Apparently skin colour evolution is linked to folate and vitamin D. The most amusing part of this excellent article is that people with dark skin in higher latitudes can develop rickets if they don't get enough sun. I didn't know that people actually got that disease anymore since the fortification of milk. I also thought that vitamin D deficiencies were related to seasonal affective disorder (SAD), but I haven't found anything about darker skinned individuals being more prone to this disorder. In fact they seem to be just as prone as the general population. strange.
Related to the podcast discussion (part III of episode 6 yet to come out) on continuing human evolution I wonder if there is any selective pressures on skin colour due to vitamin D and/or folate in any parts in the world today.
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from wikipedia: Various etiologies have been suggested.[2] One possibility is that SAD is related to a lack of serotonin and that exposure to full-spectrum artificial light may improve the condition by stimulating serotonin production, although this has been disputed.[3],[4] Another theory is that melatonin produced in the pineal gland is the primary cause,[5],[6] since there are direct connections between the retina and the pineal gland. Some studies show that melatonin levels do not appear to differ between those with and without SAD. However, mice incapable of synthesizing melatonin appear to express "depression-like" behaviors[7],[8] and melatonin receptor ligands produce antidepressant-like effect in mice[9].
Speaking of skimmed milk. Ever wondered how they manage to add fat soluble vitamins to skimmed milk? Well it turns out that the skimming process removes most of these vitamins while reducing fat content to 0.1%, so the milk needs to be fortified. However what ends up happening is that the vitamin content per g of fat just goes up. It concentrates in the few droplets available. But this is true for cholesterol too, so don't think you're getting less of it with skimmed milk!!!
I was reading up a little on Vitamin D and seasonal affective disorder (SAD) in preparation for a similar post. There are a few studies, some of which conflict, but it seems to me from what I've read that there's not a definitive link between vitamin D deficiency and SAD. It seems that phototherapy is the most commonly prescribed (and most effective) treatment. There are only a handful of research papers dealing with vitamin D and mood. In one published in 1999, the authors compared Vitamin D supplementation to phototherapy in SAD treatment. In both cases vitamin D levels were increased (moreso in the vitamin supplement treated group), but only the vitamin treated group showed significant improvement of depression scale measures. On the other hand, a more recent report in the same journal (2006) showed no improvement in mental health scores of elderly women treated with vitamin D compared to untreated women.
Wicked PLoS paper on skin colour.
If you look at the research coming out lately from various countries where this is being studied, there is a notably higher incidence of SAD in countries where Nordic bloodlines are present. If you think about this in terms of skin pigments it makes sense -- Nordics are fair-skinned, but tan easily (perfect adaptation for the land of the midnight sun). Light skin soaks up vitamin D extremely efficiently, dark skin doesn't. Your ancient Norseman also ate one heckuva lot of fish, especially in winter when other foods were less available, very rich in vitamin D (this diet is why scientists think the SAD rate in Iceland is so much lower than you would expect, they eat about 5 times as much fish as folks in USA or Canada). Now transplant/timewarp your ancient Norseman from the Arctic Circle to the USA or central Europe today and stick him in a cubicle, without all those winter fish in his diet -- what is going to happen? SAD, that's what.
I'm convinced that there is a genetic element involved here, it's not just a question of skin color or diet or geographic location. I'd love to see a study done that looked at the SAD rates between people who tanned easily and everybody else.
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