Epidemiology suggests that melanin inhibits cutaneous vitamin D3 synthesis by UVR. Laboratory investigations assessing the impact of melanin on vitamin D production have produced contradictory results. We determined the effect of melanin on vitamin D3 photosynthesis in healthy young volunteers (n = 102) of Fitzpatrick skin types II-VI (white to black). Participants, irrespective of skin type, were exposed to the same suberythemal UVR dose, to 85% body surface area, using solar simulated UVR or narrowband UVB (311 nm). This was repeated five times with intervals of 3-4 days between UVR exposures. Blood was taken before, during, and after the irradiation and assessed for serum 25-hydroxyvitamin D3 (25[OH]D3) as a marker of vitamin D3 status. Linear UVR dose-dependent increases in 25(OH)D3 were highly significant (P ≤ 7.7 x 10-11). The ratios of regression slopes of the different skin type groups were compared, and only skin type II was significantly steeper than the other groups. Comparisons between extreme skin types II and VI showed melanin inhibition factors of approximately 1.3-1.4, depending on the UVR source. We conclude that the inhibitory effect of melanin on vitamin D3 synthesis is small, compared with erythema, but that this difference may be sufficient to explain the epidemiological data.
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