One study claims that more than one billion people worldwide lack a protein called α-actinine-3 found in muscle fibers due to a modified gene. A particularity that would allow them to be more resistant to the cold, proof in support.
A variant of a gene is a gene that is very close to another gene but has mutation-related differences. Researchers at the Karolinska Institute have discovered a special property that would allow its carriers to be more resistant to low temperatures. They recall that skeletal muscle (under the voluntary control of the nervous system) is made up of fast-contracting muscle fibers and slow-contracting muscle fibers that are more resistant to fatigue. However, the protein α-actinine-3, present in fast-contracting muscle fibres, is absent in almost one person in five, or 1.5 billion individuals, due to a mutation in the gene that codes for the protein.
A genetic mutation linked to the evolution of the human species
The reason is evolution: the prevalence of the mutated gene increased as humans migrated from Africa to colder climates in central and northern Europe.
"This indicates that people who lack α-actinine-3 are better able to stay warm in a harsh climate, but there was no direct experimental evidence for this before. Now we can show that the loss of this protein gives increased tolerance to cold and we have also found a possible mechanism for this," says Prof. Håkan Westerblad.
Body temperature drops less quickly
The researchers proceeded by recruiting 42 men aged between 18 and 40 years, whether or not they were carriers of the variant. They were allowed to sit in cold water (14°C) up to the neck until their body temperature dropped significantly and the experiment was completed. Individuals were taken out of the bath every 20 minutes and rested for 10 minutes at room temperature and then repeated the experiment for the next 20 minutes. This procedure continued until the rectal temperature dropped to 35.5°C or a maximum of 120 minutes of immersion in cold water. The electrical activity of the muscles was studied by electromyography (EMG) and muscle biopsies were taken to study the protein content and the composition of the fiber types.
The results showed that the percentage of individuals able to maintain their temperature above 35.5°C during full exposure to cold water was higher in the "variant" group (69%) than in the control group (30%).
In addition, the researchers found that the skeletal muscles of people without α-actinine-3 contain a higher proportion of slow muscle fibers. Once cooled, they were better able to maintain their body temperature at a certain level: instead of activating the fast fibres, which cause shivering, they tended to produce heat by increasing the basic activity (tone) in the slow fibres.
Sports performance is affected
Clearly, their muscles respond with slower contraction and increased muscle tone generating energy-efficient heat rather than shivering. "The mutation probably brought an evolutionary benefit when migrating to a colder climate, but in our modern society, the ability to save energy may instead mean an increased risk of Western diseases (obesity, type 2 diabetes and other metabolic disorders)," notes Prof. Håkan Westerblad. An assumption that the researchers would now like to explore further, especially since their study also found a surprising difference in these people and their sports performance.
For another question was how the lack of this muscle protein α-actinine-3 could affect the body's response to exercise. According to Prof. Westerblad, "People who lack α-actinine-3 are rarely really good at sports that require strength and explosiveness, whereas this would tend to increase their capacity in endurance sports. However, the scientific team points out that one of the limitations of the study is that in experiments conducted on humans it is more difficult to study the mechanisms in the same level of detail as when they are carried out on animals and cells.
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