Vitamin D — Problems With the Latitude Hypothesis

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Virtually everything we know about vitamin D and latitude might be wrong.

When I wrote “Seafood to Sunshine: A New Understanding of Vitamin D Safety” in 2006, I took it for granted that the conventional beliefs about the effect of latitude on vitamin D synthesis were true.  Here is what I wrote:

The amount of UVB radiation available depends on the angle at which the sun’s rays strike the earth, the presence of clouds and buildings, ozone and aerosol pollution, altitude and reflective surfaces such as snow (18). Because of the effect of the sun’s angle, Webb and colleagues showed in 1988 that, even in completely clear skies, synthesis of vitamin D in the skin is impossible for four months of the year in Boston, Massachusetts and six months of the year in Edmonton, the capital of Alberta, Canada. The Webb team found that such a “vitamin D winter” occurred during at least part of the year at any latitude greater than 34 degrees (19).  More recently, one group of researchers used a computer model to suggest that in the nearly unattainable condition of truly clear skies, the vitamin D winters are shorter than Webb’s team suggested, but that under some environmental conditions, vitamin D winters can occur even at the equator (18).

The 1988 data, to which Michael Holick contributed, has been the most important data set used for understanding the vitamin D winter.  They measured vitamin D production in a handful of cities using isolated pieces of skin or 7-dehydrocholesterol mixed into a test tube concoction.

According to models developed from that evidence, no vitamin D synthesis whatsoever occurs outside of the summer at far latitudes such as 90 degrees (the north and south poles), and none whatsoever occurs during the winter at latitudes beyond 50 degrees (Antarctica, most of Greenland and Alaska, and the northern parts of Canada, Russia and Europe).  The models suggested that very little vitamin D production occurs outside of the summer in all of these northerly places and that even small migrations from equitorial regions cause huge decreases in vitamin D synthesis during all of the non-summer months.

These assumptions have fueled two important hypotheses: first, that our emergence from Africa has necessitated the evolution of whiter skin in order to make it easier to obtain vitamin D and that those of us who wear clothes in these regions are vulnerable to massive deficiency; and second, that the reduced risk of many diseases that occurs as we approach the equator is a result of improved vitamin D status.

Throughout my 2006 article, I accepted these hypotheses as likely to be true.  This had little impact on the content relating to the interactions between vitamins A, D, and K, which is the most important part of the article, but it had a major impact on my suggestions of what the ideal dose and blood level of vitamin D was likely to be.

Since I wrote that article, the vitamin D movement has grown much stronger and made bolder and bolder claims that have penetrated much deeper into mainstream consciousness, but the state of evidence for the need of these high levels has remained at the hypothesis stage.  I have thus grown more conservative, in part from studying the philosophy of science and statistics, and in part because the tables have now turned and the see-saw has now flipped, with vitamin D hitting the mainstream.  I now wonder if the lion unleashed may need to be tamed.

I recently pointed out in my post “Are Some People Pushing Their Vitamin D Too High?” that there is very little scientific evidence that we need 25(OH)D levels higher than 30-35 ng/mL (75-88 nmol/L).  Even the evidence for 30-35 ng/mL is primarily observational, meaning that we have very strong reasons for promoting the hypothesis, but no solid confirmation.

Research that has emerged since 2006 has threatened to turn the “latitude hypothesis” of vitamin D on its head.

In a 2007 paper, “Location and Vitamin D synthesis: Is the hypothesis validated by geophysical data?” (1), an Australian group of researchers created an index of ultraviolet (UV) radiation in the vitamin D range and analyzed how much vitamin D could be produced in seven locations across the United States using UV measurements collected by the US EPA.  They came to the “startling” conclusion that latitude was only related to vitamin D production during the coldest four months of the year.

They then developed a computer model that suggested vitamin D could be effectively synthesized at tropical rates across the entire globe for three quarters of the year and that the ability to synthesize it dropped off gradually between 40 and 70 degrees latitude during the winter months, and only regions between 70 and 90 degrees latitude had a complete vitamin D winter.

Another 2007 study conducted in Adenes, Norway (2) provided limited evidence suggesting that even at this far north latitude of 68 degrees vitamin D production begins in late February.  The study was not anywhere near as rigorously controlled as Webb and Holick’s test tube study, but it was conducted in live human beings.

These studies shed some major light on the form of Eskimo hysteria known as pibloktoq.  In “The Pursuit of Happiness,” and in my 2008 Wise Traditions conference lecture, “The Fat-Soluble Vitamins and Mental Health,” I described how this form of hysteria, possibly resulting from severe calcium and vitamin D deficiency, developed in Inuit who lived in regions without year-round access to dried fish and fish bones during the late winter and early spring.

If vitamin D synthesis is limited to summer in this region as previously thought, why would pibloktoq only occur in the late winter and early spring?  And how would all of the animals obtain sufficient levels of vitamin D for themselves let alone to feed the humans that would eventually prey on them?

The recent research suggesting vitamin D sythesis proceeds optimally for most of the year in this region helps explain this scenario.  It would also suggest that the Inuit must obtain vitamin D from food because the cold weather leads them to wear a great deal of clothing, and not because the UV-B light is usually unavailable.

Both of these studies directly contradict the predictions about the magnitude and geographical extent of the vitamin D winter developed from Webb and Holick’s earlier data.  Why did Webb and Holick find no vitamin D production in Boston for four months a year and none in Edmonton for six months of the year if in fact plenty of vitamin D can be produced even further north for most of the year?  Perhaps pollution, city buildings, and differences between test tube isolates and real humans made the critical difference.

A major analysis published in 2009 (3) pooled together the results of 394 studies examining vitamin D levels in over 30,000 people all across the globe in order to investigate the effect of latitude on vitamin D status.  The authors only included people who were native to the area in which they were living, and who were free-living.  They concluded that there was only an effect of latitude in Caucasians.  There was no effect of latitude in people with non-Caucasian ancestry.

The reason this deals such a major blow to the latitude hypothesis is that it is precisely people with white skin who dwell outside the equatorial regions who are supposed to be among the most vulnerable, but Caucasians actually had 45% higher levels of vitamin D than non-Caucasians!

If, in fact, the “original” humans best adapted to their environments are those who never came “out of Africa,” we must wonder why they have, on average, lower vitamin D status than Caucasians living in more northerly regions.

Granted, none of this refutes the notion that people who move outside of the environments to which they are native might suffer from vitamin D deficiency as a result.

About 20 percent of the studies found average 25(OH)D levels above 30 ng/mL, but only 4 percent of subgroups within those studies had levels above 40 ng/mL.  The authors did not report how many had average levels above 50 ng/mL, but certainly it must have been negligible.

The authors further pointed out in their discussion that the Inuit have genetic adaptations that increase their production of calcitriol, the active hormone form of vitamin D, and that Asian Indians have genetic adaptations that increase their detoxification of calcitriol (either that, or non-Asian Indians have developed adaptations that decrease detoxification).

Thus, people seem very adapted to the conditions for vitamin D synthesis that exist in the region they are from, but the evidence seems very scant that the “natural” levels of 25(OH)D are 40-60 ng/mL and even if they are in some tropical regions, people outside those regions may not be genetically adapted to having vitamin D levels so high.  In other words, a white American or an Inuit might make much more calcitriol from a 25(OH)D level of 40 ng/mL than an Indian would make.

There are, of course, problems with pooling together almost 400 studies.  For example, the studies used different assays to measure vitamin D, they were conducted during different seasons, there were fewer studies from equatorial regions, and many of the studies were likely not to have been perfectly random samples of their populations.  But when the authors took into account the type of assay, the year of publication, or the season in which the measurements were taken, these factors had no effect.  And while the sampling may not have been perfect, such a massive collection of data should help smooth things out.

Nevertheless, despite the imperfections, this is much more comprehensive an analysis than looking at a handful of tropical lifeguards and assuming that we evolved as “naked apes” bathing in the sun all day long, dreaming of the millenium in which we’d be able to blow whistles and surf out to save our drowning brethren.

The reality is even when our healthy ancestors may have been living with reduced clothing because of the heat in the tropical regions, they still may have been seeking shade or even using primitive forms of sunscreen.

Consider, for example, what Weston Price reported (NAPD, p. 104) from his visits to the Pacific Islands:

While the missionaries have encouraged the people to adopt habbits of modern civilzation, in the isolated districts the tribes were not able to depart much from their native foods because of the infrequency of the call of the trader ship.  Effort had been made in almost all of the islands to induce the natives to cover their bodies, especially when in the sight of strangers.  In several islands regulatory measures had been adopted requiring the covering of the body.  This regulation had greatly reduced the primitive practice of coating the surface of the body with coconut oil, which had the effect of absorbing the ultra-violet rays thus preventing injury from the tropical sun. This coating of oil enabled them to shed the rain which was frequently torrential though of short duration.  The irradiation of the coconut oil was considered by the natives to provide, in addition, an important source of nutrition.  Their newly acquired wet garments became a serious menace to the comfort and health of the wearers.

We know that the Pacific Islanders Price studied were healthy.  We don’t know too much of the health of the hypothetical naked ape-like creature from whom we evolved.  And unfortunately, we have no idea what the 25(OH)D levels of these healthy Pacific Islanders were.  We can, however, deduce from this that the assumption that our healthy ancestors did not actively reduce their exposure to UV light in certain ways is nonsense.

Price’s comment that they believed they derived nutritional benefit from the irradiation of the coconut oil is curious.  Several likely candidate hypotheses jump to mind:

  • Irradiation of the polyphenols creates some type of nutritional substance.
  • The absorbance spectrum of the polyphenols interferes with UV-mediated destruction of vitamin D but not its production, thereby increasing vitamin D status.
  • The polyphenols serve to curb excess production of vitamin D and thereby spare vitamins A and K.

Price does not elaborate on this, so it is possible that they simply perceived the prevention of sunburn as a “nutritional” effect or that the use of a sunscreen simply allowed them to spend more time in the sun.  However, his wording seems to suggest that the natives believed that, all things being equal, their state of nutrition increased when they applied the coconut oil to their skin during their ordinary exposure to sunlight.  Studying the effect of topical coconut oil on vitamin D production might provide us with some preliminary guess-work about the vitamin D status of these healthy natives.

In conclusion, evidence has accumulated over the last few years that strongly challenges the “latitude hypothesis” of vitamin D and that should cause us to question with great skepticism the idea that we evolved as naked apes in the summer sun with blood levels similar to tropical lifeguards and that anyone living outside the equatorial regions must supplement with vitamin D for most of the year in order to achieve such evolutionary concentrations.

These data in no way whatsoever show that levels above 30 ng/mL or even levels above 50 ng/mL are not superior to measly ol’ 30 ng/mL.  But they do show the necessity of quickly producing dose-finding, randomized, controlled trials with clinical endpoints to satisfy the vitamin D debate definitively.

And, in the mean time, to self-experiment and carefully monitor the results.  And to share these results with the paleo/traditional foods/vitamin D enthusiast communities, whether they are good results or bad results.  After all, we all want perfect health now and not in five to ten years when these trials are completed.

Stay tuned for my review of the IOM vitamin D report!

Read more about the author, Chris Masterjohn, PhD, here.

51 Responses to Vitamin D — Problems With the Latitude Hypothesis

  1. Kutta says:

    It seems to me that one extremely important factor is left out from the latitude studies. For example, if you compare middle-class Brazilians with middle class Germans, the latitude effect might be greatly diminished by the Brazilians similar propensity to work and live indoors and travel by closed vehicles (and wear clothing & use sunscreen). The total amount of UV one receives could be well much more correlated with total time spent outdoors. If this was indeed the case, then that would explain why Caucasians with lighter skins have better vitD status. Obviously hominid ancestors have often sought refuge from the sun (just like basically all tropical and subtropical mammals!), but their total UV exposure still had to be enormously greater than that of contemporary folk living in somewhat civilized condition, e.g. with no need to hunt for game or forage in the wilderness.

    I will be more eager to update beliefs on a VitD serum study if it controlled for time spent outdoors and/or included tropical hunter-gatherer subjects or detailed anthropological considerations for ancestral sun exposure.

  2. Kutta says:

    It seems to me that one extremely important factor is left out from the latitude studies. For example, if you compare middle-class Brazilians with middle class Germans, the latitude effect might be greatly diminished by the Brazilians similar propensity to work and live indoors and travel by closed vehicles (and wear clothing & use sunscreen). The total amount of UV one receives could be well much more correlated with total time spent outdoors. If this was indeed the case, then that would explain why Caucasians with lighter skins have better vitD status. Obviously hominid ancestors have often sought refuge from the sun (just like basically all tropical and subtropical mammals!), but their total UV exposure still had to be enormously greater than that of contemporary folk living in somewhat civilized condition, e.g. with no need to hunt for game or forage in the wilderness.

    I will be more eager to update beliefs on a VitD serum study if it controlled for time spent outdoors and/or included tropical hunter-gatherer subjects or detailed anthropological considerations for ancestral sun exposure.

  3. Chris Winter says:

    It is disturbing how often we are told “health advisories” made by medical experts based on scientific evidence, (ie not to eat eggs due to their cholesterol –and everyone knows that is bad), only to find out those advisories were based on flawed assumptions.
    We have become cynical of “Big Pharma” efforts to make our lives healthier. Why is there so little funding for ‘pure science’ that is not adulterated by self interests?
    Thanks for your efforts, it is refreshing (and unusual that you aren’t selling something.)

  4. STG says:

    Another thoughtful commentary from Chris. I don’t think the latitude hypothesis should be discarded, but it should be questioned. As Chris suggests, at this point in time there are not long-term clincal trials so self-experiments and monitoring v-D levels are reasonable approaches. I will assume the risks and continue to keep my vitamin D levels in the 50-70ng/ml range. In the past when I sunbathed I used coconut oil and I got a very deep, dark tan or burned if I stayed in the sun too long, so for me it did not function as a sunblock.

  5. ben says:

    another fantastic post, Chris. Whether its on facebook, your blog, or here on WAP, i follow your articles feverishly. I love how you refrain from dogma, even when its more or less “on our side.” Great critical thinking and well-done keeping an always open mind. Be well.

  6. Ned Kock says:

    Thanks for this post Chris. I think skin production of D is frequently underestimated in empirical studies. Often people will spend a little time in the sun, with only legs and arms exposed, and their blood levels will go up significantly. This seems to also happen in the winter, as long as exposure is not too early or late in the day.

  7. Kutta,

    Thanks for your comments. I agree that propensity to be outdoors is a huge factor. But skin pigmentation also is and and genetic variations corresponding to ethnicity might be, so the third study seemed to fit nicely with the idea that each region has a skin pigmentation bringing them to an appropriate vitamin D level. That said, there is no reason to assume that tropical folks living modern lives have the ideal vitamin D level, so you might be right that modern lifestyles have reduced vitamin D levels even in the tropics.

    That said, “enormously greater”? I doubt it. The first two studies I discussed in the first >2/3 of the post suggested that the UV-B radiation is similar across the whole globe for most of the year. Thus, propensity to be outside and wear fewer clothes or sunscreen would be most significant, and our healthy ancestors may have been, as you say, seeking the refuge of shade and wearing primitive sunscreen. In any case, this post is not meant to demonstrate that latitude is irrelevant, but to show just how tenuous the assumptions about it are. All we are relying on right now for >50 ng/mL is the assumption that our ancestors were naked sun-worshippers.

    Chris, thanks! Unfortunately it is human nature that no science is ‘pure’ and it is always muddled by our desire for greatness, to make a name for ourselves, to succeed in publishing many papers, etc. There is a dearth of basic research programs and not enough funding for them relative to drugs. Yet, many across the globe achieve freedom from disease with a few cows, a place to fish, and/or a few coconut trees, so it is ironic that our problem is we need more funding.

    STG, thanks! I support your self-experimentation. Heck, I don’t have any scientific evidence that my mother is my mother but I still love her. If we relied on definitive evidence to believe and act, our lives would fall apart. I only have a problem when people announce “we now have definitive scientific evidence that the optimal level of vitamin D is 350,000 ng/mL based on an astrology chart” or marginally more scientific proclamations I’ve seen. 😉

    Ben, thanks! Wellness to you too!

    Ned, good point. How little we know!


  8. John L says:

    On the point about calcitriol, do we know the 25(OH)D levels of the Inuit or Indians? Perhaps we need further indicators to assess D status.

  9. chris says:

    today’s december 24. i am caucasian and i sat in the sun for about 45 minutes, getting a reddish tint on my face and chest. no one can tell me that at 40 degrees it is impossible to synthesize vitamin d. i’m delighted to finally see some good research into this important topic.

  10. John L says:

    Wait a second, I’m not sure why this is necessarily surprising and/or contradictory to the hypothesis: 25(OH)D levels decrease in whites as the latitude increases, which is supportive, no? Also, we would expect them to have higher levels at any latitude, which is also true. The only odd observation is that non-caucasians’ levels don’t decrease as latitude increases.

  11. John L,

    Apparently these polymorphisms in calcitriol metabolism have been identified in these populations, according to literature cited by the authors. I did not follow up that literature yet, since it was a minor point.

    The hypothesis is contradicted on two levels. First, the UV-B availability predictions appear to be massively wrong, and latitude appears to only be a factor during winter, and a lesser factor than previously thought. Second, when all populations are considered together, latitude has almost no effect, with the slope of the line being close to zero and the significance being essentially totally absent. Only when analyzed as a subgroup do Caucasians exhibit latitude variation. The hypothesis is essentially that 1) dark skin was disadvantageous at higher latitudes so we evolved light skin and 2) this adaptation is insufficient, especially given clothing and indoor living. However, 1) there is no effect of latitude in other races, i.e. including the original races that are presumed to have preceded Caucasians in this sequence, and 2) Caucasians at high latitudes have higher levels than other races at lower latitudes, indicating the possibility that vitamin D levels have increased, rather than decreased, as humans have migrated away from the equator.

    None of this is definitvie of course because of caveats already noted in the post and in the comments.

    Chris, unfortunately you can get reddened skin from UV-A without any vitamin D production, so it’s hard to tell by skin alone. But yes, it appears the assuptions about vitamin D winters are likely wrong.


  12. Jacqueline says:

    I’m with John on not seeing how the bit about the Caucasians having higher levels than other races contradicts the hypothesis. If white skin can synthesise the same amount of D in 20 minutes that dark skin does in 2 hours then surely that supports the hypothesis that in the region where white skin evolved that might have been a survival advantage (or at least not a disadvantage if it was a fluke mutation e.g. associated with red hair and the defective form of melanin). You said that in the studies people were only studied where they were ‘native to the area’ and ‘free-living’. So is there any information at all about, say, sub-continental Asians or Africans living in say, the UK? Wouldn’t the latitude effect for other races only be disproved if these people still had levels equal to the levels they had if living in their ‘evolutionary home’ e.g. Africa, India – places at lower latitudes.
    You see I think that Europe – especially Northern/North West Europe and the British Isles- where presumably white skin (and red hair) actually evolved is a special case and there are two reasons: cloud and the gulf stream. The Norse word from which we get the word sky actually means grey or cloud or something. The reason (IMHO) that Caucasians evolved the ability to make so much D so quickly is that long sunny hours are relatively rare. There was an advertisment in Ireland in the last year or so which featured the ‘sun coming out’ and everyone rushing from the shops and their offices to go and get some sun, only by the time they all got there, it had gone in again.
    As for the gulf stream – it is warmer, much warmer at a given latitude in Western Europe than at the same latitude in North America or Siberia/Northern Asia – places where people have stayed darker skinned despite living so far north. This means that historically, clothing might not have been such a factor.

  13. Jacqueline says:

    Having just read the abstracts to refs (1) and (2) I now think that what they say is not much different from what I thought to be the case anyway. I thought the hypothesis was that you couldn’t get much vit D from sunlight between October and March which is roughly what (1) says – and their actual data only goes as far north as 44 degrees. I am at 53 degrees north and I would say from my experience that it is not possible to get sunburned here between October and March. At the moment around the winter solstice the sun here barely gets to 35 degrees high in the sky. Ref. (2) IMHO does not suggest very strongly that it is possible to get much vit D from sunlight in the very early spring in general. I note with interest that it was the individuals with very low levels who responded the most. Also don’t forget how quickly the position of the sun changes at these higher latitudes. Sunset here moves about 5 hours total between midsummer and midwinter.

  14. Cynthia Fredrick says:

    Chris, little by little the dogma of Hollick and Cannel is being challenged. Due to my negative experiences with Vit D, perhaps due to my Scotch/Welsh ancestry, and many others I’ve met, I knew that the whole Vit D discussion as framed by H & C was far too pat and simplistic. I knew eventually information would emerge to show that their absolutes were flawed, and I knew you would be in the forefront of that happening. Keep up the great work Chris, and I hope you never grow weary of researching and writing, because you truly have a gift.

  15. Peter Haynes says:

    Firstly, just a small point Chris. Aren’t your conversions a little out? Shouldn’t 30-35 ng/ml equate to about 75-88 nmol/L? I make this point because over the past 4 months I have measured 25(OH)D in patients attending my Rheumatology clinic (residents of a coastal holiday resort in northwest of England) with ‘polyarthralgia’ (widespread aches and pains). I found that of 126 caucasian patients, only 8% were above the ‘normal’ (>75 nmol/L). The vast majority had levels below 50 nmol/L!!

    I recommend vitamin D3 supplementation in winter, not to get them up to Baywatch levels but merely to try and achieve what you and many of your readers would regard as very modest hydroxy D levels.

    My rationale is that adequate levels of vitamin D will at least protect their bones. At the moment, I have to take it on faith that achieving adequate levels will reduce their risk of various cancers, MS, various infections etc. All we have at the moment to bolster our ‘faith’ are study after study showing associations. I know there is no money in it for anyone, but we need some intervention RCTs!!

  16. Andrew says:

    i live at 41 south in australia and have been sunbathing at solar noon over summer for several years with a uvb meter (a solartech 6.4 to measure what i am getting from the sun

    my observations are that that main variables are cloud which can amplify or reduce uvb, stratospheric ozone which you can read on the canadian worldwide ozone map and of course sun altitude (height of the sun above the horizon)

    imo if you have a good scrub before sunbathing you make quite a bit more vitamin D

    sun altitudes below about 62 degrees (above the horizon) are not worth bathing in and indeed it’s really sun angles above 69 degrees that rock !

    bilirubin processing is a not insignificant benefit of higher sun altitude angles

    both supplemental and solar skin D are needed over the year, but sun does something special that just straight oral vitamin D doesn’t, i don’t think the biochemistry is fully worked out at all

    • Brian Sandle says:

      Andrew wrote: “imo if you have a good scrub before sunbathing you make quite a bit more vitamin D”

      But vitamin D is produced by irradiation of sterols on the skin, or in the skin. Was the surface ones off, more will have to migrate out/ be produced. There may not be so much there to irradiate, so less ready to convert.

  17. Steve Mackin says:

    There is a science to this… based on intensity of vitamin D effective UV which is a narrow weighted portion of the UVB spectrum. This intensity of course decreases as latitude increases, but it does not completely disappear as the so-called vitamin D “winter” term would imply.

    Knowing the effective irradiance dose rate, coupled with the primary confounding factors of exposure time (dose), percent body area exposed, skin type and age… yields a quite accurate value of D3 IU per minute from either sun or UV lamp.

    This value also provides total IU for a given exposure time as automatically calculated from an interactive excel spreadsheet. Please see the D3 Meter page on our website to understand how this works.

    Yes you can still synthesize D3 in many areas above the “winter” latitudes… as uncomfortable as it may be to expose skin in the cold weather.

    One reason why BU studies showed no %C of 7-DHC in vitro may be due to the “glass” covering skin/solution samples. Unless it was pure quartz or fused silica much of the D-eff irradiance would have been absorbed before reaching the solution. Even with quartz both surfaces of the covering reflect away as much as 20% of the UV depending on sun angle.

    Sorry to “spam” our site… but apparently there are not many D3-interested people aware of the potential this device offers.

  18. John Cameron says:

    Chris, Your excellent post has convinced me to get more sun and reduce vitamin D supplementation.

    Dr. Price noted that Pacific Islanders believed that they received nutritional benefits from applying coconut oil to the skin. This belief is consistent with studies that have shown that neonates massaged with coconut oil gain weight faster than those massaged with mineral oil (PMID 16208048 ). Another study showed that neonates massaged with coconut oil had increased serum levels of saturated fat, while those massaged with safflower oil had increased levels of EFAs (linolenic and arachidonic acid) (PMID 16269830)

  19. Tom J says:

    Great job rethinking established ideas, Chris.

    1. I’m a little confused as to why you think the Hagenau et al. meta-analysis provides any evidence for optimal serum 25-OH-D levels. Since most of the cross-sectional studies they included were no doubt performed in developed countries, it very likely that a large proportion of the study subjects were not outdoor workers and did not spend as much time outdoors as traditional peoples. That very few of them had levels above 40 ng/mL is unsurprising and does not suggest to me that below 40 is optimal.

    2. Here in Wisconsin, it’s pretty sunny in the winter. With snow on the ground, I wouldn’t be surprised if effective UV-B radiation were doubled due to the reflection. I was talking to a friend recently who said that his dad has gotten significant sunburns in the winter from ice fishing on a snow-covered lake. Going by the mnemonic “UV-A ages, UV-B burns” this would suggest there is UV-B getting through. Latitude is around 45° here.

    Cloud cover and skin covering/clothing could well be the biggest factors. Our ancestors had to be outside even on cold winter days. Also, I’ve heard the notion that skin color in the British Isles is lighter than that in most of Northern Europe because the weather in the UK is especially cloudy. It would make sense.

  20. Andrew says:

    the uvb/uva ratio is also important in my experience

    the higher the ratio, the better

    also there is a complex inducted immune response, oxidative stress and bilirubin processing paradigm that varies with visible/blue/uva/uvb
    red/infrared ratios and absolute values

    the overall utility of sunlight on skin strongly favours high sun altitudes, maybe 71 to 76 degrees as optimal ?

  21. Steve Mackin says:

    @ Tom J: Yes… reflected UV (albedo) from white snow is nearly 90%. If you aim a UVB meter at the sky in winter… it will read only about 25% (or less) of summer reading. But if you aim it down at the snow it will read nearly the same as up at sun. Adding the two together gives about half the UVB dose rate of summer.

    At high altitudes (ski slopes) you might get a similar dose rate as sea level in summer! The vitamin D will not be so “good” however – because your dose is only on your face (about 5% of body surface).

    One minimal erythemal dose (MED) adjusted for skin type provides ~16.6 IU of D3 per minute on 10% body surface for type 2 untanned skin fyi… based on studies indicating 1000 pill-equivalent IU for 1 MED dose on face/hands/forearms correlated with delta 25(OH)D blood level change at Boston U.

    After about a 2-MED dose the D3 synthesis slows down and stops… a “regulation” process where 7-DHC converts to inert by-products to prevent overdose. Some people call this “destruction” of D3 related to UVA… but it is simply our body’s way of keeping exposure levels beyond moderation under control.

  22. I will respond to everyone individually, but I would like to make a general clarification about my conclusions first, as this has come up in several comments.

    I do not consider any of the data presented here as informative about the optimal level of vitamin D or as a definitive refutation of the idea that lighter skin evolved to facilitate greater vitamin D production. I do, however, consider the data to achieve the following:

    1)The UVB data argue strongly against the previous paradigm of vitamin D winters both in the magnitude and the temporal and geographical expanses of those winters and the relative role that geographical variations in UVB availability play in vitamin D synthesis.

    2)The data, while not refuting the hypothesis about the evolution of white skin, are very unsupportive of that hypothesis, because they do not show any relationship between latitude and 25(OH)D in dark-skinned people. If light-skin populations evolved from dark-skinned populations, the latitude relationship would have to be present in dark-skinned people in order to exert the selective pressure required to favor the dominance of light skin. However, I say that this does not refute the hypothesis because we have no way of knowing that this latitude relationship did not exist under very different conditions that could have characterized previous eras. Thus I say it is unsupportive of the hypothesis rather than saying it refutes it.

    3)The data contradict the use of latitude as a surrogate for 25(OH)D in ecological studies.

    4)The data are very unsupportive of the concept that vitamin D status has declined as humans have moved away from the equator. These data are more consistent with the concept that vitamin D status has increased as we have moved away from the equator due to the evolution of lighter skin. However, as in #2, the data do not refute the hypothesis because we cannot infer 25(OH)D levels of people living in past eras from those of people living in present eras. Therefore, the data are simply unsupportive of the hypothesis.

    5)I do not believe the data offer any insight into the optimal status of vitamin D. However, since they are very unsupportive of the “naked ape” hypothesis of optimal status, which already had scant evidence to begin with, and since this hypothesis essentially rests on the assumption about a Paleolithic lifestyle that includes the non-use of sunscreen, which is very clearly a nonsensical assumption when we consider that pre-modern Pacific Islanders used coconut oil as a sunscreen, these data emphasize the lack of support for the “naked ape” hypothesis of optimal vitamin D status. Again, the data do not refute the hypothesis, but simply emphasize the lack of evidence supporting it.

  23. See introductory remarks in last comment.

    [b]Response to Jacqueline[/b]


    I hope what I stated above clarifies my stance on the evolution of white skin. As I stated in the article, I agree that it is still a possibility that someone may move out of their native location and develop vitamin D deficiency as a result. I agree that climate could have local impacts due to cloud cover, snow cover, and so on, but that is not really part of the ‘latitude hypothesis,’ which focuses on the angle of the sun as a function of latitude and the resultant changes in UVB availability. I reviewed the previous models of UVB availability within my post, which were also based on extrapolations from several locations, and reference 1 completely and radically contradicts those models. In reference 2, the subjects only had their face exposed, so you would expect only small amounts of D produced that would show up quantitatively in the people with lowest D status. The study provides what I called “limited evidence” that UVB is available in late February, when in fact previous models predicted no availability for months to come.

    [b]Response to Cynthia[/b]

    Cynthia, thanks! I do think, though, that within the vitamin D movement Holick is among the least dogmatic and most careful.

    [b]Response to Peter[/b]

    Peter, thanks! That was a typo and I fixed it. Your recommendations, practice, and analysis sounds very sensible to me. Keep up the great work!

    [b]Response to Andrew[/b]

    Andrew, thanks for sharing your data. I did not know about the bilirubin connection. I agree that the UVB/A ratio is critical. I think you raise the possibility that light skin evolved for reasons other than D production, given that there are other benefits of sunlight.

    [b]Response to Steve[/b]

    Steve, thank you for sharing the thoughts and resource.

    [b]Response to John[/b]

    John, thanks! Price did say that the irradiation of the coconut oil was the nutritional factor, so it seems it must be something other than the fatty acids, but these studies do provide proof of principle that topical oils reach the bloodstream.

    [b]Response to Tom[/b]

    Tom, I hope my introductory remarks clarified my thoughts about the optimal level. I think that UVA can burn, as it contributes to redness, but the new models would predict substantial UVB in the winter in Wisconsin. Excellent points.


  24. Northern D says:


    Do not confuse recent dietary chages of the Inuit with the ancestral diet.

    What has happened to the Inuit through European settlers replacing the Inuit ancestral dietary foods with vitamin D poor foods should not be confused.

    The Inuit and their food changes is not unlike black slaves brought to North America 300 years ago. It has created a vitamin D deficiency crucible of adaptational change.

    The slave decendants of slave in North America have a very different relationship with vitamin D than those blacks that never left Africa. Through natural selection we now have a race of blacks in North America who tolerated vitamin D deficiency very well. However, the first waves of slaves died in droves of rickets, and vitamin D deficiency affected their fertility so badly slaves could not produce children.

    The Inuit are in a similar crucible of vitamin D deficiency now and their biology and relationship to vitamin D is changing because of a shift away from the ancestral diet.

    The Inuit have a gut adaptarion that excretes more vitamin D. This works well with their ancestral diet, but sets them up for vitamin D deficiency with the altered diet.

    You are missing the forest for the trees.

  25. Steve Mackin says:

    Primary significant reason why so many whites become D-deficient is from covering up with sunscreen creams 24-7 as hawked by dermatologists getting kickbacks and boondoggle conventions financed by big-pharma and beauty mags.

    Slip-Slop-Slap your way to horribly low D3 levels! SPF 15 blocks ~98% of D-eff UVB rays.

  26. Sheila says:

    New theory on why dark skin protects in the tropics:

  27. Sheila says:

    Interesting new theory about melanin and pathogens. Skin colour may have nothing to do with Vit D.

  28. Gigi Allaway says:

    Thank you for your comprehensive article, it is satisfying trying to work with the information here and linking it to Stephanie Seneff’s excellent work (see Could you comment on the relative availability and use of fermented CLO, which seems to be much lower in Vitamin A,D (I realize this was a necessity given changes in manufacturing processes), but still.
    Best, Gigi

  29. Kilton says:

    Thanks everyone — great post & comments.

    I’ve been taking UVB measurements with a Solarmeter 6.2 since July here in DC (38.5N). At solar noon in July the UVB measurement was over 400uW/cm2; at solar noon on the winter solstice it was 105. (All measurements taken on cloudless days.)

    I’m taking these measurements to get an idea of how strong the sun is at various times of the year. The oft-repeated claim in Vitamin D circles that you can’t make Vitamin D in the winter has always seemed bizarre to me — as if there’s a specific UVB irradiance value that is the cutoff. I don’t think 105 uW/cm2 is too shabby at all in December. You may not be able to make much but surely you can make something. I sure wish I had Steve’s spreadsheet! 🙁

    Another common claim is that you can’t make Vitamin D if your shadow is longer than you are tall. More over-simplistic nonsense. In the summer and fall I was getting values in 300uW/cm2 territory with the sun below 45 degrees in the sky. (Interstingly enough when the sun is below 45 degrees, you’ll get more UVB rays when you’re standing up because they hit you at a more direct angle. Whereas at above 45 degrees, laying down gives a more direct hit.)

    I’ve also seen the claim that Vitamin D production requires heat on the skin. Anyone know if this is the case? If it is, making it in cold days would be impossible regardless of UVB irradiance.

  30. Kilton says:

    One more thing. Would be interested in everyone’s thoughts on this study:

    Doesn’t look like Coconut Oil blocks very much UV.

  31. “These data in no way whatsoever show that levels above 30 ng/mL or even levels above 50 ng/mL are not superior to measly ol’ 30 ng/mL.”

    Wake up you guys. Go to and also to “UCSD School of Medicine and GrassrootsHealth bring you this series on vitamin D deficiency.”

    Try getting levels above 60 or even 80. See above, 70+% reduction in cancer.

  32. Hotwhathow says:

    Several studies show that a lack of certain vitamins and minerals may put you at risk for depression.

  33. Hill says:

    This article and study is one of the most comprehensive articles I have ever seen on vitamin D absorption. Thank you so much. I hope that you will get the recognition that you deserve and that your thoughts and ideas will be taken to heart more seriously than in the past. I also believe that you have done a tremendous amount of research and testing

  34. Asim says:

    Dr. AYers has challenged the latitute hypothesis as can be seen here:

    As is obvious, diet that creates inflammation may impact the body’s ability to maintain vitamin D levels.

  35. Pat Snyder says:

    I have been back and forth on taking cod liver oil for the last 6-7 years. When I take it (today I took 1 ml of high vitamin clo and high vitamin butter oil) I always get dopey brained. Please help. My cardiologist wants me on 6000 IU’s of vit d and 1800 IU’s of fish oil and 200 mg magnesium/day (not cod liver oil). I have been on this regimen for about 2 years; sneaking in clo when I feel like trying it again when I read your information. Last night I felt like I had a mild heart attack??? I want to do whats right. Do you know why I would get dopey/foggy brained when I take it? I am a 911 dispatcher and it affects my job. Please respond with any info you can give me. Thanks

  36. Greg says:


    The one fact you are leaving out of this argument is the very strong correlation between MS rates and latitude, and the several studies showing a correlation between UV exposure and MS rates, vitamin D intake and MS rates, and EAE rates and UV exposure (the mouse version of MS.)

    Ditto for diabetes, breast and prostate cancer.

    These correlations are very suspicious to me and DO support the latitude hypothesis.

    What is your opinion on these disease rates and their strong correlation to latitude?


    • Rubs says:

      Is the latitud Hypothesis or just that in northern climats people just don´t go outside because of the cold weather?. They could make some Vitamin D because there´s some UVB but it would be worthless because who can get out in the sun, if there´s some, with little clothes to get the D?. Also you can get some UVB but how much time would you have to be out to get a considerable amount of D?

  37. theresa says:

    Gawd! I am more confused than ever after reading this dense article… It’s this but it could be that it’s that but it could be this, who’s to say? We need mnore research…. JEEZ!

  38. ravi says:

    Perhaps insignificant, (or not…?) but as i live in a passive solar house and had to specially order NON-UV block windows to make it all work properly, it occurred to me that most people’s houses have these UV-Block type glass windows. If winter is weaker sun, you don’t get as much skin exposure outside and additionally inside when less-dressed you get substantially reduced UV exposure from what light does get in – then northern climate types would definitely get even that much less good sunlight –

    i believe our family gets lots in the winter as we are more often very lightly dressed due to the efficiency of our 70% NON-UV glass southern exposed solar house.

    • Brian Sandle says:

      You may be lucky to get glass which does not block UVB. I think ordinary glass allows through UVA which actually can destroy vitamin D on the skin. It is said to be a way nature avoids overdosing us.

      In colder climates I presume people outdoors tend to put on clothes sometimes before the UVA has done too much damage? Could be a reason why clothesless people in hot climates do not go so high on vitamin D?

      • Rubs says:

        I think people in hot climates are not so high in D is because the have our same habits of being indoor. But in their places they can be high in D when they go outside with little clothes unlike us that are always low in D not matter if we go out with little clothes.

  39. sue ingram says:

    Having experienced tragic melanomas in both family and friends, I decided to research more about sunscreens and 3 years later ended up launching my own. What an interesting journey so far and I am sure will continue to be as debates such as this one evolve. We have focused on obtaining as high as possible UVA protection, rather than high UVB, or only UVB as some sunscreens still do. UVB being the Vit D enabler, we also put out the message that good, controlled sunshine exposure is the best thing for Vit D creation.
    The ongoing research and studies done wrt Vit D are fascinating and there are some wonderful stories of people radically improving their health when increasing their Vit D levels. Articles such as yours are just what is needed to shake some of the thinking and challenge the theories as Vit D sure seems to be a miracle the way it is being hyped up. What a pleasure if it all holds true though. Sunshine is fabulous!
    So I shall look forward to all future contributions here.

    • Rubs says:

      Hi SUE INGRAM, when you say you researched about sunscreen you mean you found they were a possible melamona cause?. Thank you.

  40. Chantelle says:

    I was quite excited to read this article and read every comment as well! It is not surprising to me as to these more recent findings. Intuitively, I have found it impossible to supplement my Vitamin D, preferring instead to focus on getting sufficient sun exposure that would help keep my levels optimal. I live in Arizona, so this makes this seem to likely be an even more viable and practical option than it might have seemed to those in other climes, however, in spite of this, I just have found myself questioning the need to supplement, even for those in colder climes. (I was born and raised in Iowa) I found supplementation “impossible” in that I kept purchasing CLO yet every time I would go to open it and take it, it never felt “right”, so in the course of many years I have purchased several bottles of CLO only to have them expire unopened or to give them away so that they would not expire. Your article backs up what my gut sense is on the matter. I do have a Biology degree and try to keep up to some degree with current research. Anyway, I just wanted to say that your article was a “relief” in a sense to read to me because it felt like , to some degree, validation at least of what I have felt on the matter. I was very excited to read every word and look forward to finding out even more. To me, it just seems that not everyone could be as deficient as some of the past studies seemed to indicate. At the same time, I do think getting as much sun as possible is a HUGELY beneficial thing and I have seen that in my own life and health.

  41. bananabender says:

    Modern Westerners spend almost their entire day indoors. That is why the have low Vitamin D levels.

  42. Rubs says:

    After being diagnosed with a Vitamin D deficiency in march with a level of 11 ng/ml and all kind of symptoms I began to sunbathe most of the time totally naked for about 30-40 minutes everyday and at the end of summer my level was 81 ng/ml. If my body can produce such amounts of D how can a level of 30-35 ng/ml be normal?. By the way I don´t tolerate supplements, they always induce me a magnesium deficiency with panick attacks and anxiety attacks. This does not happen with the sun, So I think the supplements and the sun are not the same after all. I´m from south Spain and I live at 36º N Latitud.

  43. Rubs says:

    Hi, Chris. Don´t you participate anymore in this forum? Is it dead?

  44. Ahmed Hussein says:

    I live in Vancouver, BC Canada. Latitude 51 – How much sun would I need to expose myself to considering I would like to attain 5000-10000 IU in Vitamin D a day?

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