It is well established that vitamin D is vital to calcium homeostasis by enhancing calcium and phosphorus absorption from the intestine and by mobilization of calcium from the bone. Low vitamin D status has been considered to be significantly related to musculoskeletal disorders, including bone and muscle health.1 An association between 25-hydroxyvitamin D (25(OH)D) serum concentration and levels of bone turnover markers have been reported in children.2 Thus, serum 25(OH)D levels may potentially have a significant role in osteoporosis and the development of various fractures in an aging population.

There is a general consensus that serum 25(OH)D concentration is the best indicator of vitamin D nutritional status.3 Although there is still no consensus regarding the definition of vitamin D deficiency and sufficiency, serum 25(OH)D levels of 50 nmol/L (20 ng/mL) and 75 nmol/L (30 ng/mL) have been proposed as the threshold for deficiency and sufficiency respectively.4,5 As such, serum 25(OH)D levels below 20 ng/mL have been shown to be associated with lower bone mineral density (BMD).6, 7, 8, 9, 10 In contrast, systematic reviews of randomized clinical trials did not show a significant effect on BMD11 or reduced fracture risk12 after vitamin D supplementation. The recent Vitamin D and Omega-3 Trial (VITAL) suggested that vitamin D3 supplementation did not result in a significantly lower risk of fractures than placebo among generally healthy midlife and older adults.13 However, these results may be driven by findings from institutionalized individuals as shown in a systematic umbrella review.14 Thus, the actual effects of vitamin D on osteoporosis need to be defined.

In an effort to characterize the influence of vitamin D levels on osteoporosis/fractures in an Asian population, we prospectively examined serum 25(OH)D concentrations in patients recruited from four medical centers. In this cross-sectional study, the association of serum 25(OH)D status with BMD and fracture risk in those with osteoporosis/fracture was investigated. We then evaluated the role of 25(OH)D level in the association of bone-related markers with BMD in osteoporosis/fracture patients.