Survival of the Sickest: The Surprising Connections Between Disease and Longevity. Jonathan Prince

      If you’re wondering how people who have dark skin make enough vitamin D despite the fact that their skin blocks all those ultraviolet rays, you’re asking the right questions. Remember, ultraviolet rays that penetrate the skin destroy folate – and ultraviolet rays that penetrate the skin are necessary to create vitamin D. Dark skin evolved to protect folate, but it didn’t evolve with a switch – you can’t turn it off when you need to whip up a batch of vitamin D. So that would seem to create a new problem for people with dark skin – even if they lived in a sunny climate – because even though they received plenty of exposure to ultraviolet rays, the skin color that protected their supply of folate would prevent them from stocking up on vitamin D.

      It’s a good thing evolution’s such a clever sort, because it took that into account – it kept room for a little guy called apolipoprotein E (ApoE4) in the gene pool of dark-skinned population groups. And guess what ApoE4 does? It ensures that the amount of cholesterol flowing through your blood is cranked up. With more cholesterol available for conversion, dark-skinned people can maximize the use of whatever sunlight penetrates their skin.

      Much farther to the north, without a similar adaptation, the light-skinned people of Europe would face a similar problem. There, instead of plenty of sunlight that was largely blocked by dark skin, they had to deal with too little sunlight to make enough vitamin D even with the benefit of their light skin. And sure enough, ApoE4 is also common throughout Northern Europe. The farther north you go up the continent, the more you’ll find it. As it does in Africans, the ApoE4 gene keeps cholesterol levels cranked up, allowing its carriers to compensate for limited ultraviolet exposure by maximizing the cholesterol available for conversion to vitamin D.