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.

© 2015 The Weston A. Price Foundation for Wise Traditions in Food, Farming, and the Healing Arts.