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

Aim: 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.

Methods: 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.

Results: In February 2004, 88% of 25(OH)D levels were below 75 nmol/L, increasing to 100% in June and July. Severe deficiency (<12.5 nmol/L) was found in 1.5% of subjects. From February to July, 25(OH)D and CaI fell and CaT rose (p<0.001,<0.01, and <0.001). There was a hyperbolic relationship between PTH and 25OHD while CaT and CaI correlated negatively with PTH (r=-0.30 and -0.33; both p<0.001). Monthly mean dUV intensity ranged from 10 kJ m-2 day-1 in Dec 2003 to 0.5 kJ m-2 day-1 in June 2004. Compartmental modelling estimated that a Christchurch person made 1200 IU/day of vitamin D in mid-summer but only 60 IU/day in midwinter. Daily supplements of 1450 or 2600 IU vitamin D3 are predicted to raise the annual minimum mean plasma 25(OH)D to 75 or 100 nmol/L respectively.

Conclusions: Most Christchurch people are vitamin D deficient most of the time and a daily supplement of 2600 IU vitamin D3 would correct this.