The relationship between vitamin D and the vitamin D receptor (VDR) has been found to be of recent importance in explaining the modulation of gene expression. The VDR helps us adapt to the climate, is epistatic with skin color genes, and so on. Due to the importance of the VDR, vitamin D, and another nutrient I’ve discussed in the past—folate—this drives the argument that the need to produce vitamin D was an important factor in the evolution of skin colors around the world as migrations out of Africa took place. It is also important to note that other competing hypotheses are not necessarily alternative hypotheses to the VDH (which is short for vitamin D-folate hypothesis), since there is significant overlap between them due to what we now know about the roles of vitamin D, folate (especially due to what we know now about how vitamin D, folate and the VDR regulate gene expression),the VDR, and skin color genes. Thus, the theories have been integrated and the updated hypothesis takes into account the other theories which has significant overlap with the VDH.
Jones et al (2018) is the most recent review of the VDH; in the review, they integrate new findings of folate, vitamin D, the VDR, and skin color genes with other supposedly competing hypotheses into a new and improved VDH model which will be discussed at the end of this article.
Vitamin D is an important hormone (since it is a steroid, not a vitamin), which is the only one that is produced exogenously (from UV rays). Vitamin D is responsible for many physiologic functions including: regulating calcium levels by increasing calcium absorption, stimulates intestinal absorption of phosphate, stimulates osteoblasts which then produce receptor activator nuclear factor (RANKL) which then stimulates osteoclastogenesis which then activates osteoclasts for bone reabsorption (DeLuca, 2004). It has been further noted that around 5 percent of the human genome is under the influence of vitamin D (Jones et al, 2018).
Folate is an important water-soluble B vitamin. Since vitamin D and folate are linked by their sensitivities to UVR, then we must look at them independently and see what they do. In the case of folate, UVR causes folate degradation through the absorption of UVRs or, on the other hand, when folate oxidizes through free radicals after UVR exposure (Jones et al, 2018). So the hypothesis proposes that skin color in high UV areas evolved due to the need for protection of folate levels due to UVR degradation. On the other hand, depigmentation occurred in order for the body to produce adequate vitamin D in low UV areas.
The potential impacts of a deficiency of these nutrients on natural selection is an ongoing debate and is a common argument raised against the vitamin D–folate hypothesis. However, these arguments often do not consider that the benefits of an adequate vitamin D and folate status on reproductive success extend far beyond their roles in maintaining reproductive health.
Vitamin D receptor
In recent years, it has been found that the VDR has had a profound influence on our adaptation to local climates our ancestors found themselves in after the trek out of Africa. Most cells and organs of the body have a vitamin D receptor (Wacker and Holick, 2013), so the importance of the VDR and certain genes involved in the production of skin color, vitamin D, and folate can be seen. Thus, evidence for the hypothesis would be differential expression of certain genes that are related to the VDR. Jones et al (2018) report on a few common VDR variants and ethnicity: FOK1 which has a lower frequency in African than European and East Asian populations, and Cdx2 which was highest in Africans and lowest in Europeans. Tiosano et al (2016) reported that multiple loci which are involved with the VDR gene display strong latitudinal clines, which is evidence for the hypothesis.
The VDR helps humans adapt to changes in UV radiation, it is “part of an evolutionary complex that adapts humans to changing UV radiation” (Hochberg and Templeton, 2010: 310). This is further corroborated by the fact that the VDR promoter and skin color genes are epistatic (Popsiech et al, 2014; Tiosano et al, 2016). Skin pigmentation levels, furthermore, determine plasma vitamin D levels and VDR autoregulation (Saccone, Asani, and Bornman, 2015).
The VDR works in concert with retinoic acid receptors (Schrader et al, 1993) which then bind to nucleotide base pairs called the vitamin D-responsive elements (VDRE) which then exert their effects on gene expression (Kato, 2000; Pike and Meyer, 2010; Janik et al, 2017).
Vitamin D elicits numerous functions on gene expression through the VDR, by binding elements of vitamin D to the target genes. Since the VDR works together with other receptors that bind to the VDRE, they can have strong effects on gene expression. Now, we know that vitamin D and folate are important for humans. We know that the VDR gene appears to be under strong selection, though only in the context of other genes (Tiosano et al, 2016). Thus, the VDR—along with folate and vitamin D—are extremely important for gene expression and the adaptation of the human body to differing climates.
Skin barrier hypothesis
The skin barrier hypothesis (SBH) proposes that dark skin color arose to protect against environmental damage. This hypothesis is based on the fact that darker-pigmented peoples posess an enchanced barrier function in comparison to ligher-pigmented people, which is mainly due to the role of melanin in the scattering of UVR across the skin (Jones et al, 2018). Jones et al state that this hypothesis is “proposed as a discrete theory to the vitamin D-folate hypothesis“, but since both vitamin D and folate both have other responsibilities in the human body such as the development of skin structure, and the development of defense mechanisms that protect against UV radiation including heat and microbial stressors.
Folate may also have another important role in the human body: regulating the production, and stabilizing tetrahydrobiopterin. Melanin supports folate from UVR degradation, which then supports folate’s influence on melanin. But, as Jones et al write, tetrahydrobiopterin also acts as a cofactor in the synthesis of nitric oxide which is important in regard to vasoconstriction (blood vessel constriction). Vasoconstriction is related to increased heat flow since blood vessels are constricted, along with an increase in heart rate. As I have noted in the past, shivering revs the body’s metabolism in cold clmates in order to produce ample heat. Jones et al (2018) write:
From an evolutionary perspective, our ability to maintain vasodilation/vasoconstriction mechanisms would have been important in surviving varying UVR environments. As these mechanisms may been seen as relatively short-term responses to temperature changes, they are likely to be of greater importance in temperate UVR environments rather than environments of high UVR. This is supported by nitric oxide dependent vasodilation shown to be reduced in darkly skinned populations . This suggests that vasodilation processes offer no advantage in extreme UVR environments but may be important in temperate UVR environments, where seasonal and daily temperature fluctuations are seen.
Thus, since there would be no advantage for this mechanism in equatorial climates, it must be for more colder, Arctic climates which further lends credence to the VDH. (Since vitamin D and folate play many roles in regard to human physiologic adaptation to climate, along with the VDR.)
Metabolic conservation hypothesis
This hypothesis proposes that our ancestors became depigmented after the migrations out of Africa since there was a need to draw energetic resources away from melanin production and move that energy that would have been for melanin production for other metabolic processes that a population would need in a colder environment. Thus, it is argued that the lighter skin of European and East Asian populations can be explained by the need energetic resources being moved away from pigmenting the skin to other, more important, metabolic processes that the ancestors of Europeans and East Asians experienced. But this hypothesis has numerous premises of the VDH, including the main premise: that human skin depigmented as we migrated into areas with fewer UV rays (Jones et al, 2018). Thus, vitamin D was extremely important in driving the effects of vasodilation/vasoconstriction.
Clearly, the role of vitamin D in the adipose tissue was important for human adaptation to colder climates. Since lighter skin can produce more vitamin D in low UV climates, this was another factor that helped when we left Africa: skin lightened for better vitamin D synthesis. Since vitamin D synthesis is related to gene expression and expression of about 5 percent of our genomes, the production of more vitamin D was beneficial. So depigmentation, while being primarily due to low UV radiation, can also be seen to allow for more efficient physiologic responses and adaptations to the newer, colder climates.
Skin mutagenesis hypothesis
The last competing theory is the skin mutagenesis hypothesis. This hypothesis proposes that skin pigmentation arose as a mechanism to protect against various skin cancers. The hypothesis is based on the fact that darker-pigmented individuals are at lower risk of developing skin cancers since their skin pigmentation can fight off UV radiation. Of course, knowing what we know about vitamin D and folate, these two agents would be involved regarding this hypothesis, since both agents have photoprotective effects. Vitamin D is extremely important to DNA repair (Graziano et al, 2016), as vitamin D reduces cell and DNA damage.
Though many authors dispute the claims of this hypothesis since the effects of skin cancer would occur after the reproductive years and would thusly not have an effect on natural selection for skin color. Though those who argue for the validity of the hypothesis propose that it would help in hunter-gatherer peoples whose old train their young their ways of life.
Since these interactions have between these variables have been verified at the molecular genetic level, this lends even more credence to the VDH. (The findings inclue the frequency of common VDR variants between different ethnic groups, to UVR and folate metabolism genes which were found to be significantly associated with the frequency of 16 common folate variants and skin pigmentation in a genomic analysis of 30,000 people which were novel relationships; Jones et al, 2018a). These findings discussed by Jones et al (2018b) “indicate the existence of interactions between UVR, skin type, and vitamin D and folate genes, and they provide supporting molecular evidence for the vitamin D–folate hypothesis.”
Madrigal and Kelly (2007a) tested a sexual selection hypothesis proposed by a few proponents of the sexual selection hypithesis. Madrigal and Kelly (2007a) tested the hypothesis that skin color reflectance should be positively correlated with distance from the equator. They, however, showed that the pattern in skin color dimorphism seen around the globe was not consistent with the sexual selection hypothesis, and thus their data did not lend credence to the sexual selection hypothesis. The hypothesis states that in areas with low UV radiation, environmental selection for skin color should be relaxed and there should be a higher rate of sexual dimorphism in peoples from northerly climates due to sexual selection for lighter-skinned women. Nevertheless, the data compiled by Madrigal and Kelly (2007a) do not lend credence to the hypothesis.
Frost (2007) responded that Madrigal and Kelly (2007a) presumed that sexual selection was equal in all areas, but was constrained by natural selection for dark skin. Frost (2007) also states that sexual dimorphism in human skin color may not be able to be expressed in lighter-skinned populations at higher latitudes. Frost’s objections stem from the fact that Madrigal and Kelly tested a specific hypothesis proposed by proponents of the sexual selection hypothesis, though Madrigal and Kelly hope that Frost can test his hypotheses. However, I think it’s a moot issue. Sexual selection for women occurred after selection for light skin due to vitamin D synthesis which ensured more calcium for pregnancy and lactation.
Thusly, sexual selection for lighter skin would continue to ensure ample vitamins for women and their pregnancies and lactation to feed their babies. This would further be butressed by the fact that vitamin D exerts effects on the adipocites which lends even more credence to the claim that light skin evolved first for vitamin D synthesis. Vitamin D then exerted effects on the adipocite since more vitamin D could be produced in the absence of high levels of UV, which then aided in human physiologic adaptations to climate.
Integration of current skin color theories
As can be seen from the competing theories, they are not necessarily explaining different things, and each supposed competing theory has an aspect from the VDH in it. Thusly, it is possible to integrate the so-called competing theories into a larger explanatory framework.
Jones et al (2018b) update the VDH by integrating the other theories into it, since they are similar and do not contradict the VDH (since aspects of each one can be used to explain different aspects of the VDH). The updated hypothesis is thus:
Vitamin D and folate have differing sensitivities to UVR. Vitamin D can be synthesized following UVR exposure, folate may be degraded. So the VDH proposes that the two differing skin colors (light and dark) evolved at differing latitudes as a “balancing mechanism” to maintain adequate levels of the two agents vitamin D and folate. Since adequate levels of vitamin D and folate were maintained, there would be no ill health effects after migrating into colder climates. Vitamin D and folate both act as photoprotectors of the skin and can decrease environmental stressors. Vitamin D also exerts important effects on adipocites—both types—which then further aid in human physiologic adaptations to the cold. Perhaps most importantly, the VDR and skin color genes are epistatic—the VDR is imperative in the human body’s adaptation to new climates.
The latest research (reviewed by Jones et al, 2018b) show strong support for the interaction between genes and folate/vitamin D processes with skin pigmentation and UVR.
Health disparities due to vitamin D deficiency are well-noted in the literature. Human migrations over the recent centuries and decades have caused environmental mismatches between a population’s adapted skin type and current UV level in the location the population migrated to. Many darkly-pigmented people now live in areas with low UVR, and thusly suffer from health consequences. This leads to them either not having an adequate vitamin D-folate balance along with the risk of not having the adequate skin protections for a given environment, since UV rays influence folate and vitamin D production and so, a mismatched skin color to UV environment would cause problems for skin protection since the environment is not ancestral to that certain skin color.
As I have previously noted, it has been argued that blacks are not vitamin D deficient, and thusly not vitamin D deficient. Though, these claims rest on a slew of false arguments that have since been rebutted. It has been argued that since blacks are deficient in vitamin D, which begins in the womb, and vitamin D deficiencies cause changes in large and small arteries and arterials, that vitamin D deficiency could be the cause of higher rates of hypertension in black Americans (Rostand, 2010).
The role of vitamin D, folate, the VDR, and certain genes is under further invesitgation. This group of agents exert powerful effects on human physiology which then help with the adaptation of humans to differing climates. Folate and the vitamin D receptor play a crucial role in protecting the skin from environmental and microbial stressors. Vitamin D and the VDR are expressed in the adipose tissue, while vitamin D regulates adipogenesis and adipocite apoptosis (Abbas, 2017). Further evidence shows that there are multiple loci that are involved in the VDR that show latitudinal clines (Tiosano et al, 2016). One of the most things that lends credence to the VDH is the fact that the VDR and skin color genes are epistatic and help humans adapt to climates.
The VDH is in great shape, contrary to popular belief (Elias, Williams, and Bikle, 2016). The VDH is one of the only games in town to explain the skin color gradient noticed around the world, with vitamin D being the only agent that accounts for skin color differences. The VDH explains how and why human skin color is vastly different, and the main reason is adaptation to UV rays—or lack thereof.
Grant (2018) concludes that:
The UVB–vitamin D–cancer hypothesis has considerable supporting scientific evidence from a variety of study types: geographical ecological, observational, and laboratory studies of mechanisms, as well as several clinical trials.
Clearly, the VDH explains the incidence of the observed skin gradiation around the world the best out of the so-called competing hypotheses (which are similar enough to the VDH to where they can be absorbed into the VDH). Most importantly, the VDH predicted a novel fact—that molecular genetic evidence would show that light skin evolved independently numerous times in our lineage (Jablonksi and Chaplin, 2009).