Vitamin D apparently has nothing to do with skin color

This article is a reminder that in science, nothing is certain until you have evidence (even then, nothing is totally certain): In human evolution, changes in skin’s barrier set northern Europeans apart — ScienceDaily.

The popular idea that northern Europeans developed light skin to absorb more UV light so they could make more vitamin D — vital for healthy bones and immune function — is questioned by researchers in a new study. Ramping up the skin’s capacity to capture UV light to make vitamin D is indeed important, however, researchers concluded in their study that changes in the skin’s function as a barrier to the elements made a greater contribution than alterations in skin pigment in the ability of northern Europeans to make vitamin D.

“While is seems logical that the loss of the pigment melanin would serve as a compensatory mechanism, allowing for more irradiation of the skin surface and therefore more vitamin D production, this hypothesis is flawed for many reasons,” he continued. “For example, recent studies show that dark-skinned humans make vitamin D after sun exposure as efficiently as lightly-pigmented humans, and osteoporosis – which can be a sign of vitamin D deficiency – is less common, rather than more common, in darkly-pigmented humans.”

The Elias lab has shown that pigmented skin provides a better skin barrier, which he says was critically important for protection against dehydration and infections among ancestral humans living in sub-Saharan Africa. But the need for pigment to provide this extra protection waned as modern human populations migrated northward over the past 60,000 years or so, Elias said, while the need to absorb UVB light became greater, particularly for those humans who migrated to the far North behind retreating glaciers less than 10,000 years ago.

The data from the new study do not explain why Northern Europeans lost melanin. If the need to make more vitamin D did not drive pigment loss, what did? Elias speculates that, “Once human populations migrated northward, away from the tropical onslaught of UVB, pigment was gradually lost in service of metabolic conservation. The body will not waste precious energy and proteins to make proteins that it no longer needs.”

I’ve posted here before on the theory that people living in more northern latitudes had lighter skin in order to allow them to absorb more sunlight for vitamin D production.  The results of this study are surprising.

Still, the relationship of latitude to skin tone is so consistent and obvious, that it still cries out for some kind of explanation.  It might be that having lighter skin makes the sun warm you more, or something along those lines.*  Simply saying that darker skin wasn’t needed at higher latitudes doesn’t seem like it would lead to the observed relationship.  It might lead to people at higher latitudes having a range of different skin tones, but it seems hard to imagine that it would account for the consistently lighter skin northern populations show.  It seems like either lighter skin had to provide some benefit, or darker skin some liability in higher latitudes.

Yep, more research is needed.


 

* Disagreeable Me pointed out in the comments that dark colors absorb more heat.  I knew that but must have been temporarily confused when I wrote this sentence.

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8 Responses to Vitamin D apparently has nothing to do with skin color

  1. Yasmin Ward says:

    I studied anthropology many years ago, in part, as a response to years of questioning other people’s reactions to my mixed ethnicity and brown skin. Your post is fascinating! Thank you for sharing it. 🙂

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  2. Hi SAP,

    “It might be that having lighter skin makes the sun warm you more, or something along those lines”

    I get that this is offered only as an example of the kind of explanation you might expect to see, but I think this particular one doesn’t make sense. Dark objects absorb more warmth from light than bright objects. Bright objects are bright because they reflect so much of the energy away.

    I think it is plausible that dark skin was lost simply because it wasn’t needed. Animals that live in dark caves tend to lose their eyes for the same reason.

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    • Hi DM,
      Good to see you!

      You’re right. I was momentarily confused about the color / heat absorption thing when I wrote that comment. Good catch.

      I guess my issue with the not-being-needed explanation, is that it seems too slight to lead to no dark skinned individuals in northern latitudes with only a 60,000 year time span to work with. I might could see it leading to a variety of different skin tones during that time, but for dark skin to disappear completely in that time frame, it seems like stronger selection pressure of some kind is needed. It might be as simple as people with high melanin not tolerating the cold quite as well.

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  3. I’m still waiting on a justification for freckles.

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