Vitamin binding protein, 25-hydroxyvitamin D and bioavailable vitamin D status in the Hong Kong Chinese population

Abstract

INTRODUCTION: The association between serum 25-hydroxyvitamin D [25(OH)D] and chronic diseases like diabetes, hypertension, and cardiovascular diseases is controversial. Recently, racial differences in bone mineral density between white and black Americans were explained by the levels of bioavailable vitamin D, which suggested that it might be a better marker of vitamin D status than 25(OH)D. We aimed to investigate if bioavailable vitamin D was a better vitamin D marker than 25(OH)D in the Hong Kong Chinese population. METHODS: Serum 25(OH)D and vitamin D binding protein (DBP) levels were measured in 2674 participants (592 men, 2082 women; mean [± standard deviation] age, 49.2 ± 16.2 years) in the Hong Kong Osteoporosis Study. The levels of free and bioavailable vitamin D were calculated using the equation developed by Vermeulen et al. Their associations with serum calcium, phosphorus, and parathyroid hormone (PTH) concentrations were evaluated using ANCOVA with adjustment for age and sex. RESULTS: The mean levels of 25(OH)D and DBP were 50.3 nmol/L ± 13.7 nmol/L and 178.8 µg/mL ± 74.0 µg/mL, respectively, and calculated free and bioavailable vitamin D levels were 8.79 pg/mL ± 4.23 pg/mL and 3.50 ng/mL ± 1.67 ng/mL, respectively. Serum 25(OH)D level was significantly associated with serum calcium, phosphorus, and PTH levels (P < 0.05). In contrast, bioavailable vitamin D levels were only associated with serum calcium, while free vitamin D levels were only associated with serum phosphorus. CONCLUSION: Bioavailable vitamin D and free vitamin D correlate less well with mineral metabolism markers than 25(OH)D in the Hong Kong Chinese population. Whether genetic variations affecting 25(OH)D and DBP play a role in the association remains to be determined.