One-fifth of the world’s population has “superior resistance” to lower temperatures due to a genetic mutation that allows them to never feel cold, the study shows.
Researchers at the Karolinska Institute in Sweden had 32 healthy men between the ages of 18 and 40 in 14 degrees Celsius water until their body temperature dropped to 35.5C.
They then measured muscle electrical activity and took muscle biopsies of the volunteers to study their protein content and fiber composition.
The α-actinin-3 protein, which is found in the “fast-twitch fibers” inside muscles, is absent in 20% of people, and its absence makes them better at maintaining temperature.
These protein-free had more slow-moving muscle fibers, suggesting that the type of continuous, low-intensity activation found in this alternative to the faster version of a muscle fiber is more energy efficient in generating heat.
In turn, this allows the protein-free person to manage their heat more efficiently than someone who has protein and more “fast-twitch” fiber.
Researchers at the Karolinska Institute in Sweden had 32 healthy men between the ages of 18 and 40 in 14 degrees Celsius water until their body temperature dropped to 35.5C. Stock image
The team behind the study believes that this genetic variant could have protected modern humans from the cold while migrating from Africa more than 50,000 years ago.
Based on their study, the team estimates that approximately 1.5 billion people worldwide will wear the variant today – increasing their tolerance for colder climates.
Co-senior author Håkan Westerblad said: “Our study shows an improved cold tolerance in people lacking α-actinin-3, which would have been an advantage in evolutionary survival when moving to colder climates.
Our study also highlights the great importance of skeletal muscle as a heat generator in humans.
The findings suggest that this is due to the fact that α-actinin-3 deficiency increases cold tolerance by increasing muscle tone and leads to slower contraction muscles.
When submerged in cold water during an experiment, people with the variant had an increase in muscle tone, rather than trembling.
The loss of α-actinin-3 is caused by the loss of function variant (LOF) of the ACTN3 gene and has become more common as more people have moved to colder environments.
About 1.5 billion people worldwide today wear the ACTN3 LOF variant and therefore do not have α-actinin-3.
Although this protein deficiency is not related to muscle disease, it affects performance during power and sprint activities.
The change became more prominent as people began to move to colder climates – which researchers use as an argument as to why they could improve cold tolerance.
To test this idea, the team submerged 42 healthy men between the ages of 18 and 40, either with the LOF variant or with ACTN3 functional in 14 ° C water.
The men remained in the water for 20 minutes, broken by ten-minute breaks in the air at room temperature.
Exposure to cold water was continued until the rectal temperature reached 35.5 degrees, or for a total of two hours plus fifty minutes of breaks.
Of those men who had the genetic variant 7 out of 10, who are able to maintain their body temperature above 35.5 ° C for the full period of exposure to cold water.
However, only three and 10 of those without a variant managed to do so.
The muscles of people without protein contain a higher proportion of slow-moving fibers, which allows them to maintain their body temperature in cold environments in a more energy-efficient way.
On average, the loss of α-actinin-3 led to half the rate of temperature drop in the rectum and calf muscle.
People with the variant also showed a shift to more slow-twitch muscle fibers, causing an increase in muscle tone, rather than trembling during the dive.
In contrast, individuals without the variant had more fast-twitch muscle fibers, which doubled the rate of high-intensity blast activity.
The superior cold resistance of people with the variant was not accompanied by an increase in energy consumption.
This suggests that the continuous, low-intensity activation of slow-moving muscle fibers is an energy-efficient way to generate heat.
The results in mice showed that α-actinin-3 deficiency does not increase cold-induced brown fat adipose tissue, which generates heat in hibernating mammals and human infants.
The co-senior author of the study, Professor Marius Brazaitis, of the Lithuanian University of Sports in Kaunas, Lithuania, added: “Although there are many avenues for future research, our results increase our understanding of the evolutionary aspects of human migration.
“While energy-efficient heat generation in people without α-actinin-3 would have been an advantage when switching to a colder climate, it could actually be a disadvantage in modern societies,” he said. he.
Homes, including Nico protection, are less important and because we have relatively limited access to food, such energy efficiency and our bodies can lead to type II diabetes obesity and other metabolic disorders, Brazaitis added.
For now, it remains uncertain whether the loss of α-actinin-3 affects brown adipose tissue or the cold tolerance of human infants, whose survival would have been an important factor during human migration to colder environments.
While the variant can increase muscle fibers with slow contraction at birth, this change may not occur until later in life.
The researchers add that it is also unclear whether α-actinin-3 deficiency affects heat tolerance or responses to different types of athletic training.
The findings were published in the American Journal of Human Genetics.