One-fifth of the world’s population has ‘superior resilience’ to colder temperatures thanks to a genetic mutation, which means they will never feel the cold, research reveals.
Researchers at the Karolinska Institutet in Sweden got 32 healthy men aged 18 to 40 to sit in 14 degrees Celsius water until their body temperature dropped to 35.5 degrees Celsius.
They then measured the electrical activity of the muscles and took muscle biopsies from the volunteers to study their protein content and fiber composition.
The α-actinin-3 protein, which is found in ‘fast muscle fibers’ in the muscles, is absent in 20% of people and its absence makes them more able to maintain the temperature.
Those without the protein had more slow 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 energetically effective at generating heat.
This allows the person without the protein to control their heat more effectively than someone with the protein and more ‘fast moving’ fibers.
Researchers at the Karolinska Institutet in Sweden got 32 healthy men aged 18 to 40 to sit in 14 degrees Celsius water until their body temperature dropped to 35.5 degrees Celsius. Stock image
The team behind the study believes this genetic variant 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 about 1.5 billion people worldwide will wear the variant today, increasing their tolerance for colder climates.
Co-senior author Håkan Westerblad said: ‘Our study demonstrates improved cold tolerance in people without α-actinin-3, which would have been an evolutionary survival benefit when moving to colder climates.
‘Our study also emphasizes 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 their muscle tone and leads to slower muscle twitching.
When immersed in cold water during an experiment, people with the variant had more muscle tone than shivers.
The loss of α-actinin-3 is caused by the loss-of-function (LOF) variant of the ACTN3 gene and became more common as more people moved to colder environments.
About 1.5 billion people worldwide today carry the ACTN3 LOF variant and therefore lack α-actinin-3.
While this protein deficiency is not linked to muscle disease, it decreases performance during strength and sprint activities.
The change became more prominent when people started moving to cooler climates – which researchers use as their argument for why it can improve cold tolerance.
To test this idea, the team immersed 42 healthy 18- to 40-year-old men with the LOF variant, or working ACTN3, in 14 ° C water.
The men remained in the water for 20 minutes, interrupted by 10 minutes pauses in room temperature air.
Exposure to cold water was continued until rectal temperature reached 35.5 degrees, or a total of two hours plus fifty minute breaks.
Of the men who had the genetic variant, 7 out of 10 were able to maintain their body temperature above 35.5 ° C for the full exposure period to cold water.
Only three and ten of those without the variant were able to do so.
The muscles of people without the protein contain a higher proportion of slow muscle fibers, which allows them to maintain their body temperature in cold environments in a more energy-efficient way
Loss of α-actinin-3 resulted, on average, in half the rate of temperature drop in the rectum and on the calf muscle.
People with the variant also showed a shift to slower muscle fibers, increasing muscle tone instead of shivering during immersion.
In contrast, individuals without the variant had more muscle fibers with rapid twitching, doubling the rate of high-intensity bursting 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 muscle fibers is an energetically effective way to generate heat.
Results in mice showed that α-actinin-3 deficiency does not increase the cold-induced brown adipose tissue, which generates heat in hibernating mammals and human infants.
Co-senior study author Professor Marius Brazaitis, from the Lithuanian Sports University in Kaunas, Lithuania, added: “While there are many opportunities for future research, our results increase our understanding of evolutionary aspects of human migration.
“While the energetically efficient heat generation in people without α-actinin-3 would have been an advantage in moving to colder climates, it could even 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 as energy efficiency and our bodies can even lead to obesity, type II diabetes and other metabolic disorders, Brazaitis added.
For now, it remains uncertain whether the loss of α-actinin-3 affects brown adipose tissue or cold tolerance of human babies, whose survival would have been a major factor during human migration to colder environments.
While the variant may increase slow muscle fibers at birth, this shift may not occur until later in life.
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 are published in the American Journal of Human Genetics.