A fifth of the world’s population has “superior resistance” to colder temperatures thanks to a genetic mutation, allowing them to never feel cold, the study reveals.
Researchers at the Karolinska Institutet, Sweden, caused 32 healthy men between the ages of 18 and 40 to sit in water at 14 degrees Celsius until their body temperature dropped to 35.5 degrees.
They then measured muscle electrical activity and took muscle biopsies from the volunteers to study their protein content and fiber composition.
The α-actinin-3 protein, found in ‘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 more slow-twitch muscle fibers, suggesting that the type of low-intensity, continuous activation found in this alternative to the faster version of a muscle fiber is more energy efficient in generating heat.
This, in turn, allows the person without the protein to manage their heat more effectively than someone who has the protein and more ‘fast twitch’ fibers.
Researchers at the Karolinska Institutet, Sweden, caused 32 healthy men between the ages of 18 and 40 to sit in water at 14 degrees Celsius until their body temperature dropped to 35.5 degrees. 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.
Based on their study, the team believes that about 1.5 billion people worldwide will carry the variant today – increasing their tolerance to colder climates.
Co-author Håkan Westerblad said: ‘Our study shows an improved cold tolerance in people lacking 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, increasing muscle tone and leading to slow-twitching muscles.
When immersed in cold water during an experiment, people with the variant experienced an increase in muscle tone instead of tremors.
The loss of α-actinin-3 is caused by the loss of function (LOF) variant of the ACTN3 gene and has become more common as more humans have moved into colder environments.
About 1.5 billion people worldwide are carriers of the LOF ACTN3 variant today and therefore lack α-actinin-3.
Although this protein deficiency is not linked to muscle disease, it impairs performance during strength and sprint activities.
The change became more prominent as humans began to move to colder climates – which researchers use as an argument for why they can improve cold tolerance.
To test this idea, the team immersed 42 healthy men aged 18 to 40 years with the LOF or ACTN3 variant running in water at 14 ° C.
The men remained in the water for periods of 20 minutes, interrupted 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 the genetic variant, 7 out of 10 where they are able to maintain body temperature above 35.5 ° C during the entire period of exposure to cold water.
However, only three and ten of those without the variant were able to do this.
The muscles of people without protein contain a higher proportion of slow-twitch fibers, which allows them to maintain body temperature in cold environments more efficiently in terms of energy
On average, the loss of α-actinin-3 resulted in half the rate of decline in temperature in the rectum and calf muscle.
People with the variant also showed a shift towards slow-twitch muscle fibers, causing an increase in muscle tone instead of tremors during immersion.
On the other hand, individuals without the variant had more fast-twitch muscle fibers, which doubled the rate of high-intensity explosion 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 in rats showed that α-actinin-3 deficiency does not increase cold-induced brown adipose tissue, which generates heat in hibernating mammals and human babies.
The study’s co-author, Professor 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 the evolutionary aspects of human migration.
“While energy-efficient heat generation in people without α-actinin-3 would be an advantage when moving to colder climates, it can actually be a disadvantage in modern societies,” he said.
Housing including Nico protection is less important and as we have relatively limited access to food, energy efficiency and our bodies can actually result in obesity, type II diabetes and other metabolic disorders, added Brazaitis.
For the time being, it remains uncertain whether the loss of α-actinin-3 affects the brown adipose tissue or the cold tolerance of human babies, whose survival would have been an important factor during human migration to colder environments.
Although the variant may increase the muscle fibers of slow contraction at birth, it is possible that this change will only occur 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 results were published in the American Journal of Human Genetics.