Vitamin D versus sunburn: A tale of skin tones

As an addendum to my recent sunburn and sunscreen posts, I thought I’d make brief mention of something that came up in my research: the balance between skin color, sun protection, and vitamin D.

We need vitamin D. It helps keep our bones healthy, and it also bolsters our immune system, among other things. And as I mentioned in the sunburn post, our main natural source of vitamin D is…us! UVB light, the sunburning kind of ultraviolet light, triggers a biochemical pathway in our skin that ends with the production and release into the body of vitamin D.

But too much sun is bad, which is why our skin produces UVB-absorbing melanin.

So where’s the balance point between skin that’s light enough to allow sufficient vitamin D production and dark enough to fend off sunburns and skin cancer?

Map of the world showing UV light exposure

Maps of annual average UVB exposure at the earth’s surface (top) and variation in the level of exposure. Take-home message: people at the equator get exposed to a lot of constant UVB light; people in Patagonia and Greenland, not so much. (Jablonski N and Chaplin G. Human skin pigmentation as an adaptation to UV radiation. Proc Natl Acad Sci USA. 2010. doi: 10.1073/pnas.0914628107)

That’s something Penn State University anthropologist Nina Jablonski has been studying for some time. Jablonski has argued that skin color is a product of the evolutionary tension between those two needs, as well as the need to maintain levels of folate, a B vitamin broken down by UVB. This tension, she thinks, is reflected in the gradient of skin color — the “sepia rainbow,” as she puts it — one sees as you look from the equator out to the poles, a gradient that neatly matches the gradient of how much UV radiation reaches the earth’s surface (as mapped by NASA’s TOMS/EP mission).

As she says in her 2010 Proceedings of the National Academy of Sciences paper on the topic:

Skin pigmentation is probably one of the best examples of natural selection acting on a human trait. It is the product of two opposing clines, one emphasizing dark constitutive pigmentation and photoprotection against high loads of UVA and UVB near the equator, and the other favoring light constitutive pigmentation to promote seasonal, UVB-induced photosynthesis of vitamin D3 near the poles.

She also explains the idea eloquently in this TED talk:

So at the end of the day, what is skin color all about? It may largely be nature’s way of making sure you get your vitamins.

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