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

Summary 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