Seasonal variation in vitamin D levels in the Canterbury, New Zealand population in relation to available UV radiation

The optimal plasma 25-hydroxyvitamin D (25(OH)D) concentration is probably >75 nmol/L but in temperate regions lower levels are common. Few studies report the intensity of solar ultraviolet (UV) radiation when 25(OH)D is measured. We measured plasma 25(OH)D and incident solar UVB radiation in Christchurch and modelled the relationship between them. 25(OH)D, total calcium (CaT), ionised calcium (CaI) and parathyroid hormone (PTH) were measured in healthy volunteers (119 female, 82 male; median age 45 years, range 18 to 83) between February and July 2004. Vitamin D-weighted UV energy measurements (dUV) for Christchurch were from the National Institute of Water and Atmospheric Research (NIWA) UV Atlas. In February 2004, 88% of 25(OH)D levels were below 75 nmol/L, increasing to 100% in June and July. Severe deficiency (