Interdependence between body surface area and ultraviolet B dose in vitamin D production: a randomized controlled trial

Background Ultraviolet (UV) B radiation increases serum vitamin D level expressed as 25‐hydroxyvitamin‐D₃ [25(OH)D], but the relationship to body surface area and UVB dose needs investigation. Objective To investigate the importance of body surface area and UVB dose on vitamin D production after UVB exposure. Methods We randomized 92 participants to have 6%, 12% or 24% of their skin exposed to 0·75 (7·5 mJ cm^?2 at 298 nm using the CIE erythema action spectrum), 1·5 (15 mJ cm^?2) or 3·0 (30 mJ cm^?2) standard erythema doses (SED) of UVB. Each participant underwent four UVB exposures at intervals of 2–3 days. Skin pigmentation and 25(OH)D levels were measured before and 48 h after the final exposure. Results The increase in 25(OH)D after irradiation [Δ25(OH)D] was positively correlated with body surface area (P = 0·006; R² = 0·08) and UVB dose (P < 0·0001; R² = 0·28), and negatively correlated with baseline 25(OH)D (P < 0·0001; R² = 0·18), for the entire data sample. However, when analysing each body surface area separately, we found a significant UVB response correlation for 6% (P < 0·0001; R² = 0·48) and 12% (P = 0·0004; R² = 0·35), but not for 24%. We also found a significant skin area response correlation for 0·75 SED (P < 0·0001; R² = 0·56), but not for 1·5 and 3·0 SED when analysing each UVB dose separately. The relationships did not change significantly after adjustment of Δ25(OH)D for baseline 25(OH)D. Conclusion The increase in 25(OH)D depends mainly on the UVB dose; however, for small UVB doses the area of irradiated body surface is important.