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Objective: Bisphosphonates such as alendronate are widely used for postmenopausal osteoporosis. Supplemental calcium is also generally recommended. This trial directly compares alendronate to supplemental calcium and examines the effect of calcium supplementation on alendronate treatment.Methods: This 2-year, randomized, double-blind, multicenter trial enrolled healthy, postmenopausal women with low bone mineral density (BMD). Patients with a dietary calcium intake ≥ 800 mg/day received daily vitamin D 400 IU and alendronate 10 mg/calcium-placebo, alendronate 10 mg/elemental calcium 1000 mg, or alendronate-placebo/calcium 1000 mg (2:2:1). Endpoints included BMD, bone turnover markers (BTMs), and adverse events.Results: Randomized patients (N = 701) were an average of 20.4 years postmenopausal. After 24 months, increases in lumbar spine BMD differed significantly between patients receiving calcium alone (0.8%) and either alendronate alone (5.6%) or alendronate + calcium (6.0%) (p < 0.001). Significant differences were also seen at the trochanter and femoral neck (p < 0.001). BTMs were significantly lower with alendronate-containing treatments than calcium alone (p < 0.001). Addition of calcium supplementation to alendronate did not significantly increase BMD compared to alendronate alone (p = 0.29 to 0.97), but did result in a statistically significant, though small, additional reduction in urinary NTx. Adverse events were similar among treatment groups. Limitations include no assessment of vitamin D levels and a discontinuation rate of approximately 30%, although discontinuation rates were similar among treatment groups.Conclusions: In postmenopausal women with a daily intake of ≥800 mg calcium and 400 IU vitamin D, 24-month treatment with alendronate 10 mg daily with or without calcium 1000 mg resulted in significantly greater increases in BMD and reduction of bone turnover than supplemental calcium alone. Addition of supplemental calcium to alendronate treatment had no effect on BMD and resulted in a small, though statistically significant, additional reduction in NTx. [ABSTRACT FROM AUTHOR]