Other noteworthy genes included HERC2, which has implications in the expression of OCA2 (melanocyte-specific transporter protein), and IRF4, which encodes for an important enzyme in folate-dependent melanin production. In an Australian cross-sectional study that analyzed vitamin D and pigmentation gene polymorphisms in conjunction with plasma vitamin D levels, the most notable rate of vitamin D loss occurred in individuals with the darkest pigmentation HERC2 (AA) genotype.31 In contrast, the lightest pigmentation HERC2 (GG) genotypes had increased vitamin D3 photosynthesis. Interestingly, the lightest interferon regulatory factor 4 (IRF4) TT genotype and the darkest HERC2 AA genotype, rendering the greatest folate loss and largest synthesis of vitamin D3, were not seen in combination in any of the participants.30 In addition to HERC2, derived alleles from pigment-associated genes SLC24A5*A and SLC45A2*G demonstrated greater frequencies in Europeans (>90%) compared to Africans and East Asians, where the allelic frequencies were either rare or absent.1 This evidence delineates not only the complexity but also the strong relationship between skin pigmentation, latitude, and vitamin D status. The GWAS also have supported this concept. In comparing European populations to African populations, there was a 4-fold increase in the frequencies of “derived alleles of the vitamin D transport protein (GC, rs3755967), the 25(OH)D3 synthesizing enzyme (CYP2R1, rs10741657), VDR (rs2228570 (commonly known as FokI polymorphism), rs1544410 (Bsm1), and rs731236 (Taq1) and the VDR target genes CYP24A1 (rs17216707), CD14 (rs2569190), and CARD9 (rs4077515).”32
Articles With Evidence Against the Vitamin D Theory—This review analyzed the level of support for the theory that vitamin D was the main driver for skin lightening. Although most articles supported this theory, there were articles that listed other plausible counterarguments. Jablonski and Chaplin3 suggested that humans living in higher latitudes compensated for increased demand of vitamin D by placing cultural importance on a diet of vitamin D–rich foods and thus would not have experienced decreased vitamin D levels, which we hypothesize were the driver for skin lightening. Elias et al39 argued that initial pigment dilution may have instead served to improve metabolic conservation, as the authors found no evidence of rickets—the sequelae of vitamin D deficiency—in pre–industrial age human fossils. Elias and Williams38 proposed that differences in skin pigment are due to a more intact skin permeability barrier as “a requirement for life in a desiccating terrestrial environment,” which is seen in darker skin tones compared to lighter skin tones and thus can survive better in warmer climates with less risk of infections or dehydration.
Articles With Neutral Evidence for the Vitamin D Theory—Greaves41 argued against the idea that skin evolved to become lighter to protect against vitamin D deficiency. They proposed that the chimpanzee, which is the human’s most closely related species, had light skin covered by hair, and the loss of this hair led to exposed pale skin that created a need for increased melanin production for protection from UVR. Greaves41 stated that the MC1R gene (associated with darker pigmentation) was selected for in African populations, and those with pale skin retained their original pigment as they migrated to higher latitudes. Further research has demonstrated that the genetic natural selection for skin pigment is a complex process that involves multiple gene variants found throughout cultures across the globe.
