Melanin has a Small Inhibitory Effect on Cutaneous Vitamin D Synthesis: A Comparison of Extreme Phenotypes

Abstract

Epidemiology suggests that melanin inhibits cutaneous vitamin D 3 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 D 3 photosynthesis in healthy young volunteers (n ¼ 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 3e4 days between UVR exposures. Blood was taken before, during, and after the irradiation and assessed for serum 25-hydroxyvitamin D 3 (25[OH]D 3) as a marker of vitamin D 3 status. Linear UVR dose-dependent increases in 25(OH)D 3 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 1.3e1.4, depending on the UVR source. We conclude that the inhibitory effect of melanin on vitamin D 3 synthesis is small, compared with erythema, but that this difference may be sufficient to explain the epidemiological data.

Insufficient Sun Exposure Has Become a Real Public Health Problem

“…A study based on a UK climatology model recommends 9–13 min (depending on latitude) lunchtime exposure between March and September in season-appropriate clothing [ 107 ]. Exposure times should be increased with darker FST but recent studies suggest that the inhibitory effect of melanin on vitamin D synthesis is relatively small compared with erythema [ 108 ]. A maximum of 30 min unprotected exposure is advised.…”

Skin colour and vitamin D: An update

“…[28] Thus, a dark pigmentation of the skin, which is mainly due to eumelanin-laden melanosomes in keratinocytes of lower layers, seems not to inhibit significantly vitamin D 3 synthesis taking place in the upper layers. [6,30,31] Accordingly, dark skinned hunter-gatherer populations of Africa that live traditionally and expose most of their skin to intensive sun, such as the Tanzanian Hadza, have a rather high vitamin D status with average 25(OH)D 3 serum levels in the order of 110 nmol/L. [32] Like other dietary fats, vitamin D 3 obtained from diet or supplements is assembled in intestinal enterocytes into chylomicrons and passed through the lymphatic system to the bloodstream, where the vitamin is unloaded to the serum glycoprotein GC (GC vitamin D binding protein).…”

Safe, mild ultraviolet‐B exposure: An essential human requirement for vitamin D and other vital bodily parameter adequacy: A review

“…This shows the minor differences in vitamin D production in the volunteer subjects tested, of skin types I to VI, irradiated all over with solar simulated radiation, apart from under their underwear, indicating skin pigmentation does not inhibit production to a significant degree 23 , figure by kind courtesy of Professor Antony Young, but also shown by Bogh et al 9 , though pigment gene differences can also play a part (Datta et al 25 )…”

“…Further, and perhaps unexpectedly, even marked skin pigmentation generally appears as shown by two separate research groups to have at most a minor effect on ultraviolet radiation‐induced vitamin D production 9,23 . This would appear likely to be because the cholesterol‐derived vitamin D precursor is present in high enough concentrations superficially to permit vitamin D production, especially as the melanin is concentrated near the epidermal basal layer (Figure 6).…”

“…22 Further, and perhaps unexpectedly, even marked skin pigmentation generally appears as shown by two separate research groups to have at most a minor effect on ultraviolet radiation-induced vitamin D production. 9,23 This would appear likely to be because the cholesterol-derived vitamin D precursor is present in high enough concentrations superficially to permit vitamin D production, especially as the melanin is concentrated near the epidermal basal layer (Figure 6). However, Kift et al 24 avoids constant allergic contact dermatitis reactions to the known huge number of environmental allergens, both of these reactions occurring through the ultraviolet-induced suppression of adaptive immunity.…”