A randomized controlled trial of vitamin D supplementation on preventing postmenopausal bone loss and modifying bone metabolism using identical twin pairs

Vitamin D supplementation, when given with calcium, has been shown to incre ase bone mineral density (BMD) and reduce the incidence of hip fracture in elderly subjects. Despite its widespread use, the benefits of vitamin D sup plementation in younger women and as a single agent are less clear. We perf ormed a randomized co-twin, placebo-controlled, double-blind trial over 2 y ears to measure the effect of vitamin D, supplementation on bone density an d bone metabolism in young postmenopausal women. Seventy-nine monozygotic ( MZ) twin pairs (mean age, 58.7 years; range, 47-70 years) were recruited. F or each twin pair, one was randomized to 800 IU cholecalciferol/day for 2 y ears and the other was randomized to placebo. BMD was measured at the spine and hip and heel ultrasound at baseline, 12, 18, and 24 months. Samples we re collected at 0, 3, and 6 months to measure serum calcium, 25-hydroxyvita min D [25(OH)D], parathyroid hormone (PTR), osteocalcin, and urinary deoxyp yridinoline (DPD). In total, 64 pairs completed the study. No differences i n baseline characteristics were seen between the groups. At 6 months, the t reatment group had an increase in serum vitamin D [mean +/- SEM intrapair d ifference, 14.1 +/- 2.4 mug/liter (p < 0.001)]. There were no significant d ifferences in other serum measurements or bone markers at 3 months or 6 mon ths. At 24 months, no significant treatment effect was seen on BMD or calca neal ultrasound change within pairs. Subanalysis of treatment response by v itamin D receptor (VDR) genotype revealed no significant difference in effe ct on BRID variables with treatment. On the basis of these results, vitamin D supplementation, on its own, cannot be recommended routinely as an osteo porosis prevention for healthy postmenopausal women with normal vitamin D l evels under the age of 70 years.