: “If you're American or European it's hard to realize this, but being able to digest milk as an adult is one weird genetic adaptation.”
While pausing to reflect of this point, a few tongue-in-cheek observations are in order: given the observation that this “weird genetic adaptation” may be skewing the perception of the American public, perhaps the USA Today should be urging teachers to add lactose intolerance education to the laundry list of ‘sensitivities’ to be foisted on Johnny and Susie in the public schools? And what about the school lunch programs pushing a “Euro-centric” diet of milk and cheese? (Actually, cheese is very low in lactose and can usually be eaten by those who are lactose intolerant, according to the National Institue of Health.) In the surreal landscape of American politics, how long will it be before the “Got Milk?” advertising campaign is accused of being a racist taunt? Will healthcare reforms take into account the millions suffering from Lactose Intolerance Syndrome? Surely animal-rights activists will want to get in on the action, too, now that they can identify a long history of Northern European ‘exploitation’ of dairy cattle due to a “weird genetic adaptation”? The possibilities seem almost endless for the various resentment lobbies.