A revised action spectrum for vitamin D synthesis by suberythemal UV radiation exposure in humans in vivo

Action spectra are important biological weighting functions for risk/benefit analyses of ultraviolet (UV) radiation (UVR) exposure. One important human benefit of exposure to terrestrial solar UVB radiation (∼295 to 315 nm) is the cutaneous synthesis of vitamin D(3) that is initiated by the photoconversion of 7-dehydrocholesterol to previtamin D(3). An action spectrum for this process that is followed by other nonphotochemical steps to achieve biologically active vitamin D(3) has been established from ex vivo data and is widely used, although its validity has been questioned. We tested this action spectrum in vivo by full- or partial-body suberythemal irradiation of 75 healthy young volunteers with five different polychromatic UVR spectra on five serial occasions. Serum 25-hydroxyvitamin D(3) [25(OH)D(3)] levels, as the most accurate measure of vitamin D(3) status, were assessed before, during, and after the exposures. These were then used to generate linear dose–response curves that were different for each UVR spectrum. It was established that the previtamin D(3) action spectrum was not valid when related to the serum 25(OH)D(3) levels, as weighting the UVR doses with this action spectrum did not result in a common regression line unless it was adjusted by a blue shift, with 5 nm giving the best fit. Such a blue shift is in accord with the published in vitro action spectra for vitamin D(3) synthesis. Thus, calculations regarding the risk (typically erythema) versus the benefit of exposure to solar UVR based on the ex vivo previtamin D(3) action spectrum require revision.