Changes in the vitamin D endocrine system and bone turnover after oral vitamin D3 supplementation in healthy adults: results of a randomised trial

Healthy adults aged 19–48？years living in Oslo, Norway (59°N) were randomised to receive a daily dose of 10？μg vitamin D3 given as fish oil capsules or multivitamin tablets during four weeks in late winter. Serum samples from baseline and after 28？days were analysed for 25-hydroxyvitamin D (s-25(OH)D), 1,25-dihydroxyvitamin D (s-1,25(OH)2D), intact parathyroid hormone (s-iPTH), and osteoclast-specific tartrate-resistant acid phosphatase 5b (s-TRACP). Fifty-five eligible participants completed the intervention (74% of those randomised).S-25(OH)D increased by mean 34.1 (SD 13.1) nmol/l, p？<？0.001; s-iPTH decreased by mean 1.2 (SD 2.5) pmol/l, p？=？0.001; s-1,25(OH)2D increased by mean 13 (SD 48) pmol/l, p？=？0.057; and s-TRACP increased by mean 0.38 (SD 0.33) U/l, p？<？0.001. For all these parameters, there was no difference between fish oil and multivitamin formulation. Baseline concentrations were the only independent predictors of changes in biochemical parameters.Four weeks of daily supplementation with 10？μg vitamin D3 decreased mean s-iPTH and increased s-TRACP concentration, and this did not differ by mode of administration. Our results suggest an increased bone resorption following vitamin D supplementation in young individuals, despite a decrease in parathyroid hormone levels.ClinicalTrials.gov: NCT01482689