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found that accumulation of 25-hydroxyvitamin D is not affected by skin pigmentation. I would like to point to a possible explanation to the seeming contradiction between their results and those of many observational investigations, which have found that vitamin D status is better and vitamin D production proceeds more readily in people with fairer skin.
measured the increase of plasma 25-hydroxyvitamin D caused by unfiltered radiation from Philips TL12 lamps containing very shortwave components, including a significant component at wavelengths <290nm. Such radiation is efficiently absorbed by 7-dehydrocholesterol (7-DHC, provitamin D3) in the upper epidermis, outside the most pigmented skin layer. Daylight, on the other hand, contains effectively no radiation below 290nm. The transmission of the stratum corneum can exceed 90% at 300nm and increases with wavelength (
), so some previtamin D3-producing radiation, even that of very short wavelength, can reach the outer epidermal layers. The stratum granulosum does not contain much melanin, but some provitamin D3, which can therefore be converted to previtamin D, even in heavily pigmented individuals.
Compared with light-skinned people living at the same latitude, dark-skinned persons generally have lower vitamin D status. The spring rise in vitamin D status is also lower in dark-skinned people (
). Exposure of isolated human skin to summer sunlight and exposure in vivo of humans to sunbeds simulating sunlight showed that Caucasian skin is more efficient at forming previtamin D3 than is African American skin (
Figure 1Radiation spectra for Philips TL12, Houva, and daylight in Boston for clear sky at noon on 21 June 2006. The vertical scale is in Wm−2nm−1 for daylight, arbitrary for lamps.
Figure 2 shows an absorption spectrum (in relative units) for the previtamin D3 precursor, 7-DHC, together with transmission spectra for human stratum corneum and epidermis redrawn from
found 58, 393, and 303ngcm−2 in the stratum corneum+granulosum, stratum spinosum, and stratum basale, respectively). Short-wavelength radiation does not penetrate far into the epidermis, but can efficiently convert the 7-DHC in the superficial layers, as this radiation is efficiently absorbed by 7-DHC. UVR of longer wavelength is not efficiently absorbed by 7-DHC, but can penetrate deeper and reach more of the 7-DHC present. Therefore, previtamin D3 synthesis induced by daylight or other UVR of longer wavelength is more sensitive to the melanin content of the skin than is that induced by short-wavelength radiation. One can speculate that the rate of synthesis may decrease when the the skin acclimatizes and pigment moves toward the surface, and there is some indication of this (Figure 2 of
, is that pale skin has evolved when humans migrated northward out of Africa to areas where UV is less intense, or, in his own words, “the inhabitants of the interior highlands of the far north not using a fish diet or any other generous source of food vitamin D must needs get most of their antirachitic vitamin from sunlight. If their skins are not sufficiently white to admit large enough amounts of solar radiation for normal bone metabolism, they are subject to rickets”. It is now known that other problems than rickets may arise even with slight vitamin D deficiency. Murray's theory has recently been challenged by
) in which it was found that vitamin D status is equally improved by UV irradiation irrespective of skin pigmentation, but these seem to be exceptions.
tested very few individuals, and does not specify the radiation.
ACKNOWLEDGMENTS
I thank Dr Richard L McKenzie for valuable comments and Professor Mary Norval for lingustic corrections, and acknowledge the use of the Quick TUV calculator at 〈http://cprm.acd.ucar.edu/Models/TUV/Interactive_TUV/〉 for calculation of the daylight spectrum.
Statement: The submitted paper is based on an original idea and literature data as described in the paper (except for the absorption spectrum of 7-DHC, which is based on my own measurements).
REFERENCES
Armas L.A.G.
Dowell S.
Aktar M.
et al.
Ultraviolet-B radiation increases serum 25-hydroxyvitamin D levels: The effect of UVB dose and skin color.
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