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Vitamin D Synthesis May Be Independent of Skin Pigmentation Only with UV of Short Wavelength

      Abbreviations

      7-DHC
      7-dehydrocholesterol (provitamin D3)
      TO THE EDITOR
      • Bogh M.K.B.
      • Schmedes A.V.
      • Philipsen P.A.
      • et al.
      Vitamin D production after UVB exposure depends on baseline vitamin D and total cholesterol but not on skin pigmentation.
      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.
      • Bogh M.K.B.
      • Schmedes A.V.
      • Philipsen P.A.
      • et al.
      Vitamin D production after UVB exposure depends on baseline vitamin D and total cholesterol but not on skin pigmentation.
      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 (
      • Philp J.
      • Allcock C.
      The ultraviolet microscopic_transmission_of_human_stratum corneum.
      ), 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 (
      • Harris S.S.
      • Dawson-Hughes B.
      Seasonal changes in plasma 25-hydroxyvitamin D concentrations of young American black and white women.
      ). 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 (
      • 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.
      ;
      • Chen T.C.
      • Chimeh F.
      • Lu Z.
      • et al.
      Factors that influence the cutaneous synthesis and dietary sources of vitamin D.
      ).
      The radiation spectra of the lamps used by
      • Bogh M.K.B.
      • Schmedes A.V.
      • Philipsen P.A.
      • et al.
      Vitamin D production after UVB exposure depends on baseline vitamin D and total cholesterol but not on skin pigmentation.
      and
      • 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.
      are shown in Figure 1, together with a daylight spectrum calculated for the conditions of
      • Chen T.C.
      • Chimeh F.
      • Lu Z.
      • et al.
      Factors that influence the cutaneous synthesis and dietary sources of vitamin D.
      in their daylight experiment. It can be seen that
      • Bogh M.K.B.
      • Schmedes A.V.
      • Philipsen P.A.
      • et al.
      Vitamin D production after UVB exposure depends on baseline vitamin D and total cholesterol but not on skin pigmentation.
      , who did not find any effect of pigmentation, used treatments with a larger proportion of short-wave UV radiation than did
      • 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.
      and
      • Chen T.C.
      • Chimeh F.
      • Lu Z.
      • et al.
      Factors that influence the cutaneous synthesis and dietary sources of vitamin D.
      , who found such an effect.
      Figure thumbnail gr1
      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.
      Philips TL12 redrawn from
      • Bogh M.K.B.
      • Schmedes A.V.
      • Philipsen P.A.
      • et al.
      Vitamin D production after UVB exposure depends on baseline vitamin D and total cholesterol but not on skin pigmentation.
      ; Houva redrawn from
      • Morison W.L.
      • Pike R.A.
      Spectral power distributions of radiation sources used in phototherapy and photochemotherapy.
      , with permission from Elsevier. Daylight calculated by “Quick TUV” (see cprm.acd.ucar.edu/Models/TUV/Interactive_TUV).
      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
      • Bruls W.A.G.
      • Slaper H.
      • van der Leun J.
      • et al.
      Transmission of human epidermis and stratum corneum as a function of thickness in the ultraviolet and visible wavelengths.
      . 7-DHC occurs throughout the epidermis (
      • Holick M.F.
      • MacLaughlin J.A.
      • Clark M.B.
      • et al.
      Photosynthesis of previtamin D3 in human skin and the physiologic consequences.
      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
      • 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.
      ).
      Figure thumbnail gr2
      Figure 2Transmission spectra for unexposed stratum corneum and epidermis (the latter expanded 10x) according to
      • Bruls W.A.G.
      • Slaper H.
      • van der Leun J.
      • et al.
      Transmission of human epidermis and stratum corneum as a function of thickness in the ultraviolet and visible wavelengths.
      and absorption spectrum for 7-DHC (provitamin D3) (the latter in arbitrary units).
      A commonly held view, originating from
      • Murray F.G.
      Pigmentation, sunlight, and nutritional disease.
      , 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
      • Robins A.H.
      The evolution of light skin color: Role of vitamin D disputed.
      , who cites three investigations (
      • Stamp T.C.B.
      Factors in human vitamin D nutrition and in the production and cure of classical rickets.
      ;
      • Lo C.W.
      • Paris P.W.
      • Holick M.F.
      Indian and Pakistani immigrants have the same capacity as Caucasians to produce vitamin D in response to ultraviolet radiation.
      ;
      • Brazerol W.F.
      • McPhee A.J.
      • Mimouni F.
      • et al.
      Serial ultraviolet B exposure and serum 25 hydroxyvitamin D response in young adult American blacks and white: no racial differences.
      ) 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.
      • Brazerol W.F.
      • McPhee A.J.
      • Mimouni F.
      • et al.
      Serial ultraviolet B exposure and serum 25 hydroxyvitamin D response in young adult American blacks and white: no racial differences.
      used FS40 lamps, which emit as much short-wavelength radiation as the TL12 used by
      • Bogh M.K.B.
      • Schmedes A.V.
      • Philipsen P.A.
      • et al.
      Vitamin D production after UVB exposure depends on baseline vitamin D and total cholesterol but not on skin pigmentation.
      ;
      • Lo C.W.
      • Paris P.W.
      • Holick M.F.
      Indian and Pakistani immigrants have the same capacity as Caucasians to produce vitamin D in response to ultraviolet radiation.
      used a FSX72T12 UVB:HO lamp, which has a similar spectrum.
      • Stamp T.C.B.
      Factors in human vitamin D nutrition and in the production and cure of classical rickets.
      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).

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