Optimal vitamin D status and its relationship with bone and mineral metabolism in Hong Kong Chinese.

Abstract

Although 25-hydroxyvitamin D (25[OH]D) is commonly used to define vitamin D status, there is no consensus on the cutoff levels for vitamin D deficiency and insufficiency. In this study, we aimed to identify the 25(OH)D threshold that maximally suppressed parathyroid hormone (PTH) in Hong Kong Chinese population. The study included 5276 participants (70% female) of the Hong Kong Osteoporosis Study aged 20 or above who had total 25(OH)D measured. Three-phase segmented regression was used to identify the optimal break-point between 25(OH)D and PTH. The prevalence of vitamin D deficiency observed was 43.8% and the prevalence of insufficient (<75nmol/L) or deficient (<50nmol/L) vitamin D levels was 90.1% in our study population. Using unadjusted three-phase segmented regression, the estimated first and second break-point of 25(OH)D on PTH suppression were 32nmol/L (95% CI: 29-35) and 89nmol/L (95% CI: 77-101) with an r² of 0.048, whereas the estimated first and second break-point of 25(OH)D were 27nmol/L (95% CI: 24-30) and 47nmol/L (95% CI: 37-56) after adjusting for factors affecting bone and mineral metabolism. In addition, the relationship between 25(OH)D and PTH significantly differed by sex and age. The threshold for 25OHD at the point of maximal suppression of PTH estimated in this study was lower than the suggested threshold of vitamin D deficiency in the literature, perhaps due to race or assay differences, and the relationship between vitamin D and PTH changed with sex and age. Standardization in the methodology of searching for the optimal break-point is desirable so that a consensus on cutoff points can be obtained.