How to measure the Vitamin-D-synthetic activity of UV lamps used in phototherapy?

Abstract

Ultraviolet lamps are widely used in phototherapy, and the positive effect of ultraviolet radiation is mainly associated with the synthesis of vitamin D in human skin. Nevertheless, to avoid harmful effects the biological efficacy of UV lamps is still evaluated based on the erythema action spectrum using UV detectors with an output in erythema units. Evaluation of vitamin D synthesis on this basis is inadequate because of the difference between the erythema and the Vitamin D synthesis action spectra. Hence, direct measurement of the vitamin-D-synthetic activity is a missing link in the metrology of UV lamps that are used for medical and/or cosmetic purposes. This paper presents original methods based on the same photoreaction in vitro by which vitamin D is synthesized in human skin via photo-and thermo-induced conversions of 7-Dehydrocholesterol (provitamin D 3). The UV photons are absorbed by provitamin D molecules in solution or embedded in specially designed UV transparent and stable matrix mimicking biological samples. Three operation modes of varying complexity have been developed to follow the photoreaction course in real time, and the results are presented of measuring the vitamin D synthesizing activity of several UV lamps, as well as the first performed comparative studies on direct measurements of the vitamin D level in blood and in solution.