Discrepant influence of vitamin D status on parathyroid hormone and bone mass after two years of calcium supplementation.

ABSTRACT

OBJECTIVE: To investigate the influence of vitamin D status on parathyroid hormone and bone mass after a 2-year supplementation of calcium alone. PATIENTS AND METHODS: Randomized, double-blind, placebo-controlled clinical trial, in healthy postmenopausal women without osteoporosis three hundred and thirty-six subjects aged 60-97 years were studied and randomized to receive elemental calcium 500 mg/day (n = 175) or placebo (n = 161) for 2 years. MEASUREMENTS Changes in parathyroid hormone (PTH) and bone mineral density (BMD) from baseline and vitamin D status. Values are presented as means +/- SD. RESULTS: After 2 years, subjects with calcium supplementation had significant decrease in plasma PTH level (4.4 +/- 1.7 vs 4.7 +/- 1.9 pmol/l, P < 0.01), improved lumbar BMD (1.031 +/- 0.12 vs 1.004 +/- 0.12 g/cm(2), P < 0.001) and total hip BMD (0.890 +/- 0.10 vs 0.883 +/- 0.10 g/cm(2), P < 0.001) without change in femoral neck BMD. In the placebo group, PTH level significantly increased (4.8 +/- 1.6 vs 4.5 +/- 1.5 pmol/l, P < 0.001), lumbar BMD slightly increased (1.027 +/- 0.14 vs 1.018 +/- 0.14 g/cm(2), P < 0.001), total hip and femoral neck BMD decreased (0.876 +/- 0.11 vs 0.887 +/- 0.11 g/cm(2), P < 0.001 and 0.783 +/- 0.10 vs 0.798 +/- 0.10 g/cm(2), P < 0.001, respectively). When subjects were classified according to baseline 25-hydroxyvitamin D [25(OH)D] levels into those with 25(OH)D in the lower tertile (lowVitD) and those in the middle and upper tertiles combined (normVitD). The degree of PTH suppression after calcium supplementation was significantly higher in the normVitD compared to the lowVitD groups (-5.6 +/- 26.7%vs 1.3 +/- 27.2%, P < 0.05). No effect of vitamin D status on the change in lumbar BMD after calcium supplementation was demonstrated. Despite the higher suppression of PTH, there was a slight decrease in femoral neck BMD after calcium supplementation in the normVitD group while femoral neck BMD was more or less maintained in the lowVitD group (-0.6 +/- 3.2%vs 0.5 +/- 2.9%, P < 0.05). CONCLUSION: Calcium supplementation appears to affect femoral bone mass less in Thai postmenopausal women with adequate vitamin D status, despite higher suppression of PTH.