Red hair is the rarest natural hair color, expressed in only 1-2 percent of the world’s population. Of course, that number does not include salon-goers who seek to mask their original hair color.
Red hair’s rarity lies in the variations in the MC1R gene. This gene provides the necessary instructions for our body to make a protein called the melancortin-1 receptor (MC1R). This receptor plays an essential role in pigmentation, and is found on the surface of melanocytes. Melanocytes are the melanin-producing cells of the skin. Melanin is what gives our hair, eyes, and skin color - and protects our skin from sun damage. Specifically, the MC1R controls which type of melanin is produced in the body: eumelanin or pheomelanin. Individuals who produce more eumelanin typically have darker complexions and darker hair. Eumelanin protects the skin from UV-radiation, preventing skin damage when exposed to sunlight. Conversely, those who produce more pheomelanin have skin that is freckled and fairer and have red or blond hair. Unlike eumelanin, pheomelanin does not protect the skin from UV-radiation, making these individuals at a greater risk for sun damage.
Of course, as all genes are, the MC1R gene is subject to variations. This is where the science behind red hair lies. In 1995, the specific alleles linked to red hair were discovered. Simply, an allele is one version of the DNA sequence found on a gene. At any one genomic location, we inherit two alleles – one from each parent. The number of alleles that may code for red hair has only increased in recent years, and they are referred to as the red hair color (RHC) alleles. It is estimated that 80% of people with red hair have a variant of the MC1R gene associated with red hair color.
Interestingly, these variations don’t only contribute to their ginger locks, fair complexion, and freckles: they may even impact responses to pain. Research has observed that among mice and humans who have these MC1R gene variations, their acute pain reception is different from those who lack these variations.
One 2021 study, led by Pierre Fontanillas, found that women with red hair have a higher sensitivity to pain. In this study, 25,321 participants filled out a pain sensitivity questionnaire, with 6,853 completing an at-home cold pressure test. This specific test sought to assess cold pain threshold (when the subject first noted discomfort) and cold pain tolerance (the time between the cold pain threshold and when the subject removed their hand from the water). When analyzing the results from this evaluation, the researchers found that red-haired women had a far higher pain sensitivity score than women with other hair colors. This difference was not noted among male subjects. While an interesting finding, this study was largely based on self-report data, and at-home testing. It is important to remember that no matter how robust the methods seem in writing, they are of course subject to error in practice. This is also the case in this study, underscoring the importance of interpreting results with caution.
Another area of research that is extremely relevant to pain is investigating why red hair individuals may require greater amounts of anesthesia. In a 2005 study published in the journal, Anesthesiology, researchers investigated individuals’ tolerance to both subcutaneous and topical lidocaine – a commonly used local anesthetic for dental procedures. What researchers found, a result later confirmed in Fontillas’ paper, was that redheads expressed a greater sensitivity to temperature-related pain than non-redheads. They also found that subcutaneous lidocaine (lidocaine injected with a shot) was significantly less effective in the redhead subjects. Topical lidocaine, however, was slightly less effective in this group, but not significantly so. These findings fell in line with collected anecdotal evidence from subjects.
While studies on this topic span several decades, much of the research is limited by small sample sizes. For example, the study examining lidocaine only had 60 total participants. Another, arguably more important, shortcoming found in this body of literature is the lack of MC1R genotyping (the process of determining someone’s genetic makeup) among participants. Rather than grouping people simply based on having red hair, utilizing genotyping enables researchers to identify biological differences between the control and experimental groups (in this case, having an MC1R variant or not). This allows researchers to be more precise with their findings and attribute said findings to specific genetic mechanisms that may influence pain responses. Although anecdotal claims that redheads experience pain more intensely by seem convincing, the clinical evidence falls short.
However, if we were to accept that variations in the MC1R gene intensify feelings of pain, how this happens remains an even greater looming question.
Eva Kellner is a recent graduate from the Faculty of Arts and Science, with a major in Environment. Her research interests include urban green spaces, urban agriculture, and outdoor community spaces - all as promoters of climate resilience among city-dwellers.
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