One-fifth of the world’s population has “superior resistance” to colder temperatures thanks to a genetic mutation, which allows them to never feel the cold, the study reveals.
Researchers at the Karolinska Institute in Sweden had 32 healthy men aged 18 to 40 sitting in water at 14 degrees Celsius until their body temperature dropped to 35.5 ° C.
They then measured muscle electrical activity and took muscle biopsies from the volunteers to study their protein content and fiber composition.
The protein α-actinin-3, which is found in the “fast-twitch fibers” within muscles, is absent in 20% of people and its absence makes them better at maintaining temperature.
These without the protein had slower-contracting 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 for generating heat.
In turn, this allows the person who has no protein to manage their heat more efficiently than someone who has the protein and more “fast-twitch” fibers.
Researchers at the Karolinska Institute in Sweden had 32 healthy men aged 18 to 40 sitting in water at 14 degrees Celsius until their body temperature dropped to 35.5 ºC. Stock image
The team behind the study believes that this genetic variant may have protected modern humans from the cold when they migrated out of Africa more than 50,000 years ago.
According to its study, the team believes that approximately 1.5 billion people worldwide will wear the variant today, increasing their tolerance to colder climates.
Co-senior author Håkan Westerblad said: “Our study shows better cold tolerance in people who do not have α-actinin-3, which would have been an evolutionary survival advantage 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 because 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 increased muscle tone rather than trembling.
Loss of α-actinin-3 is caused by the loss of function variant (LOF) of the ACTN3 gene and became more frequent as more humans moved to colder environments.
About 1.5 billion people worldwide carry the ACTN3 LOF variant today and therefore lack α-actinin-3.
Although this protein deficiency is not related to muscle disease, it affects performance during power and speed activities.
The change became more important as humans began to move to colder climates, which researchers use as an argument why it can improve cold tolerance.
To test this idea, the team submerged 42 healthy men aged 18 to 40 with the LOF or ACTN3 variant running in 14 ° C water.
The men remained in the water for periods of 20 minutes, divided by a ten-minute break 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 genetic variant 7 out of 10 where they are able to maintain body temperature above 35.5 ° C during the full period of exposure to cold water.
However, only three and ten of those who did not have the variant were able to do so.
The muscles of people without protein contain a higher proportion of slow-twitch fibers, allowing them to maintain body temperature in cold environments in a more energy-efficient way.
On average, loss of α-actinin-3 caused half the rate of decrease in temperature in the calf’s rectum and muscle.
People with the variant also showed a shift toward slower-contracting muscle fibers, causing an increase in muscle tone rather than trembling during the dive.
In contrast, individuals without the variant had faster-twitch muscle fibers, which doubled the rate of high-intensity burst activity.
The superior cold resistance of people with the variant was not accompanied by an increase in energy consumption.
This suggests that continuous, low-intensity activation of slow-twitch muscle fibers is an energy-efficient way to generate heat.
The results obtained in mice showed that α-actinin-3 deficiency does not increase cold-induced brown fat tissue, which generates heat in mammals and hibernating infants.
The author of the co-senior study, Marius Brazaitis, of the Lithuanian Sports University in Kaunas, Lithuania, added: “While there are many avenues for future research, our results increase our understanding of evolutionary aspects. of human migration.
“While energy-efficient heat generation in people who don’t have α-actinin-3 would have been an advantage in moving to colder climates, it could actually be a disadvantage in modern societies,” he said.
Housing, including Nico protection, is less important and because we have relatively limited access to food, such energy efficiency and our bodies can cause obesity type II diabetes and other metabolic disorders, Brazaitis added.
It is not yet known whether the loss of α-actinin-3 affects brown fat 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 may increase slow-twitch muscle fibers 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 have been published in the American Journal of Human Genetics.