Thursday, 18 June 2009

Vitamin D and homeostasis

In my previous posts, I argued that a homeostatic mechanism keeps the level of vitamin D in our bloodstream within a certain range. When UV-B light is always intense, as in the tropics, the level seems to be 50-75 nmol/L in young adults and progressively lower in older age groups. The more sunlight varies seasonally, the more the body will produce vitamin D in summer in order to maintain at least 50 nmol/L in winter—a level well below the recommended minimum of 75 nmol/L and even further below the 150 nmol/L now being advocated by vitamin-D proponents.

This homeostatic mechanism breaks down if we daily ingest 10,000 IU of vitamin D or more (Vieth, 1999). It seems that the human body has never naturally encountered such intakes, at least not on a continual basis.

In a recent review article, Robins (2009) presents evidence for a second homeostatic mechanism. Even when the level of vitamin D varies in the bloodstream, the second mechanism ensures that these divergent levels will translate into the same concentration of the biologically active 1,25-(OH)2D metabolite.

Matsuoka et al. (1991) demonstrated that after single-dose, whole-body UVB exposure black subjects had distinctly lower serum vitamin D3 levels than whites, but differences between the two groups narrowed after liver hydroxylation to 25-OHD and disappeared after kidney hydroxylation to 1,25-(OH)2D. These findings suggest that there is a compensatory mechanism whereby, in the presence of vitamin D3 suppression by melanin, the liver and kidney hydroxylating enzymes are activated in tandem to ensure that the concentration of the biologically active 1,25-(OH)2D metabolite is normalized and kept constant regardless of ethnic pigmentation (Matsuoka et al., 1991, 1995).

Robins (2009) goes on to note that nearly half of all African Americans are vitamin-D deficient but show no signs of calcium deficiency. Indeed, they “have a lower prevalence of osteoporosis, a lower incidence of fractures and a higher bone mineral density than white Americans, who generally exhibit a much more favourable vitamin D status.” He also cites a survey of 232 black (East African) immigrant children in Melbourne, Australia, among whom 87% had levels below 50 nmol/L and 44% below 25 nmol/L. None had rickets (McGillivray et al., 2007).

References

Matsuoka, L.Y., Wortsman, J., Chen, T.C., & Holick, M.F. (1995). Compensation for the interracial variance in the cutaneous synthesis of vitamin D, Journal of Laboratory and Clinical Medicine, 126, 452-457.

Matsuoka, L.Y., Wortsman, J., Haddad, J.G., Kolm, P., & Hollis, B.W. (1991). Racial pigmentation and the cutaneous synthesis of vitamin D. Archives of Dermatology, 127, 536-538.

McGillivray, G., Skull, S.A., Davie, G., Kofoed, S., Frydenberg, L., Rice, J., Cooke, R., & Carapetis, J.R. (2007). High prevalence of asymptomatic vitamin-D and iron deficiency in East African immigrant children and adolescents living in a temperate climate. Archives of Disease in Childhood, 92, 1088-1093.

Robins, A.H. (2009). The evolution of light skin color: role of vitamin D disputed, American Journal of Physical Anthropology, early view.

Vieth, R. (1999). Vitamin D supplementation, 25-hydroxyvitamin D concentrations, and safety, American Journal of Clinical Nutrition, 69, 842-856.

14 comments:

RG said...

Peter, do we have any evidence that the UV-induced DNA damage repair system is more active in europeans than africans ?

Usually, dark brown or black africans as well as the fairest europeans have nice textured skins, if we exclude the extreme tanning behaviors of the mad english people.
However, I have noticed that many metis of african-european mixt show many bumps, spots and melanomes on their skin.
It would make sense if we imagine a mix between a system optimized for more Vit. D production, no passive protection but a more active DNA damage repairs, and a system optimized for just the opposite. Some mixt individuals will miss both, the DNA repair system and the passive protection from melanine, thus the spots and melanomes.

Anonymous said...

Hmmm,

I had assumed that Vitamin D deficiency was one of the things depressing AA IQs.

Seems I was wrong, and it is all genetic.

Tod said...

UV-Index for Melbourne 3 months at '11'. In Britain it never goes above '8'. Those who beliieve that warnings about skin cancer have lead to widespread vitamin D deficiency in Australia should look at (McGillivray et al). East Africans can't be suspected of being worried about sun.

"In the late 1980s, I interviewed elderly French-Canadian farmers who remembered how things were before the 'sunshine movement' took hold during the interwar years. In their youth, they practiced sun avoidance to a degree that would now seem unthinkable and even ridiculous"

In FWDM the stress seemed to be on the sun avoidance of women. One of those interviewed said that she only remembered a few men who wore the gloves and broad brimmed hat.

Women, the elderly, and ethnic minorities tended to have lower vitamin-D levels in the blood.(1)
Very few women avoid the sun today women; lower levels of the form of 'D' being measured may well be natural for women.

The vitamin D enthusiasts may convince women to try and attain serum levels that are unlikely to be beneficial.



(1)Martins D, Wolf M, Pan D, et al. Prevalence of cardiovascular risk factors and the serum levels of 25-hydroxyvitamin D in the United States. Data from the third National Health and Nutrition Examination Survey. Arch Int Med 2007; 2007; 167:1159-1165.
Article Here

Tod said...

Re. RG's comment on adaptation to UV as explaining characteristics of black African's skin. Apart being much thicker, black Africans' skin is very dark even where it has never been exposed to the sun (this is may be unique to them). The extra melanin may be related to health but the Bushmen's lack of it despite comparable sun exposure to field workers casts doubt on the relevance of UV as the primary factor.
(Less mate choice means weaker sexual selection for light skin in women and, hence, less counterbalancing of natural selection for dark skin in either sex to protect against sunburn and skin cancer (Aoki, 2002; Frost, 2007; Frost, 1994).) Hmmm.

West Africa is extremely humid and perhaps as a result it's the most unhealthy place in the world with myriad tropical diseases and parasites.

Have you read John Manning's book yet ?

(Manning's book has some holes in it; Finland's average digit ration is equal to the lowest in the world. Then again the Finnish climate may not be conducive to disease.

RG said...

Tod,
No I havn't read
Manning's book, but he has a short resume of it at:
http://www.human-nature.com/nibbs/02/manning.html
I was aware of this sexual dimorphism a couple of years ago through its mediatization to homosexuality. It is another subtil anatomic difference between ethnic groups that is supposedly related to testosterone level.
Is there a connection with vitD ?

Back to today's post, it is indicated that vit.D level is lower in africans despite higher bone density. What I understand, the point of all these posts about vitD is to show that vitD variations among population has nothing, or very little to do with skin color selection and therefore that sexual selection is the only thing left to explain skin color (as opposed to what is said in most school textbooks).
Am I correct ?

A small anecdoctical story about that. I was substituting a science class 2 months ago, and the assignment was to show a video of a History Channel episode about human consomption of things. By 'human',understand 'americans'. So the video went to the topic of milk consumption and showed a huge amount of bottles of milk, an ocean of milk, in front of each american house. All this suggested a vague feeling of guilt, because of the amount of trash generated and the relative waste in calory efficiency, i.e, 'we should all eat grass'. At this point, to relieve this feeling of guilt, I could not resist to mention that milk consumption is so high because a mutation occured about 10 or 9 thousand years ago that makes lactose tolerance possible in adults, but that not all human ethnic groups have the mutation. For example few asian people have it. I also suggested that culinary traditions could represent such genetic differences, i.e. if europeans developed hundreds of different cheese (the 400 french cheeses!), creame and milk receipes, that is most likely because they had a taste for it, as simple as that! But that taste came from the capacity to tolerate lactose, of which they have no control.
But my class was very ethnically diverse, and I saw some frowned faces at the mention of genetic differences between races. When I ask an asian kid if milk was a common ingredient in asian cuisine he defensively assured me that yes, there was no differences, milk was heavily used in asian cooking...
If lactose tolerance is already inapropriate to mention, forget about 'other' differences. The bottom line is that school textbooks are not ready to change. UV light and vitamine D will stay for a long time 'the only' selection mechanism for skin tone. Sexual selection, especially on neotenic traits and color tones, is too reminiscent of Eugenism, and therefore racism, to be mentioned in ethnically diverse public schools.

Tod said...

Sorry RG - I referred to your comment by way of broaching an off topic issue (well I think it's an issue) with Peter.

Maybe there is something in the idea milk production has been over promoted and subsidized. Before it was popularized by advertising from commercial interests only children drank it, even farmers preferred buttermilk. Milk was for infants 150 years ago, and it had a well earned reputation for being dangerous to them.

RG said...

Yes my first comment was slightly off topic but not too much.
Since vitamineD synthesis seems more and more irrelevant to explain skin color, I just wanted to make a new hypothesis. My idea was that skins with no passive protection like melanine must develop a better DNA repair system against UV damages or else they get skin cancer. But maybe a side effect of that would be a more efficient DNA repair system in general in all the body, not just the skin, and maybe that would translate into a higher life expectency, or a lower aging rate. So sexual selection for a neotenic trait such as fair skin would create individuals who stay younger longer, that's a perfect feedback loop. Speculations, I know, but since vitamine D is loosing its status as a positive selection mechanism, there must be something else.

Tod, about farmers or adults not drinking milk, true, but they eat a lot of cheese and yogourt.

Peter Frost said...

RG,

I did a quick google search, but I didn't find any evidence of ethnic differences in DNA repair of skin cells (although I found an article on ethnic differences in DNA repair of lung cells).

Tod,

There was more sun avoidance among women than among men. But even men normally wore hats until the early 1940s. You mention a quote from FWDM about a woman's brother-in-law who wore a hat and long gloves, with the implication that this was unusual for a man. The informant, however, was born in 1925, so that quote would refer to a man from the postwar years.

I haven't read Manning's book cover to cover. His writing style is not always easy to follow.

RG,

Yes, I'm trying to document that vitamin-D deficiency is, at best, a weak selection pressure. Dark skin does not seem to be a serious constraint on adequate vitamin D production. On the one hand, even black skin can produce sufficient vitamin D during a short period of sun exposure. On the other hand, tropical human populations seem to use vitamin D more efficiently, i.e., they can maintain adequate levels of calcium even with very low levels of vitamin D (see Sedrani, S. (1984) Low 25-Hydroxyvitamin D and normal seroum calcium concentrations in Saudi Arabia: Riyadh region, Annals of Nutrition & Metabolism, 28, 181-185).

I agree that it will be some time before people accept that sexual selection has strongly influenced human variation in skin color. Opinions will begin to change when two things happen:

1. Firm dating of the changes to European skin color. It will be shown that European skin whitened very rapidly and long after the entry of modern humans into Europe, probably around 15,000 to 10,000 years ago. None of the competing theories can explain such a rapid metamorphosis, especially if it coincides with other phenotypic changes (i.e., diversification of eye and hair color, lengthening of head hair, changes to facial morphology).

2. Vitamin D revisionism. It will be shown that vitamin D is not as wonderful as people now think it to be. Harmful effects probably appear at sustained levels as low as 130 nmol/L. Yes, the skin can naturally produce much higher levels, but only when sun exposure is discontinuous. It will also be shown that the body can maintain adequate vitamin D homeostasis regardless of skin color. Most reports of vitamin-D deficiency among African Americans will be shown to be exaggerated or simply data misinterpretation.

Tod said...

Allowing that UV causes enough skin cancer and sunburn to be a selective force for darker skin; it would also be a selective force for instinctive avoidance of the strongest UV. African women in the fields could - and surely would - schedule tasks to avoid the midday sun and not just on their own account. Many of them would be mothers accompanied by infants who they would avoid exposing to to the sun at its strongest.

Melanin has many effects in the body, (black workers rarely suffer from industrial deafness for example), but the amount that black Africans have is overkill as far as the danger of UV is concerned. Compare the light skinned Bushmen; the idea that they don't spend much time working has been discredited; they're out in intense UV for a comparable time to garden farming females. Moreover among San the women and the men get this exposure; they should have been impacted by selection for UV protection to a greater extent than where men do less work in the sun.

My conclusion is that Bushmen are nowhere near as dark as they should be if your recent paper's explanation is correct. (Sexual selection has been suggested to have resulted in certain characteristics of female San, I suppose it could be argued that the skin of both sexes has been lightened somewhat by such a process.)

The mission headmaster quoted (FWDM) in aid of the longevity of darker people did not specifically mention light skinned women as being short lived - although it would be noticeable one would think - nor did he mention skin cancer as what killed off paler villagers.

The coast of West Africa is the most unhealthy part of the continent. While I believe the point that darker skin confers better health, simple resistance to sunburn and skin cancer would not reduce mortality enough to make older 'yellow' villagers so rare.

Anonymous said...

From personal observation, East Asians have thicker skin than caucasians.

Tod said...

"10 Aug 2008 ... Penn State University said that experiments on mice suggest that high levels of testosterone may be a key factor in spreading disease. ..."

Where do we we find high T. and a lot of diseases that reduce reproductive fitness by mortality and the effects of infectious infertility?

"The geography of infertility shows different zones of low fertility in Africa with a large zone of hypofecundity extending through most of West Africa".

Peter Frost said...

Tod,

I substantially agree with you. Black skin, like white skin, is an outlier that's hard to explain in terms of natural selection alone. I quoted that mission headmaster in full because I wanted my readers to form their own opinions. There is certainly a perception among Africans that darker skin is associated with greater strength and resistance. My main argument is simply that the high incidence of polygyny among sub-Saharan agriculturalists reduces sexual selection of women. So any selection for lighter-skinned, 'yellow' African women would be diminished accordingly.

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