Introduction: A high peak bone mass may be essential for reducing the risk of osteoporosis later in life and a sufficient vitamin D level during puberty may be necessary for optimal bone accretion and obtaining a high peak bone mass. Dietary intake and synthesis during winter of vitamin D might be limited but the effect of vitamin D supplementation in adolescence on bone mass is not well established.Objective: To investigate the effect of supplementation with 5 and 10 mu g/day vitamin D-3 for 12 months in 11- to 12-year-old girls on bone mass and bone turnover as well as the possible influence of VDR and ER genotype on the effect of the supplementation.Methods: The girls (n = 221) were randomized to receive either 5 mu g or 10 mu g vitamin D-3 supplementation per day or placebo for 12 months. Whole body and lumbar spine bone mass measured by DXA and pubertal status were determined at baseline and after 12 months whereas physical activity and dietary intake of calcium and vitamin D were assessed at baseline. Serum (S) 25-hydroxyvitamin D (25OHD), S-osteocalcin, S-parathyroid hormone, S-calcium, S-inorganic phosphate, urinary (U) pyridinoline (Pyr) and deoxpyridinoline (Dpyr) were measured at baseline and after 6 and 12 months.Results: The S-25OHD concentration increased (p < 0.001) relative to the baseline values in the groups receiving either 5 mu g/day (mean +/- SD; 11.0 +/- 10.3 nmol/l, baseline 41.9 +/- 17.6 nmol/l) or 10 mu g/day (13.3 +/- 11.8 nmol/l, baseline 44.4 +/- 16.6 nmol/l) vitamin D-3 for 12 months compared to placebo (-3.1 +/- 9.8 nmol/l, baseline 43.4 +/- 17.1 nmol/l). There was no effect of vitamin D-supplementation on biomarkers for bone turnover or on whole body or spine bone mineral augmentation. However, vitamin D supplementation increased whole body bone mineral density (BMD) (p = 0.007) and bone mineral content (BMC) (p = 0.048) in the FF VDR genotype but not in the Ff or ff VDR genotypes.Conclusion: Supplementation with vitamin D (5 or 10 mu g/day) over 12 months increased the S-25OHD concentration but there was no effect on indices of bone health in the entire group of girls. However, there was an effect on BMD for a subgroup with the FF VDR genotype indicating an influence of genotype. (C) 2009 Elsevier Inc. All rights reserved.