Plasma 25-hydroxyvitamin D (25[OH]D), total calcium (CaT), ionised calcium (CaI) and parathyroid hormone (PTH) concentrations in healthy volunteers (11 9 female, 82 male; median age 45 years), measured between February and July 2004, were modelled in a relationship with vitamin D-weighted ultraviolet (UV) energy measurements (dUV) for Christchurch, obtained from the National Institute of Water and Atmospheric Research UV Atlas. 25(OH)D levels were <75 nmol/L in 88% of subjects in February 2004 and in 100% of subjects in June and July. Levels were severely deficient (<12.5 nmol/L) in 1.5% of subjects. Between February and July, there were significant decreases in 25(OH)D and CaI concentrations and a significant increase in the CaT concentration. There was a hyperbolic relationship between PTH and 25(OH)D 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 December 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. The authors suggest that a daily supplement of 2600 IU vitamin D3 would correct vitamin D deficiency.
