Effect of monthly high-dose vitamin D on bone density in community-dwelling older adults substudy of a randomized controlled trial

Background: Severe vitamin D deficiency causes osteomalacia, yet trials of vitamin D supplementation in the community have not on average demonstrated benefit to bone mineral density (BMD) or fracture risk in adults.

Objective: To determine whether monthly high-dose vitamin D supplementation influences BMD in the general population and in those with low 25-hydroxyvitamin D levels.

Methods: Two-year substudy of a trial in older community-resident adults. A total of 452 participants were randomized to receive monthly doses of vitamin D3 100 000 IU, or placebo. The primary end-point was change in lumbar spine BMD. Exploratory analyses to identify thresholds of baseline 25-hydroxyvitamin D for vitamin D effects on BMD were prespecified.

Results: Intention-to-treat analyses showed no significant treatment effect in the lumbar spine (between-groups difference 0.0071 g cm^-2 , 95%CI: -0.0012, 0.0154) or total body but BMD loss at both hip sites was significantly attenuated by ~1/2% over 2 years. There was a significant interaction between baseline 25-hydroxyvitamin D and treatment effect (P = 0.04). With baseline 25-hydroxyvitamin D ≤ 30 nmol L^-1 (n = 46), there were between-groups BMD changes at the spine and femoral sites of ~2%, significant in the spine and femoral neck, but there was no effect on total body BMD. When baseline 25-hydroxyvitamin D was >30 nmol L^-1 , differences were ~1/2% and significant only at the total hip.

Conclusions: This substudy finds no clinically important benefit to BMD from untargeted vitamin D supplementation of older, community-dwelling adults. Exploratory analyses suggest meaningful benefit in those with baseline 25-hydroxyvitamin D ≤ 30 nmol L^-1 . This represents a significant step towards a trial-based definition of vitamin D deficiency for bone health in older adults.