OUTLINE: This is a multicenter study. Participants are stratified according to baseline serum 25(OH)D level (< 20 ng/mL vs 20-32 ng/mL or < 50 nmol/L vs 50-80 nmol/L), baseline mammographic density (11-50% vs > 50%), and designated biopsy site (yes vs no). Participants are randomized to 1 of 2 treatment arms.