Vitamin D dosing downer: implications for bone density

Vitamin D has been a major focus in the context of musculoskeletal conditions, especially osteoporosis and bone fractures, due to its role in bone formation. Vitamin D is required for the optimal formation of bone tissue and functions largely because it modulates key bone cells called osteoblasts and osteoclasts^[1]. Vitamin D also works indirectly to improve bone mineral density (BMD) by increasing intestinal calcium absorption^[2], and other ways laid out in Figure 1. As such, supplementing with vitamin D, specifically D₃, the most biologically active form, has been investigated as an intervention to improve BMD.

Previous trials have yielded disparate results, and a recent meta-analysis^[3] suggested that vitamin D supplementation does not have any meaningful effect on bone mineral density or the risk of fractures among adults. One of the major shortcomings of earlier trials and those included in the abovementioned meta-analysis is their reliance on dual x-ray absorptiometry (DXA) scans to measure BMD, which does not address all the important components of bone strength: bone mass, morphology, and architecture. Recently, more sensitive measurements have been developed that can provide a more accurate representation of bone strength: peripheral quantitative computed tomography (HR-pQCT). This method can measure aspects of bone structure related to bone strength beyond density. Also, measurements derived from this method have been found to predict^[4] fracture risk independent of BMD.

The study under review explored the dose-response effects of vitamin D supplementation among healthy community-dwelling adults over 36 months using HR-pQCT to assess BMD and bone strength. This study used relatively high doses at 4000 and 10000 IU per day.

Vitamin D plays a key role in bone formation and bone strength. Previous studies have found that vitamin D supplementation does not improve bone strength among otherwise healthy adults. However, more sensitive and accurate measures have been developed that may provide a more accurate view on exactly how vitamin D supplementation might affect bones in otherwise healthy adults.