Calcium has been shown to have positive effects on bone mineral density in postmenopausal women. However, these effects are small, it is unknown whether they are sustained with long-term use, they have not been shown with intention-to-treat analyses, and the evidence for fracture prevention with calcium monotherapy is inconsistent.

A randomized controlled trial of calcium (1 g/day as the citrate) in 1471 healthy postmenopausal women (aged 74±4 years) was performed to assess the effects on bone density and fracture incidence over 5 years.

Follow-up was complete in 90% of subjects, and average medication compliance was 55% to 58%. Calcium had a significant beneficial effect on bone density (intention-to-treat analysis), with between-groups differences at 5 years of 1.8% (spine), 1.6% (total hip), and 1.2% (total body). Effects were greater in a per-protocol analysis (5-year differences of 2.3%, 2.8%, and 1.8%, respectively). A total of 425 fractures occurred in 281 women. Hazard ratios, based on time to first fracture, were 0.90 (95% confidence interval [CI], 0.71-1.16) for any symptomatic fracture, 0.72 (95% CI, 0.44-1.18) for vertebral, 3.55 (95% CI, 1.31-9.63) for hip, and 0.65 (95% CI, 0.41-1.04) for forearm fracture. Per-protocol analysis found respective hazard ratios of 0.86 (95% CI, 0.64-1.17), 0.62 (95% CI, 0.33-1.16), 3.24 (95% CI, 0.65-16.1), and 0.45 (95% CI, 0.24-0.87). Height loss was reduced by calcium in the per-protocol population (P=.03). Serum alkaline phosphatase and procollagen type-I N-terminal propeptide were lower in the calcium group at 5 years, but constipation was more common.

Calcium results in a sustained reduction in bone loss and turnover, but its effect on fracture remains uncertain. Poor long-term compliance limits its effectiveness.