Gene Expression

Vitamin D: why evolution may matter

Mark of Denialism left a comment below re: Vitamin D deficiency:

I wouldn’t call that evidence thin biff. The role of vitamin D in tuberculosis was actually pretty well nailed by studies of immigrants in the UK which demonstrate that latent TB infections will reactivate in the sunless climate. The demonstration of the role of Vit D in making defensins seems the likely physiological explanation.

I am wary though of some of the more hyperbolic claims I’ve seen about vitamin D lately. It’s fascinating stuff, sure. But until replicated in RCT rather than epidemiologically I think it’s too early to recommend universal supplementation with a vitamin that does have a toxic syndrome. Many vitamins have appeared similarly astounding in such studies and the benefits failed to pan out. This is likely due to certain epidemiologic biases that tend to show people who actively take care of themselves (take supplements, exercise, etc.) are generally wealthier, and healthier


First, in regard to toxicity, this site (NIH affiliated) states that 2,000 IU is the upper bound for Vitamin D daily intake. It recommends 200 IU for the young adult, and up to 600 IU for the elderly. I have brown skin, and it turns out that that means I’m about 1/6 as productive in producing Vitamin D via sun exposure as a white skinned person, so I doubt that the 200 IU recommendation really applies to me. To clear up my Vitamin D deficiency I consumed 3,000 IU per day for a month (3 1,000 IU tablets) and now am going through a regime of 2,000 IU in winter and 1,000 IU in summer. Note, even with the toxicity caution this is below that threshold.

The second issue is a serious one. People do tend to overreact to medical hypotheses and neglect confounds. That being said, I think this is a case where evolution has something to say; multiple evolutionary genomics papers have suggested that skin color is a trait that has been under very powerful selection of late in the northern hemisphere. One locus, OCA2, is the third longest haplotype in the European genome, and has a noticeable affect on skin color. Another locus, SLC24A5 has recently fixed in Europeans and seems to have been subject to selection on the order of 10% per generation across many populations. It accounts for 1/3 of the skin color difference between Europeans and Africans. Skin color has also been driven by selection in East Asia.

The most parsimonious explanation for these selective events, some of the most powerful sweeps detectable in the human genome, is that there were Vitamin D deficiency issues across Eurasia. I have posited that the rise of agriculture resulted in a concomitant decline in intake of Vitamin D through diet and an increase in the frequency and virulence of epidemics because due to increased population density. In any case, the point is that there is a strong likelihood that Vitamin D had selective implications in the past, and I think that should weigh in as a prior when considering its health implications. The possibility that people with skin color in the range of my own were simply out-bred by their lighter skinned relatives 10,000 years ago across vast swaths of Eurasia suggests a major fitness penalty, and also makes me weigh my priors in assessing the validity of current medical research. That being said, just because the Vitamin D thesis is the best we have right now does not mean it is a definite answer (I have started to suspect that not all the light skin producing alleles were selected because of the light skin, and ergo Vitamin D production), and that uncertainty reduces the weight that one can place upon this avenue of insight.

Comments

  1. #1 _Arthur
    December 27, 2007

    As I mentionned on this blog earlier this year, there’s a “Vitamin D Council” that makes outrageous claims about vitamin D health benefits, coincidentally the mostly same claims that we heard about vitamin C.

    They start reommending that people get more sunshine, but the doses they are recommending are megadoses, that cannot be reached with neither food nor sunshine. They also hold the belief one cannot overdose vitamin D. Their leader often makes the claim that he can, and do, gobble a full bottle of Vitamin D pills with no ill effect.

  2. #2 Marc
    December 27, 2007

    Response to Arthur: In fact, no one, especially John Cannell of the Vitamin D Council, recommends toxic levels of vitamin D consumption. The statements he makes are documented to the nth degree from medical/nutritional research. Secondly, twenty minutes of sunlight on Caucasian skin can produce up to 20,000 IU of vitamin D with no toxicity whatever. How, then, can the recommendation for 3,000-5,000 IU daily be considered toxic? It takes about 40,000 IU per day for long periods to produce toxicity. And as to the idea that the science is skewed because wealthier, healthier people are the ones taking vitamin D, consider this: a study from Creighton University school of nursing about August 2007 showed that women who took supplements for four years reduced their risk of all cancers by 60-77%. The women in the experimental and control groups were of the same background and the study was not epidemiological; it was a double-blind, placebo-controlled interventional study that assured the women had similar characteristics. This study is considered the gold standard of research and eliminates the problems of bias based on economics, health habits, etc. Another similar study done on flu among African American women showed that vitamin D supplementation cut the risk of winter flu to almost zero. Before you slam the science on which the vitamin D council bases its statements, you should read two books: THE UV ADVANTAGE and SOLAR POWER FOR OPTIMAL HEALTH. Then, write an apology to Dr. Cannell at the Vitamin D council.

  3. #3 arby
    December 27, 2007

    Thanks for the post and comments. I read SciBlogs in Google reader, so don’t really pay much attention to who is writing what, just whether or not the particular post interests me. I had noticed that some blogger(s?) seemed to be anti-D (and anti-fish oil as well), but never really knew why. I am as an anti-woo as anyone, but never included vitamin D or DHA in that category, and had no idea what these people had agin ‘em. No doubt there are over the top advocates, there always are, but I had never read them. It seemed like a fight without an enemy from out here. The studies I had heard about seemed valid, and the evolutionary and epidemiological evidence is persuasive, if not difinitive.
    Anyhow, I gather their main complaint is toxicity? Non issue, as you make clear. Science News has done a lot on vitamin D over the last few years, there are lots of articles and tons of links to the original research. sciencenews.org, but some or all may be behind a subscription wall. Thanks again, rb

  4. #4 deang
    December 27, 2007

    Are there studies that discuss selection for lighter skin/Vitamin D intake in indigenous populations in the higher latitudes of the Americas, where skin colors were generally darker than in Eurasia? Early photos I’ve seen of pre-European-admixture Inuit, Gwich’in, Yahgan, Ona, and other high latitude indigenous of the Americas almost always showed skin tones of what seemed to me Mongolian levels of darkness. How does this jibe with that you’re saying?

  5. #5 arby
    December 27, 2007

    The Inuit, etc are the few northern people who get sufficient vitamin D from diet, think fish and fish eating mammals, especially the livers. They can afford to fight melanoma by maintaining dark skin. For the rest of us the risk of melanoma is the lesser of two evils. rb

  6. #6 razib
    December 27, 2007

    what arby said. i think diet is a big issue.

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