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Serum 25(OH)D concentration over time. Box plots of serum 25(OH)D concentration (ng/mL) before random assignment to vitamin D supplementation of 200, 1000, 2000, or 4000 IU/day and 3, 12, and 24 months later. To convert 25(OH)D ng/mL to nmol/L, multiply ng/mL by 2.5.
