Almost one in five people lacking this protein are better at having warm and, energy-wise, at enduring a tougher climate, but there hasn’t been any direct experimental evidence for this before
Skeletal muscle fibers can be classified as “fast” and “slow”. Due to the increased content of myoglobin, “slow” muscles have a richer red color, while “fast” ones remain whiter. Muscle tissue in humans is characterized by a reduced amount of white fibers, and in some it is especially small. The fact is that the work of these fibers is partly provided by the protein actinin-alpha-3, and about 20 percent of people carry a mutation that stops the work of the gene encoding this protein.
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It is known that this mutation began to spread ever since people migrated from Africa to regions with colder climates. Indeed, Swedish scientists recently demonstrated that the loss of actinin-alpha-3 increases resistance to cold and reduces energy expenditure to maintain body temperature.
A team of scientists from Karolinska Institute in Sweden, led by professor Håkan Westerblad conducted experiments with 42 healthy volunteers aged 18 to 40 years. They were placed in cold (14°C) water in “sessions” of 20 minutes with 10-minute breaks. In total, the subjects spent two hours in the cold, while the scientists tracked their muscle contractions using electromyography (EMG). In addition, samples of muscle tissue were taken from them to assess its composition and the content of actinin-alpha-3.
As might be expected, people lacking actinin-alpha-3 had more “slow” red muscles. At the same time, during the cooling period, only 31 percent of the body temperature dropped below 35.5°C, while among the volunteers with actinin-alpha-3 and a large number of “fast” fibers, there were only 30 percent of them. Thus, it can be concluded that the absence of this protein increases the body’s resistance to cold.
Indeed, both types of muscle fibers are capable of maintaining body temperature. However, the “fast” do it with quick, sharp contractions, which is manifested by tremors – not the most effective and rather energy-consuming method. But the “red” fibers warm the body with slow tonic contractions, much more economical.
Obviously, a mutation that disrupted the synthesis of actinin-alpha-3 and increased the content of red fibers compared to white was beneficial in the relocation of ancient people to cold regions. And, of course, it affects personal physical characteristics:
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“People who lack α-actinin-3 rarely succeed in sports requiring strength and explosiveness,” says Professor Westerblad, “but they do better in terms of endurance.”
The results of the study were published in the American Journal of Human Genetics.