Safety, Feasibility, and Biomarker Effects of High-Dose Vitamin D Supplementation Among Women at High Risk for Breast Cancer.

Vitamin D deficiency is a potentially modifiable risk factor that may be targeted for breast cancer prevention. We examined the safety, feasibility, and biomarker effects of high-dose vitamin D among women at high risk for breast cancer. Forty high-risk women, defined as a 5-year breast cancer risk ≥1.67% per the Gail model, lobular or ductal carcinoma in situ, were assigned to a 1-year intervention of vitamin D3 20,000 IU or 30,000 IU weekly. Participants were monitored for toxicity every 3 months, underwent serial blood draws at baseline, 6 and 12 months, and a digital mammogram at baseline and 12 months. Biomarker endpoints included serum 25-hydroxyvitamin D [25(OH)D], 1,25-dihydroxyvitamin D [1,25(OH)₂D], parathyroid hormone (PTH), insulin-like growth factor (IGF-1), IGF binding protein (IGFBP-3), and mammographic density (MD) using Cumulus software. From November 2007 to January 2011, we enrolled 40 women; 37 were evaluable at 6 months and 30 at 12 months. One patient was taken off study for hypercalciuria; otherwise, the intervention was well tolerated. From baseline to 12 months, mean serum 25(OH)D and 1,25(OH)₂D rose from 20.0 to 46.9 ng/ml and 69.7 to 98.1 pg/ml, respectively (p<0.01). Serum PTH decreased by 12% at 6 months and IGF-1/IGFBP-3 ratio decreased by 4.3% at 12 months (p<0.05). There was no significant change in MD regardless of menopausal status or dose level. We demonstrated that 1 year of high-dose vitamin D3 was associated with a significant increase in circulating vitamin D levels and favorable effects on IGF signaling, but no significant change in MD.