Vitamin D insufficiency in New Zealanders during the winter is associated with higher parathyroid hormone concentrations: implications for bone health?

BACKGROUND: Parathyroid hormone concentration (PTH) is elevated in vitamin D insufficiency and when prolonged, this condition leads to reduced bone mass and possibly osteoporosis. The threshold of 25-hydroxyvitamin D above which PTH plateaus, is a criterion often used to define vitamin D adequacy. AIMS: To determine whether the higher rates of vitamin D inadequacy reported in the winter than summer months in New Zealand also result in higher PTH concentrations. Also to explore the relationship between 25-hydroxyvitamin D and PTH concentrations in a New Zealand population to determine if a threshold exists for plasma 25-hydroxyvitamin D concentration.
METHODS: Plasma 25-hydroxyvitamin D and PTH concentrations were determined in 342 volunteers living in Invercargill and Dunedin (latitude 45-46 degrees S) in late summer (February) and early spring (October).
RESULTS: Mean plasma 25-hydroxyvitamin D concentration was higher in the late summer versus early spring (79 vs 51 nmol/L; p<0.001). The lower plasma 25-hydroxyvitamin D in early spring versus summer was associated with a 0.2 pmol/L (p<0.001) higher PTH concentration. A threshold of 61 nmol/L was estimated for plasma 25-hydroxyvitamin D, above which there was no further decrease in PTH concentration. DISCUSSION: The higher PTH concentration in winter than summer suggests that the low 25-hydroxyvitamin D concentration in the winter months may be having an adverse effect on bone health. Many New Zealanders have 25-hydroxyvitamin D concentrations less than 62 nmol/L, especially in winter. Strategies to improve the vitamin D status of the population such as supplementation and food fortification may be needed.