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Friday, September 19, 2008

New Study Adds Finer Details to History of Lactose Tolerance

I'm written on several occasions that lactose tolerance was probably developed first in the Middle East, after dairying was invented. There people processed milk into forms that would keep longer in the heat, like yogurt, kefir, and cheese. Each of these also happens to be relatively low in lactose as well.

You can follow a gradient of lactose tolerance across Europe. The lowest percentages of tolerance (highest percentages of lactose intolerance) are in the south and east. The highest percentages of tolerance are in the north and west. This has enormous implications for food and genetic history. Drinking of higher lactose products, like milk straight from the cow, increased and dairy products spread throughout all foods. Apparently milk products and possibly even lactose tolerance produced healthier humans who could out-reproduce those who stayed lactose intolerant. The ability to better process calcium may have made up for the lack of sunlight, and the vitamin D it contains.

Doing large-scale surveys of lactose tolerance or intolerance was difficult in the past because few people would want to take a lactose tolerance test if they didn't have to. You'd need very large samples to determine finer shading of tolerance within a country, though. Genetic testing today is easy and large genetic samples exist.

That's what stimulated an article published in the European Journal of Human Genetics. "Lactase persistence-related genetic variant: population substructure and health outcomes," by George Davey Smith et al. (advance online publication 17 September 2008; doi: 10.1038/ejhg.2008.156).

The abstract said in part:

We genotyped the C/T-13910 variant (rs4988235) that constitutes the putatively causal allele for lactase persistence (T allele representing persistence) in a general population sample of 3344 women aged 60–79 years from 23 towns across Britain. We found an overall frequency of 0.253 for the C (lactase non-persistence) allele, but with considerable gradients of decreasing frequency from the south to the north and from the east to the west of Britain for this allele.

In simpler language, the same pattern that stretches across Europe can be found in miniature across Britain. Those in the north and west, which have even less sun, have higher rates of lactose tolerance than those in the south. This also correlates with the traditional invasion patterns, with the earlier inhabitants often descended from Scandinavian countries, known to have very high percentages of lactose tolerance, having been displaced to the north and west by later influxes.

This bit of correlation to earlier studies helps clarify what is still a fuzzy picture of the spread of lactose tolerance.

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