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The prevalence of rickets and osteomalacia due to vitamin D deficiency is probably low in New Zealand.1,2 Of greater public health concern is that lesser forms of vitamin D deficiency, often termed insufficiency, may increase the risk of chronic diseases such as osteoporosis, multiple sclerosis, Type 1 diabetes, and certain types of cancer.3,4 The circulating concentration of 25-hydroxyvitamin D is the best indicator of vitamin D status.
Serum 25-hydroxyvitamin D concentrations of New Zealanders are suboptimal.5-7 Nearly 50% of adult (≥15 y) participants in the 1997 NNS had vitamin D insufficiency, based on a serum 25-hydroxyvitamin D <50 nmol/L.6 Rates of insufficiency were higher in the winter than summer months and higher amongst Pacific People than Maori and Europeans, presumably due to their darker skin colour.
Despite evidence of a high rate of vitamin D inadequacy, we lack evidence that low 25-hydroxyvitamin D concentrations are negatively impacting New Zealanders' health. In the absence of this evidence a public health programme to improve vitamin D status is unlikely to be implemented.
Circulating parathyroid hormone (PTH) concentrations have been inversely associated with 25-hydroxyvitamin D concentrations up to a threshold, above which PTH concentrations plateau at a minimum level.8
PTH is elevated in vitamin D insufficiency and when prolonged, this condition leads to reduced bone mass and an increased risk of osteoporosis.3 If the high rates of vitamin D inadequacy reported in New Zealand are having an adverse effect on bone health, PTH concentrations should be higher in the winter than summer, given the marked seasonal variation in 25-hydroxyvitamin D concentrations.
The threshold of 25-hydroxyvitamin D above which PTH concentrations plateau is a criterion often used to define vitamin D adequacy. This threshold has varied considerably by population studied but has not been estimated in a New Zealand population.8
The purpose of this study was to determine whether the higher rates of vitamin D inadequacy reported in the winter than summer months in New Zealand also resulted in higher PTH concentrations. To do this we measured plasma 25-hydroxyvitamin D and PTH concentrations in a cohort of people living in the south of NZ (latitude 45-46°S) in late summer (February) and early spring (October). We also explored the relationship between 25-hydroxyvitamin D and parathyroid hormone concentrations in this population...