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A mutation that is common in northern Europe is less so in Africa.

A genetic variation that increases muscle tone also makes people more tolerant of cold conditions, according to a study of volunteers dunked for long stretches in chilly water.

More than 1.5 billion people globally have two non-functional versions of the gene ACTN3, which encodes a muscle protein. The non-functional gene is more common in the colder climates of central and northern Europe than in Africa.

To find out why, Håkan Westerblad at the Karolinska Institute in Stockholm, Marius Brazaitis at the Lithuanian Sports University in Kanaus and their colleagues recruited 27 men who had functional ACTN3 and 15 who did not. Participants were immersed in water at 14 ˚C in 20-minute bursts until their body temperature had fallen below 35.5 ˚C, or they had spent a total of 120 minutes in the cold water.

The authors found that 69% of the participants with non-functional ACTN3 could maintain their body temperature above 35.5 ˚C, compared with only 30% of those with the functional variant. Those lacking functional ACTN3 — and the protein it encodes — seemed to preserve heat not by shivering but by tensing their muscles, temporarily increasing muscle tone.

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