The relation between sunscreen layer thickness and vitamin D production after ultraviolet B exposure: a randomized clinical trial

Background Sunscreens absorb ultraviolet B (UVB) and it is a major concern that sunscreen use may lead to vitamin D deficiency. Objectives To investigate the relation between the amount of sunscreen applied and the vitamin D serum level in humans after UVB exposure under controlled conditions. Methods Thirty-seven healthy volunteers with fair skin types were randomized to receive an inorganic sunscreen with sun protection factor (SPF) 8 of 0 mg cm(-2), 0.5 mg cm(-2), 1 mg cm(-2), 1.5 mg cm(-2), or 2 mg cm(-2) thickness on the upper body, approximately 25% of the body area. Participants were irradiated with a fixed UVB dose of 3 standard erythema doses 20 min after sunscreen application. This procedure was repeated four times with a 2- to 3-day interval. Blood samples were drawn before the first irradiation and 3 days after the last to determine the serum vitamin D level expressed as 25-hydroxyvitamin D3 [25(OH)D]. Results The vitamin D serum level increased in an exponential manner with decreasing thickness of sunscreen layer in response to UVB exposure. For all thicknesses of sunscreen, the level of 25(OH) D increased significantly after irradiation (P < 0.05), except for the group treated with 2 mg cm(-2), in which the increase in 25(OH)D was not statistically significant (P = 0.16). Conclusions Vitamin D production increases exponentially when thinner sunscreen layers than recommended are applied (< 2 mg cm(-2)). When the amount of sunscreen and SPF advised by the World Health Organization are used, vitamin D production may be abolished. Re-evaluation of sun-protection strategies could be warranted.