Cold air arrives one after another, catching people off guard. It makes one wish to wrap themselves in a blanket every day before heading out to work or school.
However, there are always a few people in the crowd who dress very lightly, as if living in summer. They stand out conspicuously, making those who see them shiver involuntarily:
"Why are some people so unfazed by the cold? I wish I could be as cold-resistant as them!"
"Why, in the same weather, can they wear short sleeves and shorts while I need a down jacket plus three layers of thermal leggings?"
"Do people who love winter swimming have some kind of superpower?"
"What causes us to be left behind, frozen at the starting line?"
The reasons are actually somewhat complex. Human tolerance to cold depends on genetic factors, subcutaneous fat thickness, body size, and more. Some scientists point out that when body temperature drops, the rate at which we release energy from muscle cells also slows down to maintain body heat.
But some people genuinely have an innate advantage in cold resistance—genetic variation!
Human skeletal muscle is mainly composed of two fiber types: slow-twitch skeletal muscle fibers and fast-twitch skeletal muscle fibers.
Slow-twitch muscle fibers contract slowly and are generally responsible for slower aerobic exercises. They are durable and have better tolerance to low temperatures. Fast-twitch muscle fibers contract quickly and are responsible for more powerful explosive or anaerobic movements like jumping and sprinting. They can consume energy rapidly but also fatigue more easily.
Taking chicken as a simple example might help us understand the difference between these two muscle fibers.
A chicken's legs are used for walking over long periods, so slow-twitch fibers and myoglobin are more densely packed. Since myoglobin is rich in iron, the leg meat appears dark red.
Chicken breast muscles are only needed for brief, intense, anaerobic explosive activities when birds flap their wings. With a high density of fast-twitch fibers, chicken breast meat is light pink in color.
The proportion of slow-twitch and fast-twitch muscle fibers in each person's muscles varies from individual to individual. During exercise, people automatically select the appropriate motor units according to their needs.
In non-athletic individuals, the fiber type ratio is close to 1:1. Strength athletes have a higher proportion of fast-twitch muscle fibers; for example, sprinters have 70%–75% fast-twitch fibers. Endurance athletes (like long-distance runners) have 80%–85% slow-twitch fibers.
This also explains why some people are better suited for powerlifting, which requires explosive strength, while others are more suited for long-distance cycling.
Is there a deeper connection between slow-twitch, fast-twitch muscle fibers and the human body's ability to withstand cold? Indeed, there is!
A 2021 paper published in the American Journal of Human Genetics mentioned that researchers recruited 42 healthy men aged 18 to 40. They were asked to soak in 14°C cold water for 20 minutes, then rest at room temperature for 10 minutes, repeating this cycle until their body temperature dropped to 35.5°C.
During the immersion, researchers used electromyography (EMG) to measure the participants' muscle electrical activity, assessed the amount of shivering, and collected muscle biopsies to study protein content and fiber type composition.
Researchers from the Karolinska Institute in Sweden found:
1. 20% of the participants lacked alpha-actinin-3 (ACTN3) in their fast-twitch muscle fibers, meaning they were genetic variants.
2. Among participants with normal genes, only 30% could maintain a body temperature above 35.5°C. The genetic variants performed excellently, with 69% able to maintain a temperature above 35.5°C even after cold water exposure. Their body temperature dropped slowly, and they exhibited very little shivering.
So, how do these genetically variant individuals maintain body heat and resist cold?
The study found that this group activates slow-twitch muscle fibers and releases calcium ions, generating more heat by increasing muscle tension.
However, the human body doesn't rely solely on skeletal muscle for heat production. Did other thermogenic organs—brown adipose tissue—produce heat during the experiment and affect the results?
To eliminate confounding factors, researchers conducted experiments using mice lacking alpha-actinin-3 as subjects. They observed that the characteristics of brown adipose tissue in the genetically variant mice did not change at all.
This means that the ability of the genetic variants to maintain body temperature during the experiment is attributable to the activation of slow-twitch muscle fibers. The warming effect of brown adipose tissue is negligible.
Fifty thousand years ago, human ancestors who migrated from Africa to Europe lost alpha-actinin-3 in their fast-twitch muscle fibers. It was precisely this loss that helped them better cope with cold climates, efficiently using slow-twitch fibers to conserve energy rather than wasting it on shivering.
But in today's society, evolutionary advantages coexist with health risks.
The researchers stated: "Those with the genetic variant are more prone to obesity, type 2 diabetes, and other metabolic disorders. They also have a higher risk of falls in old age because, compared to people with normal genes, they possess fewer fast-twitch muscle fibers, which are less suited for controlling strength and explosive movements."
After weighing the pros and cons, which ability would you prefer to have?
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