A study revealed that one-fifth of the world’s population are “extremely resilient” to low temperatures thanks to a genetic mutation, which allows them to never feel cold.
And researchers at the Karolinska Institute in Sweden had 32 healthy men between the ages of 18 and 40 sitting in 14 degrees Celsius of water, until their body temperature drops to 35.5 degrees Celsius.
Then they measured the electrical activity of the muscles and took muscle biopsies from the volunteers to study their protein content and fiber composition.
In 20% of people, α-actinin-3, which is found in “fast twitch fibers”, is absent, and its absence makes them better at maintaining temperature.
And these, without the protein, contained slow-twitch muscle fibers, indicating that the kind of continuous low-intensity activation found in this alternative to the faster version of the muscle fibers is more effective in generating heat.
This, in turn, allows a person without protein to manage heat more effectively than someone with faster protein and fiber.
The team behind the study believes this genetic variant may have protected modern humans from the cold when they migrated from Africa more than 50,000 years ago.
Based on their study, the team believes that approximately 1.5 billion people around the world will carry the variant today – increasing their tolerance for cold climates.
Senior co-author Håkan Westerblad said: “Our study shows an improvement in cold tolerance in people who lack α-actinin-3, which may have an evolutionary advantage for survival when moving to cold climates. Our study also highlights the great importance of skeletal muscle as a thermogenic in humans. “.
The results suggest that this is because a lack of actin 3 promotes cold tolerance by increasing their muscle tension, and leads to more slow muscle twitching.
When immersed in cold water during the experiment, people who had the alternative had an increase in muscle tone rather than twitching.
Loss of actin 3 is caused by a loss of function (LOF) in the ACTN3 gene, and is becoming more common as more people move to cooler environments.
About 1.5 billion people around the world carry the ACTN3 LOF variant today and thus lack actin 3.
Although this protein deficiency is not associated with muscle disease, it impairs performance during strength and jogging activities.
The change became more prominent when humans began to move to colder climates – which is what the researchers use as an argument for why it has improved withstand the cold.
To test this idea, the team submerged 42 healthy men between the ages of 18 and 40, using either type LOF or ACTN3 working in 14 ° C of water.
The men were kept in the water for 20 minutes, with a ten-minute break in room temperature air.
Exposure to cold water was continued until the temperature reached 35.5 degrees, or for two hours plus fifty minutes of pausing.
On average, the loss of α-actinin-3 reduced the average temperature of the rectum and calf by half.
People who had the alternative also showed a shift towards slow-twitch muscle fibers, which caused increased muscle tone rather than twitching during immersion.
In contrast, individuals without the variant had fast-twitch muscle fibers, which doubled the rate of high-intensity blast activity.
And resistance to supercooling of people with the variant was not associated with an increase in energy consumption.
This indicates that continuous, low-intensity activation of the slow-twitch muscle fibers is a strongly effective method of heat generation.
Results in mice showed that alpha-actinin-3 deficiency does not lead to increased cold-induced brown adipose tissue, which generates heat in mammals and infants hibernating.
“Although there are many avenues for future investigation, our findings increase our understanding of the evolutionary aspects of human migration,” added study co-author Professor Marius Prazaitz of the University of Sports in Kaunas, Lithuania.
“While generating heat more efficiently in people who lack actinin-3 would have been an advantage when moving to cooler climates, it might actually be a disadvantage in modern societies,” he said.
At present, it remains uncertain whether the loss of α-actinin-3 affects brown adipose tissue or cold tolerance of infants, whose survival was an important factor during the human migration to colder environments.
It is also not clear whether alpha actinin 3 deficiency affects heat tolerance or responses to different types of exercise, the researchers add.
The results are published in the American Journal of Human Genetics.
Source: Daily Mail