ELEVATION OF BLOOD VITAMIN-D(2) LEVELS DOES NOT IMPEDE THE RELEASE OF VITAMIN-D(3) FROM THE SKIN

The mechanism for the transfer of fat-soluble vitamin D3 from the avascular basal cellular layers of the epidermis to dermal capillaries and peripheral circulation is unknown, although vitamin D-binding protein (DBP) is thought to mediate this process. To evaluate the effect of increased occupancy of vitamin D carrier(s) on vitamin D3 removal from the skin, serial serum vitamin D2 and D3 concentrations were determined in three groups of six healthy volunteers given combinations of an oral dose of vitamin D2 (50,000 IU) and a fixed dose of UVB radiation (27 mJ/cm2). Serum vitamin D3 levels increased significantly following UVB (time effect, P < .01 by ANOVA), but the response remained unchanged after pretreatment with vitamin D2, increasing from 3 ± 1 to 14 ± 5 ng/mL (mean ± SEM), versus UVB alone, 5 ± 1 to 16 ± 5 ng/mL. Elevation of serum vitamin D2 levels was also similar in the groups given vitamin D2 alone (< 1 to 64 ± 8 ng/mL) and vitamin D2 + UVB (< 1 to 45 ± 8 ng/mL). There was no time or treatment effect for changes in serum levels of 25-hydroxyvitamin D, 1,25-dihydroxyvitamin D, or (DBP) levels (P > .1). We conclude that vitamin D3 egress from the skin is not affected by elevated circulating vitamin D concentrations; thus, the cutaneous release of vitamin D is probably mediated by a protein such as DBP with a high carrying capacity for the vitamin. © 1992.