As I changed races in Me-hee-co last week something I read in Stephanie Seneff's blog post on VitD and sunscreens, etc. that got me thinking about my negative vitamin D experience a few years back. For those who haven't read me drop a comment on it, here's the Cliff's Notes version. I rather foolishly decided to jump on the megadoses of VitD bandwagon a few years back and began taking somewhere in the 5000-10000 IU/day range. I started getting weird "dizzy spells" but only when lying down and shifting positions (like rolling over). I also had my first ever "high" BP reading at my "well woman" doc visit. So I figured I should stop taking all supps and sort that out. I've since added back in the other supps and identified it was the D. I now only take 1000 IU and no more than 2000 IU, definitely not daily (currently I'm experimenting with whether the Estroven is doing anything at this point and it has D in it).
So anyway, Seneff talks a lot about how poorly dark-skinned immigrants from more equatorial regions fair when they move to northern latitudes. The dark skin prevents the sun's rays from penetrating to stimulate D synthesis. So this got me thinking about why we tan at all? I have to think that our ancestors knew enough to find shade/caves/etc. during peak hours of sun exposure and didn't experience sunburn. I doubt they spent several days in a row, as I just did, in the hot sun to maximize the vanity tan (sorry, I'm an unapologetic sun worshiper!) . I don't burn much, I will when I overdo it, and I can burn through a tan if that makes any sense. So while I'll burn less if I've got a base over fresh off the plane, the tanning doesn't have a totally protective quality. Instead, I do wonder if perhaps this was nature's way of limiting VitD synthesis to avoid excessively high levels. IOW, large doses of D may be indicated and effective for those who are deficient, but folks living closer to the equator are darker skinned than those living nearer the poles. It seems a reasonable hypothesis that skin pigmentation evolved/adapted over time so that VitD wasn't synthesized in limitless quantities based solely on one's exposure to the sun, but rather pigmentation may have some correlation to one's D status for seasonally appropriate sun exposure. I know there is some literature on excessive levels of VitD, but I haven't had much time to look into that. It certainly seems we humans have a mechanism by which to limit our own synthesis.
I imagine paleolithic and even up through fairly recent neolithic times this was not an issue. I think I belong somewhere quite South from where I live. Unfortunately aging parents not planning to move anytime soon (or ever) make that not probable any time soon.