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Calcium absorption theory 'insufficient' to explain milk tolerance: Study

By Mark Astley+

23-Jan-2014
Last updated on 23-Jan-2014 at 17:14 GMT

Calcium absorption theory 'insufficient' to explain milk tolerance

Analysis of DNA data from the 5,000-year old skeletal remains of eight Iberian farmers suggests that the widely cited theory of calcium absorption was not the only reason Europeans evolved a tolerance for milk.

A multinational team of researchers from Sweden, the UK, France and Spain claim to have “debunked” the calcium assimilation hypothesis - the theory that North Europeans began drinking milk to offset the calcium and vitamin D deficiency caused by a lack of sunlight.

Roughly one third of adults worldwide can drink milk without suffering stomach problems. This trait, known as lactase persistence, is a genetically-determined characteristic whereby the enzyme lactase - necessary for the digestion of lactose, the main carbohydrate in milk - is expressed throughout adult life.

In Europeans, lactase persistence is “strongly associated with, and probably caused by” a single mutation of the lactase gene, -13,910*T, the study said.

In their study, Direct estimates of natural selection in Iberia indicate calcium absorption was not the only driver of lactase persistence in Europe, the researchers claimed that the calcium assimilation hypothesis is “insufficient” to explain the spread of lactase persistence in Europe over the last 10,000 years.

Milk drinking gene

To test the calcium assimilation hypothesis, the researchers analysed DNA data from the skeletal remains of eight late Neolithic Iberian individuals.

They did not expect the skeletons to exhibit poor vitamin D and calcium status “because of relatively high incident UVB-light levels” in the region.

The tests showed that the milk drinking gene spread as quickly in sun-drenched Spain as it did elsewhere. This, according to the researchers, suggests that milk must have been beneficial in the region for some other reason than its vitamin D content.

“We conclude that the calcium assimilation hypothesis is insufficient to explain the spread of lactase persistence in Europe,” the study said.

Protection against malaria

The study, which was published in the journal Molecular Biology and Evolution, detailed other theories that have been proposed.

Some, according to the study, have suggested that a diet rich in milk may offer some protection against malaria “or that lactase persistence spread as prestige class behaviour.”

“In missed economies when crops have failed, fermented milk products are an alternative source of nutrients to lactase persistence and non-lactase persistence individuals alike,” the study said.

“However, as lower lactose content milk products are consumed so only high lactose content products (milk and yogurt) would be left. In famine conditions the consequences of high lactose food consumption in non-lactase persistence (particularly diarrhoea) would be more severe than in well-nourished non-lactase persistence individuals, perhaps leading to high but episodic selection differentials.”

Source: Molecular Biology and Evolution

“Direct estimates of natural selection in Iberia indicate calcium absorption was not the only driver of lactase persistence in Europe”

Author: O Sverrisdóttir, A Timpson, J Toombs, C Lecoeur, P Froguel, J Miguel Carretero, J Luis, A Ferreras, A Götherström, M Thomas.