The vitamin D synthetic capacity of sunlight: In situ monitoring and model calculation

In appropriate dose solar UV radiation is beneficial for people, specifically due to production of vitamin D3 in skin from its precursor 7-dehydrocholesterol. Vitamin D3, traditionally perceived as the main regulator of calcium homeostasis, is now acknowledged as one of the body's many control systems.
   But the UV overdose may result in detrimental health effects (sunburn, accelerated ageing of the skin, skin cancer), and because of that, when calculating biologically effective solar UV irradiance, in most cases the CIE erythema action spectrum is widely used as biologic weighting function for estimation of biological activity of solar UV radiation.
   However, in view of significant difference between the action spectra of CIE erythema and vitamin D synthesis the beneficial vitamin D synthetic capacity of sunlight cannot be correctly estimated from these data. At the same time to weigh the risks and benefits of sun exposure it is necessary to determine moderate exposures that provide adequate vitamin D nutrition for people but prevent skin cancer.
   With due regard to the essential role of vitamin D3 for human health we examined the possibility of use an in vitro model of vitamin D synthesis for simplified estimation in situ of provitamin D3 photoconversion into previtamin D3 from the UV absorption spectra similar to a number of chemical UV dosimeters as, for instance, polysulphone film, that measured an accepted UV dose by the absorbance decrease at the fixed wavelength. The large-scale linear correlation (R=0.99) was found on a clear summer day in Nea Michaniona (40.47N, 22.85E) between concentration of accumulated previtamin D3 and maximum absorbance decline in the initial provitamin D3 absorption spectrum at 282 nm. However, long-term observations in Kiev (50.38 N, 30.53 E) carried out over three years during April-September showed worse (R = 0.77) correlation, and a source of ambiguity of such indirect estimation of previtamin D3 concentration is discussed in detail.
   In our opinion, the difference in the latitude of Kiev (50.38 N) and Nea Michaniona (40.47 N) together with variable ozone and weather conditions has essential effect on the short wavelength edge of solar UV spectrum that is closely linked to the rate of irreversible photodegradation of previtamin D3 causing rather large scatter in the Kiev data compared with those ones from Greece.
   In addition, taking into account the widespread of natural synthesis of vitamin D in biosphere under solar UV irradiation that induces synthesis of vitamin D from its precursor, we have introduced new algorithm for direct calculation of the vitamin D effective irradiance. Based on the First Law of Photochemistry: “Light must be absorbed for photochemistry to occur” and keeping in mind that photobiological effects are initiated by photochemistry, a straightforward procedure for calculation the vitamin D synthetic capacity of sunlight has been developed using solar UV spectra as input data to the reaction model of previtamin D photosynthesis. Performed calculations demostrate critical dependence of previtamin D3 accumulation on stratospheric ozone, season, latitude, and cloudiness.
   There are good grounds to believe that direct calculation of the vitamin D synthetic capacity of sunlight using solar spectra together with the photoreaction model is favoured over commonly used calculations based on the in vivo vitamin D action spectrum, especially in view of the fact that the vitamin D3 action spectrum is based only on the work of a single laboratory unlike the erythemic response which was developed and validated in ~20 laboratories.
   Comparison of experimental and simulation data conforms to recent findings on Europe’s darker atmosphere in the UV-B and implicates practical certainty of presented algorithm for global mapping of biologically active (antirachitic) solar UV radiation. In our opinion, this algorithm is useful for direct estimation of the vitamin D synthetic capacity of sunlight and provides a means for introduction of new UV ‘D-index’ on daily UV forecasts in addition to commonly used erythemal UV index.