The Dose-Response Effects of Vitamin D₃ on Serum 25-Hydroxyvitamin D Levels in Vitamin D-Deficient Young Indian...

Background/Objectives: Vitamin D deficiency is highly prevalent among young Indian women. This study evaluated the dose-response effects of varying levels of daily oral vitamin D₃ intake on serum 25-hydroxyvitamin D [25(OH)D] concentrations and related bone biomarkers in vitamin D-deficient and insufficient young women. Methods: In a 12-week, double-blind, randomized controlled trial conducted in urban Bangalore, 108 non-pregnant, non-lactating women aged 18-35 years with serum 25(OH)D <20 ng/mL were randomized (1:1:1:1) to receive daily vitamin D₃ at doses of 0, 400, 600, or 800 IU delivered via a fortified chocolate wafer. Serum 25(OH)D, parathyroid hormone (PTH), bone-specific alkaline phosphatase (BSAP), and osteocalcin were measured at baseline, and at weeks 4, 8, and 12. Intention-to-treat analysis was performed using mixed linear and logistic models to assess the effect of intervention. Results: At baseline, 76.4% of participants were vitamin D-deficient (<12 ng/mL) and 23.6% were insufficient (12-20 ng/mL). Serum 25(OH)D increased significantly over time in the 400, 600, and 800 IU/day groups, with a significant time × dose interaction effect (p < 0.001), demonstrating an overall graded response across doses. By week 12, vitamin D sufficiency (≥20 ng/mL) was achieved in 7.4%, 26.9%, 37.0%, and 65.4% of participants in the 0, 400, 600, and 800 IU groups, respectively. From baseline to week 12, the mean increase in serum 25(OH)D was 7.69 ng/mL in the 400 IU group, 8.83 ng/mL in the 600 IU group, and 10.23 ng/mL in the 800 IU group; no significant difference was observed between the 400 IU and 600 IU groups, whereas the 800 IU group demonstrated the greatest overall increase. PTH decreased over time without a significant time × dose interaction. No significant interaction effect was noted for BSAP and osteocalcin. Mean compliance was high (>98%) across all groups, and no serious adverse events were reported. Conclusions: Daily dietary intake of vitamin D₃ at doses of 400-800 IU for 12 weeks significantly improved serum 25(OH)D concentrations in vitamin D-deficient young Indian women. While PTH levels decreased over time, effects on bone turnover markers were modest and not dose specific. A daily dose of 800 IU/day was the most effective, achieving vitamin D sufficiency in 65% of participants.