Vitamin D
Factors that influence the cutaneous synthesis and dietary sources of vitamin D

https://doi.org/10.1016/j.abb.2006.12.017Get rights and content

Abstract

The major sources of vitamin D for most humans are casual exposure of the skin to solar ultraviolet B (UVB; 290–315 nm) radiation and from dietary intake. The cutaneous synthesis of vitamin D is a function of skin pigmentation and of the solar zenith angle which depends on latitude, season, and time of day. In order to mimic the natural environment of skin to sunlight exposure, we therefore measured serum 25-hydroxyvitamin D levels in volunteers with different skin types following repeated UV irradiation. Because melanin pigment in human skin competes for and absorbs the UVB photons responsible for the photolysis of 7-dehydrocholesterol to previtamin D3, we also studied the effect of skin pigmentation on previtamin D3 production in a human skin model by exposing type II and type V skin samples to noon sunlight in June when the solar zenith angle is most acute. Vitamin D is rare in food. Among the vitamin D-rich food, oily fish are considered to be one of the best sources. Therefore, we analyzed the vitamin D content in several commonly consumed oily and non-oily fish. The data showed that farmed salmon had a mean content of vitamin D that was ∼25% of the mean content found in wild caught salmon from Alaska, and that vitamin D2 was found in farmed salmon, but not in wild caught salmon. The results provide useful global guidelines for obtaining sufficient vitamin D3 by cutaneous synthesis and from dietary intake to prevent vitamin D deficiency and its health consequences, ensuing illness, especially, bone fractures in the elderly.

Section snippets

Human study

Healthy adults with different skin types (II, III, IV and V) were recruited in the beginning of winter to participate in our study. They were asked to lie in a tanning bed (source of UV irradiation) to expose whole body to a light similar to sunlight. Based on the manufacturer recommendation each volunteer received a total of 0.75 of MED (minimal erythema dose) in each session. To achieve that goal, the skin type II, III, IV and V received an average of 6, 8, 11, and 12 min of UV irradiation

Results

To investigate the effect of skin pigmentation on previtamin D3 synthesis in humans, we exposed type II and type V skin along with ampoules containing 7-DHC solution to noon sunlight in June on a cloudless day in Boston, Massachusetts. We found that in June, 0.67 ± 0.11% of 7-DHC in epidermis was converted to previtamin D3 in type II skin, but no detectable amount was found in type V skin samples after 5 min of sunlight exposure (Fig. 1). A small amount (0.18 ± 0.06%) of epidermal 7-DHC was

Discussion

Vitamin D3 is unique among numerous vitamins and hormones in that it is derived from a precursor, previtamin D3, which is synthesized in the skin in response to solar irradiation [14]. The photoproduction of previtamin D3 is dependent on the concentration of 7-DHC in epidermis and melanin pigmentation [23], [24]. There is an inverse relation between the concentrations of provitamin D3 in the epidermis with age [23]. The percentage conversion of cutaneous 7-DHC is also influenced by the solar

Acknowledgments

This work was supported in part by Grants (RO1AR-36963-05, R37AG6079-07, and MO1RR00053) from the National Institutes of Health, and a grant from the UV Foundation.

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    Present address: Nephrology Division, Department of Medicine, Harvard Medical School, Boston MA 02215, USA.

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