A modelling approach to determine how much UVB radiation is available across the UK for the cutaneous production of vitamin D

The UK's current nutritional guidelines assume that from school to retirement, vitamin D requirements are met by skin exposure to UVB in sunlight. However, it is well documented that at latitudes such as the UK, the sunlight in the winter can be insufficient to synthesize appreciable vitamin D levels. We are funded by the UK Department of Health to provide data to inform new national guidance on vitamin D acquisition. In this study we use a well-established radiative transfer model to map the available UVB across the UK during the last 10 years (2003-2012). A suite of data (aerosol optical properties, surface reflectivity, cloud optical depth and coverage, total ozone column and digital elevation) derived from satellite estimates are used as model inputs to calculate the spectral UV irradiance and the vitamin D dose for different time periods. The model-derived vitamin D doses are validated against spectral irradiance measurements at Reading (51.44N, 0.94E) and Manchester (53.47N, 2.23E). Results from more than 2500 days at each site reveal that the modeled dose is overestimated by 5% while the overall agreement is satisfactory (R 2 >0.9) since the ground-based measurements are not always representative of a typical satellite measurement pixel. On a monthly basis, the overall agreement is significantly improved (bias <2%, R 2 =0.99). The model results will be combined with controlled exposure studies that determined how much vitamin D is synthesized per dose of UVB in adults with different skin types. This enables an estimate of the vitamin D effective exposure available across the UK for different skin types under realistic climatological conditions. Further studies provide for a range of actual exposure patterns within the theoretical availability.