A fifth of the world’s population has a ‘superior resistance’ to colder temperatures thanks to a genetic mutation, which enables them to never feel the cold, according to a study.
Researchers at the Karolinska Institutet in Sweden put 32 healthy men aged 18 to 40 years in 14 degrees Celsius water until their body temperature dropped to 35.5C.
They then measured the muscle’s electrical activity and took the muscle biopsies from the volunteers to study their protein content and fiber composition.
The α-actinin-3 protein, which is found in ‘fast-twitch fibers’ in the muscles, is absent in 20% of people, and its absence makes it better to maintain the temperature.
These without protein had more slow 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 effective at generating heat.
This in turn enables the person to manage the heat more effectively than someone who has the protein and more fiber that pulls fast.
Researchers at the Karolinska Institutet in Sweden put 32 healthy men aged 18 to 40 in 14 degrees Celsius water until their body temperature dropped to 35.5C. Stock image
The team behind the study believe that 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 worldwide today will carry the variant – which increases their tolerance to colder climates.
Co-senior author Håkan Westerblad said: ‘Our study shows an improved cold tolerance in people who do not have α-actinin-3, which would have been an evolutionary survival benefit if they had moved to colder climates.
“Our study also highlights the great importance of skeletal muscle as a heat generator in humans.”
The findings suggest that α-actinin-3 deficiency increases cold tolerance by increasing their muscle tone and leading to more slow-moving muscles.
When immersed in cold water during an experiment, people with the variant had an increase in muscle tone rather than shivering.
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 people move to colder environments.
About 1.5 billion people worldwide carry the ACTN3 LOF variant today and therefore do not have α-actinin-3.
Although this protein deficiency is not associated with muscle disease, it impairs performance during strength and sprint activities.
The change became more prominent as people began to move in cooler climates – which researchers use as their argument for improving cold tolerance.
To test this idea, the team immersed 42 healthy 18 to 40 year old men with the LOF variant or ACTN3 in 14 ° C water.
The men remained in the water for 20 minutes, breaking up with ten-minute breaks in room temperature air.
The exposure to cold water was continued until the rectal temperature reached 35.5 degrees, or for a total of two hours plus fifty minute breaks.
Of the men who had the genetic variant, 7 out of 10 kept the body temperature above 35.5 ° C during the entire cold water exposure period.
Only three and ten of those without the variant could do so.
The muscles of people without protein contain a greater amount of slow-moving fibers, which enables them to maintain their body temperature in cold environments in a more energy-efficient way.
On average, loss of α-actinin-3 led to half the rate of drop in temperature in the rectum and on the calf muscle.
People with the variant have also shown a shift towards slower muscle fibers, causing an increase in muscle tone rather than shivering during immersion.
In contrast, individuals without the variant had more muscle fibers that pulled fast, doubling 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 the persistent, low-intensity activation of slow-moving muscle fibers is an energetic way to generate heat.
Results in mice showed that the deficiency of α-actinin-3 does not increase the cold-induced brown adipose tissue, which generates heat in mammals and infants in hibernation.
Fellow senior study author Professor Marius Brazaitis, from the Lithuanian Sports University in Kaunas, Lithuania, added: ‘Although there are many ways for future research, our results increase our understanding of evolutionary aspects of human migration.
“While the energy-efficient heat generation in people who do not have α-actinin-3 would have been an advantage if they moved to colder climates, it could be a disadvantage in modern societies,” he said.
Housing, including protecting Nico, is less important, and since we have relatively limited access to food, such is energy efficiency, and our bodies can lead to obesity type II diabetes and other metabolic disorders, Brazaitis said.
For now, it remains uncertain whether the loss of α-actinin-3 affects brown adipose tissue or cold tolerance in infants, whose survival would have been an important factor during human migration to colder environments.
Although the variation in muscle fibers that move slowly may increase during birth, it is possible that this shift only occurs later in life.
According to researchers, it is also not clear whether the deficiency of α-actinin-3 affects the heat tolerance or reactions to different types of athletic exercises.
The findings were published in the American Journal of Human Genetics.