Is it always cold, or is it just you?

A new study suggests that some humans may be genetically predisposed to tolerate sudden dips in temperature better. And the reason could go all the way back to the last Ice Age.

There are some people always armed with winter wear, no matter what the season. They reach for their jackets at the slightest dip in temperature. Genes could be responsible for this kind of hypersensitivity to cold, according to findings from a new study by researchers at the Karolinska Institutet in Sweden.

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The researchers found that individuals who had the protein a-actinin-3 were not able to adjust to lower temperatures as well as those who did not have this protein. A-actinin-3 is a muscle-binding protein found in fast-twitch muscle fibres or white muscle fibres. Those without the protein, correspondingly, tend to have more slow-twitch muscle fibres.

“Slow-twitch muscle fibres are more effective at heat-generation, improving your tolerance to low temperatures,” Håkan Westerblad, professor of cellular muscle physiology at the Karolinska Institutet and leader of one of the three teams behind the study, told Wknd. “When faced with lower temperatures, slow-twitch fibres produce heat through continuous partial muscle contractions, helping your body maintain its temperature in a sustained fashion.”

This is markedly different from the way fast-twitch muscles fibres respond to lowered temperatures. They contract rapidly, leading to shivering. Though they generate energy quickly, they are also more easily fatigued.

Slow-twitch muscle fibres metabolise oxygen to derive energy. This slow oxidation action makes them fatigue-resistant and better able to support endurance activities like cycling, swimming and distance running. Fast-twitch muscle fibres metabolise glucose to generate energy and are thus better suited to bursts of activity, like sprinting or jumping.

According to the Karolinska study — whose findings were published in the American Journal of Human Genetics in February — one in five subjects lacked a-actinin-3, which means they had more slow-twitch fibres. And why they likely ended up without the protein is fascinating.

The lack of a-actinin-3 is a result of a polymorphic mutation in the gene ACTN3. Westerblad explains that these mutations could have occurred when modern Man’s ancestors migrated to cold regions where food was scarce.

At the time, the absence of this protein would have provided a crucial evolutionary advantage. “If your muscles were more energy-efficient, you could protect your body temperature and have a greater chance of surviving,” Westerblad says. And so the body adapted, and learnt how to generate more heat while using fewer calories.

In the modern world, though, the lack of a-actinin-3 — while it means you won’t need winter wear in an air-conditioned space — could have negative consequences, mainly because most modern food is energy-rich. If your body is living in 2021 and using up energy like it’s in the midst of a barren icy tundra, you would need to adjust your intake accordingly.

Either way, it’s interesting to think that your ability to handle a sudden dip in temperature could depend on whether some distant ancestors once struggled to survive in an icy wasteland.