Gene Expression

Potential for Vitamin D synthesis

From The evolution of human skin coloration, page 12:

i-0147704b288c871db3f85362cf73cb37-Jablonski_skin_color_200012.jpg

The main reason I post is that I don’t think that intuitively people have a good idea of how far north the north of Europe is, and the fact that “temperate” East Asia is at the latitude of the Mediterranean (as is most of the United States).

Comments

  1. #1 Umlud
    April 30, 2008

    You really realize it when you spend a winter or summer there. During the winters in my undergraduate in Scotland, I would regularly have to be in class just as the sun was pallidly rising (9AM lecture) and leave the teaching lab 1.5 hours after sundown (2PM-5PM laboratory). Conversely, during the summer, I had to put up extra curtains to ensure that I could sleep through the 17-hour days.

  2. #2 pconroy
    April 30, 2008

    Yeah, in Ireland in the summer I remember driving a tractor out in the fields at 11:00 PM at night – you wouldn’t find that in the US!

  3. #3 pconroy
    April 30, 2008

    I also remember that in my teens I almost exclusively grew in stature ONLY during the summer, when I would be outdoors mostly – thereby getting enough vitamin D I suppose. In my 15th year, I grew 3″ in 3 months, and in my 16th year I grew 3.5″ in 3 months, of the summer.

  4. #4 Kevin C.
    April 30, 2008

    pconroy,

    You do find that here in Alaska, which, don’t forget, is (unfortunately, I sometimes think) part of the United States.

  5. #5 yttrai
    April 30, 2008

    Even in the balmy UP i’ve seen twilight linger until 11.15 pm. Of course, WE’RE not part of the US either ;)

  6. #6 Sandgroper
    May 1, 2008

    Don’t get me wrong, I’m a big Jablonski fan, but that map is average annual UV and it’s therefore only part of the picture. For adequate Vitamin D synthesis you need sufficient UV exposure at least a couple times a week, so really what should matter is what the available UV is in the depths of winter and whether you can get enough outdoor time to soak it up (and then whether your skin is pale enough to soak up enough).

    For 1961-1990, the mean max. temperature in London in January was 6.7 degrees Celsius and the mean average temperature was 3.5. For the same time period and month, the mean max. in Beijing was 1.6 and the mean average -4.3.
    Getting enough outdoor time in winter in northern China is problematic. Temperate is not the word I would use. What really matters is what the weather was like during the Neolithic, but the relativity is clear enough – in winter, London is a lot warmer (if that is the right word) than Beijing, despite being at higher latitude.

    http://racialreality.blogspot.com/2006/01/skin-reflectance-of-selected-world.html This gives some numbers on skin reflectance, taken from Jablonski & Chaplin. London scores 62.3, southern Chinese score 59.2. People in north-eastern China (anecdotal, eyeballing it) are noticeably more pale skinned than in the south (southern Chinese are hybrids of northern Chinese with Austronesians, who are light brownish), so by my reckoning they’d be scoring up around the same as London. Anecdotal observation is confounded somewhat by social attitudes towards tanning, so it’s only a crude substitute for proper measurements, but this falls in favour of Chinese opting for paler skin. They’re also confounded somewhat by skin tone and texture, which in this comparison would fall the other way.

    I think the fish story comes from relatively low skin reflectance in Inuit despite latitude. Any people who remained hunter gatherers have not been under the same selective pressure for pale skin that people who turned to agriculture have. Australian Aboriginals in south eastern Australia remained dark skinned because they remained hunter gatherers (and ate lots of fish), not to mention living outside mostly stark naked.

  7. #7 Sandgroper
    May 1, 2008

    Here’s Elli’s word on skin colour in humans. (It ain’t just black and white.) (The skin colour I mean, not the paper.)

    http://www.cs.stevens.edu/~elli/spie.pdf

  8. #8 Luis
    May 1, 2008

    I do realize. And I also realize that many people don’t realize that. So, yes, good post, Razib.

    Northern Europe is unique in having relatively warm weather and being at that ultra-high latitude, allowing for a population density nowhere else in the planet isfound so far North. That’s because of the Gulf Stream. That alone seems a good reason for paleness of Northern Europeans.

    Add to that the usual cloudiness because of sea influence (in comparison, most of Siberia is probably a lot sunnier, even if much cooler too).

    For adequate Vitamin D synthesis you need sufficient UV exposure at least a couple times a week, so really what should matter is what the available UV is in the depths of winter and whether you can get enough outdoor time to soak it up (and then whether your skin is pale enough to soak up enough).

    Are you sure about that?

    Anyhow, I assume that pre-Modern people were a lot of their time outdoors, even in the depths of winter. There was no TV nor Internet, heating needed to have the wood fetched and almost anything needed someone to move out and aroudn. Same in Hebei as in Sweden.

    Also, vit. D synthesis is not just the B&W extremes. People with brown or beige skin can actually synthetize it a lot better than people with black skin, while they are also better protected against UV damage than people with very pale skin (who often can’t even tan or have a hard time with that). It’s a gradual adaptation and probably most people have the best balance for the enviroment and diet of their ancestors. It may sound kind of “Mediterranean supremacist” (something I am not) but the fact is that people with intermediate skin tones and who can tan and untan easily are the ones who can adapt better to varied enviroments for this crucial health aspect. The extremes are of course “best adapted” for their ancestral enviroments… but they are less versatile too.

  9. #9 Monado, FCD
    May 3, 2008

    I recall reading somewhere that having pale european skin meant you could get enough sunlight to make Vitamin D with 15 minutes of outdoor exposure in the winter on the face alone. I imagine most people in the past would get that much outdoors tending animals, gathering firewood, and going to the latrine.

  10. #10 jaakkeli
    May 3, 2008

    Monado, that’s a silly thing to say, as in northernmost Europe, there is a significant period of time with no sun at all in the winter! Even somewhat below that, the deep winter sun is completely meaningless, so in countries like Iceland and Finland there is effectively no sunlight for a significant time. Everyone is automatically deficient if they don’t get it from the diet.

  11. #11 Luis
    May 3, 2008

    … as in northernmost Europe, there is a significant period of time with no sun at all in the winter!

    C’mon! That’s called Lappland!

    When we talk of Epipaleolithic and Neolithic Northern Europeans, we mean much more to the south normally, where the Sun may be low in Winter but it does rise for many hours anyhow: short days maybe but days anyhow.

    Even today that extreme area you mention is barely populated (except for the Russian naval base of Murmansk, a very recent endeavour). For most of its history Lappland has been inhabited by just a handful of tribal reindeer herders (the Saami) with a very specialized way of life and that are not significatively ancestral for other populations.

    In contrast, Denmark (at the latitude of Edinburgh, but also of the Hudson Bay and Kamtchatka) has been populated with some density since the Epipaleolithic.

    Even somewhat below that, the deep winter sun is completely meaningless, so in countries like Iceland and Finland there is effectively no sunlight for a significant time.

    Iceland is a “recent” colony and not ancestral to other peoples too, so we can exclude it. Finland might be the only meaningful example you have mentioned so far but mostly Finns lived in the south of the country, where the situation is not as extreme. It’s unclear if Finns are ancestral to other populations of Northern Europe but it would be in small ammounts in any case. I don’t have very clear if Finns are less pygmented (“whiter”) than other North European populations of lower latitudes but my impression is that they are roughly like the rest of that group.

  12. #12 jaakkeli
    May 4, 2008

    For most of its history Lappland has been inhabited by just a handful of tribal reindeer herders (the Saami)

    Actually reindeer herding is a relatively recent thing. It’s only stereotypical of Saamis because it’s the one specialized way of life protecting Saamis from assimilation, so there are few Saamis in other areas left.

    In contrast, Denmark (at the latitude of Edinburgh, but also of the Hudson Bay and Kamtchatka) has been populated with some density since the Epipaleolithic.

    Even in Denmark the difference between deep winter and summer sunlight dose is massive. Trivial Googling gave me this (activist site, but the source isn’t):

    http://www.eoearth.org/image/Fig._5.6._Modeled_clear-sky_daily_erythemal_UV_radiation_dose.JPG

    Note the difference for 50N. It’s even bigger at Danish latitudes, of course. In Finland (over 60N), deep winter sunlight is completely meaningless. You can sit there, stare at the sun and not notice anything. It won’t even make you feel warmer. It’s too weak.

    I don’t have very clear if Finns are less pygmented (“whiter”) than other North European populations of lower latitudes

    Actually, what you’d expect compared to coastal Western Europeans would be a greater ability to both tan and lose that tan. In Finland, the daily radiation dose in the summer is larger than it is on the equator (!); in winter, it’s meaningless. The closer you get to the pole, the more extreme the difference. Scandinavia is similar, but less extreme.

    The kind of non-tannable skin you often see on eg. British people is not very common in Finns, Swedes & Balts.

  13. #13 Luis
    May 5, 2008

    Even in Denmark the difference between deep winter and summer sunlight dose is massive.

    Maybe. But it’s still pretty different than north of the Polar Circle. Basically you were claiming that there is no diffeence between Denmark and Lappland and that is not the case.

    Also most Europeans, including many pale and blond, live south of Denmark (except maybe in Russia). Scandinavia and Scotland have historically been very marginal areas (and the more nortwards you go, the more preipherical from a historical and prehistorical viewpoint – until some centuries ago, as heavy plough first and industralization later changed that somewhat). I strongly think the basic pygmentation variant evolved not so far north but in continental Europe probably, though it was even more extremely selected as the far north latitudes were colonized in the Epipaleolithic and later on.

    Actually, what you’d expect compared to coastal Western Europeans would be a greater ability to both tan and lose that tan. In Finland, the daily radiation dose in the summer is larger than it is on the equator (!)

    That doesn’t make sense. The ability to tan is surely useful in any case but the direct sunlight radiation at 60ºN cannot be higher than at the tropics, not even in midsummer! The highest UV radiation will always be at the tropical belt because it’s the only area of Earth where the Sun ever hits vertically. And the tropic of Cancer is like 22ºN, crossing the Sahara, just south of the transition between zones 1 and 2 in Razib’s map.

    Razib’s map is very illustrative actually because it strongly suggest that, while migrating to South Asia (in zone 1) required little or o adaptation in this respect, moving further north into East, Central or West Asia (zone 2)already began to demand some lightening of the skin. That surely happened in the Upper Paleolithic.

    Zone 3 was almost not touched in the Paleolithic but there are at least a couple of exceptions: the Altai (at the border) some spots of the Eastern European region (border too) and, specially, the Rhin area (clearly inside zone 3). The Rhin area was definitively inhabited for most of the UP (except possibly the LGM) and I would think that this is the region where with the highest likehood the phenotype evolved first to the lightest versions we can see today.

  14. #14 jaakkeli
    May 6, 2008

    Scandinavia and Scotland have historically been very marginal areas

    Sure. However, marginal northernmost Europe has still developed notably lighter pigmentation, so of course you should note the particular radiation conditions.

    The ability to tan is surely useful in any case but the direct sunlight radiation at 60ºN cannot be higher than at the tropics, not even in midsummer! The highest UV radiation will always be at the tropical belt

    You’re thinking of the momentary intensity. That is greater at the equator – but the day lasts much longer in the north, so the *total daily dose* is higher at 60N in midsummer.

    Also, if you also count snowcover effects & reflected radiation, the intensity in the far north will be greater at some times than it is at the equator (although only in some areas at midsummer). Northern Finland is still covered with snow, even though the winter was record warm – and yeah, it’s a marginal area, but it also happens to be a place where the skin colour relation with latitude inverts (northerners have more tannable skin).

  15. #15 windy
    May 6, 2008

    It’s unclear if Finns are ancestral to other populations of Northern Europe but it would be in small ammounts in any case

    Not “Finns” sensu stricto, but previous Finno-Ugric populations probably contributed lots to modern Russians and Balts.

  16. #16 Luis
    May 6, 2008

    You’re thinking of the momentary intensity. That is greater at the equator – but the day lasts much longer in the north, so the *total daily dose* is higher at 60N in midsummer.

    I can’t agree with this reasoning. Short nights don’t mean greater solar radiation overall and any map of that kind will tell you that. Southern Spain may have longer nights than Denmark in summertime (only) but still gets a lot more solar radiation in that same period – and that’s why Danes go to get tanned (and sometimes burned) to the Costa del Sol, not just because prices are cheaper (that they aren’t anymore).

    The above is your own speculation but it’s in disagreement with the data.

    Also, if you also count snowcover effects & reflected radiation, the intensity in the far north will be greater at some times than it is at the equator (although only in some areas at midsummer).

    You may have a point here. Winter sun plus snow ammounts to greater radiation and need for protection (as any ski aficionado knows) and this element is maybe being overlooked. It should anyhow affect more those areas with less cloud cover (Siberia, inner Russia) than the Atlantic areas with winter cloudy skies. It might be an element accounting for the greater natural tan observed among some Siberian populations maybe.

    Finno-Ugric populations probably contributed lots to modern Russians and Balts.

    It seems that mostly to Northern Russians and Latvians. Though it’s surely true that Finno-Ugrics and Indo-Europeans expanded side by side in NE Europe (with different ecological niches though) and that may have ammounted overall to a lot of “invisible” admixture and/or cultural loans.

  17. #17 jaakkeli
    May 7, 2008

    I can’t agree with this reasoning.

    This is a simple calculation, as long as you ignore the atmosphere: just calculate the flux given the latitude and time and day and integrate over the day. It’s a simple problem, although laborous with loads of trigonometry if you begin without assuming knowledge of basic astronomical coordinates.

    I checked the astronomy textbook I own and conveniently I don’t even have to count, as they actually have this particular calculation as an example. Yes, I was right. (The book is in Finnish, but it has been translated as Fundamental Astronomy by Karttunen et al. So if you don’t believe me, you can follow the math yourself. It’s too long-winded to type on a blog.) They even include the simplest model for the atmosphere and it’s still true. (Ignoring the atmosphere, the *north pole* gets a higher daily dose at midsummer!)

    and that’s why Danes go to get tanned (and sometimes burned) to the Costa del Sol

    No, they go there because they don’t want to spend all day lying in the sun. The peak strength of the sun is stronger, but that only means that it’s stronger at about midday; at other times the northern sun is stronger. When you go further north, the hours of strong daylight get longer, even if the peak daylight isn’t as much. It all works out to a higher dose in the north. (Where I live, the midsummer dose is larger than it ever is at the equator.)

    If you’re going to work on the fields for most of the day, it’s the total dose that matters. If you’re on vacation, you’re not going to spend the whole day in the sun – but vacations and traveling to Spain for sunbathing are modern inventions.

    Whether the daily dose should translate to more tannable skin is still a little questionable, though, since AFAIK no one has really figured out convincingly just how tanning translates into a benefit.

  18. #18 Luis
    May 7, 2008

    Jaakeli: it’s very simple: in midsummer at noon, the Sun hits the tropic of Cancer from “right above” (90º), it must hit Malaga at some 70º and Denmark at not more than 50º (didn’t do the exact tangent calculations but should be quite simple anyhow if you are maths oriented). It’s (angularly) high but never higher than more to the south.

    Even yourself have posted earlier a graph that shows that the total ammount of radiation/daily sunlight increases for any time of the year as you move southwards. 50º get more radiation than 70º, etc. Therefore any Mediterranean spot at 35º degrees gets more radiation at any time of the year than any Nordic spot at 50º. This simple logic is only broken at the tropic of Cancer (circa 22ºN).

  19. #19 jaakkeli
    May 8, 2008

    It’s (angularly) high but never higher than more to the south.

    SHEESH. Integrate over time, from sunrise to sundown. Start from this page:

    http://en.wikipedia.org/wiki/Celestial_coordinate_system

    and the equation directly giving the sine of the elevation angle. That’s the equation for which angle the sun hits at and it gives it for the *whole day*. The momentary radiation level is of course directly proportional to the sine of the elevation angle, so it’s a simple integration of that equation. Hour angle is the only thing that varies with time over the day, so to integrate over the period of daylight integrate over hour angle from sunrise to sundown and hour angle for sunrise and sundown is solvable from the equation on the wiki page: they’re the moments when the sine of the elevation angle is zero.

    It’s a very simply integration and the result is, if I didn’t make any dumb errors, that the daily radiation dose at midsummer in Malaga compared to Copenhagen is… hey, you were right: it’s a whopping 0.4 % greater! I could cherry-pick other specific latitudes, though, and make it greater in Denmark. Basically, the total radiation dose received in Denmark and Spain at midsummer is *the same*, ignoring atmospheric conditions.

    This simple logic is only broken at the tropic of Cancer (circa 22ºN).

    No, such simple logic doesn’t work anywhere. I would type the radiation dose as function of declination and latitude ignoring atmosphere, but even that has a dozen trigonometric functions. It’s not simple declining function even between the tropics and polar circles.

    BTW the reason why I wasn’t entirely correct is that I didn’t realize that one of the maxima of the function seems to be at Spanish latitudes. The midsummer total dose seems to be greater in all of Spain than it is at the tropic of Cancer (ignoring atmosphere).

  20. #20 Luis
    May 8, 2008

    Whatever, Jaakkeli. You are contradicting the very graph you used some posts above to demonstrate another point. In that graph it’s evident that, in midsummer as in midwinter, the daily dose of solar radiation is much higher (almost double for midsummer) at 50ºN than at 70ºN. The same happens when you compare 30ºN with 50ºN. The length of the day is not really relevant because it does not even compensate for the much lower angle of inccidence.

    Please!

  21. #21 Sandgroper
    May 9, 2008

    My own empirical observation is derived from having lived for long periods both in southern Australia and on the Tropic of Cancer. For whatever reason, just driving and walking around, not from deliberate exposure, in summer the exposed parts of my skin go much darker in southern Australia than on the Tropic of Cancer. The only explanations I could ever come up with were (1) in summer, cloud cover is generally much greater at the Tropic of Cancer than in southern Australia, (2) the humidity is very much higher, suggesting maybe that water droplets diffuse the sunlight to some extent, (3) the airborne suspended particulate matter is very much higher, which definitely has a diffusive effect, (4) maybe, hole in the ozone layer. I haven’t tried to do any maths on any of this, so it is just impressions and observations, but photographers who don’t use automatic cameras will be very familiar with this, the available visible light in southern Australia is more than in the tropics.

    All of that is a bit beside the main point – I assume everyone will agree that in winter, higher latitudes get a lot less UV than lower latitudes. Apparently the UV required for Vitamin D synthesis is in a fairly narrow band of wavelengths, and below a certain angle of incidence this band of wavelengths is just not present at all. Above that, its availability will depend on cloud cover, exposure, etc. Reportedly, at low latitudes in summer at midday, exposure of the face, hands etc for 15 minutes twice per week is sufficient for adequate synthesis of Vitamin D. I presume this applies to all people, regardless of skin tone. Or maybe dark skinned people need to expose larger areas of their bodies – dusky maidens with bare areas of skin are absolutely in no way offensive to me.

    Reportedly, above about 30 degrees of latitude in winter, this band of wavelengths is not always sufficiently available, such that some lightening of skin and/or dietary supplementation becomes necessary to at least some extent. 30 degrees is not that much. This jibes with the recent observation that surprised everyone that in southern Australia, quite a high proportion of the population have Vitamin D deficiency, including even very light skinned people. This came as a big surprise because southern Australia has abundant bright sunshine, even in winter, people tend to favour a fairly outdoorsy lifestyle and the winters are severe nowhere in Australia near sea level, generally adequate nutrition is just not a societal issue at all, and it was always assumed that Vitamin D deficiency would not be a problem, at least for people of European ancestry. But it is. It is suspected to have become more prevalent after widespread public education about skin cancer from over-exposure to the sun and the widespread use of sun screens, but it also looks like they just got that one plain wrong – even with abundant exposure to sunlight when available in winter in southern Australia, Vitamin D deficiency can occur even in very light skinned people.

    It occurs to me that one of the health issues that may beset Australian Aboriginal people in southern Australia, who notably have much shorter life expectancy than the rest of the population, is that a lot of them no longer live outside, go naked and pursue a hunter/gatherer lifestyle. It is by no means the only issue, but it seems a possible.

  22. #22 windy
    May 9, 2008

    It seems that mostly to Northern Russians and Latvians. Though it’s surely true that Finno-Ugrics and Indo-Europeans expanded side by side in NE Europe (with different ecological niches though) and that may have ammounted overall to a lot of “invisible” admixture and/or cultural loans.

    “Side by side”?

    Whatever, Jaakkeli. You are contradicting the very graph you used some posts above to demonstrate another point. In that graph it’s evident that, in midsummer as in midwinter, the daily dose of solar radiation is much higher (almost double for midsummer) at 50ºN than at 70ºN.

    But that graph is taking into account the atmosphere, so Jaakkeli is not contradicting himself but for UV skin exposure the daily incoming radiation dose is not really relevant without considering the atmosphere.

  23. #23 Tim Borger
    May 9, 2008

    Living in an area of the world that gets a lot of UV rays doesn’t make it a sure thing that you won’t have a deficiency of vitamin D.

    I live in Phoenix AZ, I’m a very fair skinned white male, play basketball outside once a week when the temperature is below 100 degrees F, have a pool in my backyard that I float around in when the temperature is above 100 degrees F and go hiking two or three times a month (not to mention many other activities that I attend that are outdoors). I still became vitamin D deficient. The first indication was I was calcium deficient which led my doctor to believe I had parathyroid disease. After many other tests I was found to be vitamin D deficient.

    By taking vitamin D suppliments for a year I now have normal vitamin D levels.

  24. #24 Sandgroper
    May 10, 2008

    Tim, what do you take?

    I have been hunting for D3 supplements where I am and can’t find them anywhere.

  25. #25 Lab Cat
    May 10, 2008

    A couple of comments about Vitamin D synthesis that I was taught as a Nutrition undergrad.

    1) The UV waves hitting England during the winter months are the wrong wavelength for Vit D synthesis. I think for London it was from Nov to Feb, but I also recall something to do with the season of grass growing. When grass is growing white northern Europeans can make vitamin D when exposed to sunlight for a short length of time each day.

    2) Immigrants to Britain from the Indian subcontinent were more likely to suffer from Vitamin D deficiency. Part of the problem was that their production of Vitamin D is less efficient than white Europeans or perhaps needs a different UV wavelength?

    Finally, in response to Tim Borger’s comment: My Dad as a child and teenager had vitamin D deficiency. It turned out that he was not able to make it in sufficient quantities. This was in the 1930s and I don’t know if they treated it other than to tell my Grandma to feed him lots of dairy products. As an adult this problem appeared to have gone away. Mind you, now in his 80s he has developed severe osteoporosis.

  26. #26 Blind Squirrel FCD
    May 10, 2008

    Sandgroper: My local Wal-mart carries D3.

    A friend who is a recovering alcoholic Is taking D3 as part of his treatment. 50,000 units daily! WTF? I though it was one of the toxic oil soluble vitamins. So what does it take to hurt someone with the stuff? Hit them over the head with a full bottle?

  27. #27 Sandgroper
    May 11, 2008

    Thanks.