biostudies-literatureCrew KDAmerican Society of Clinical OncologyAmerican Society of Clinical Oncology (ASCO)NCI NIH HHSHHS | NIH | National Cancer InstituteHHS | NIH | National Cancer Institute (NCI)481-490https://www.ebi.ac.uk/biostudies/studies/S-EPMC6609474biostudies-literatureUnknown12(7)Observational studies have reported an inverse association between vitamin D intake and breast cancer risk. We examined whether vitamin D supplementation in high-risk premenopausal women reduces mammographic density (MD), an established breast cancer risk factor. We conducted a multicenter randomized double-blind placebo-controlled trial in premenopausal women at high risk for breast cancer [5-year risk ≥ 1.67%, lifetime risk ≥ 20%, lobular carcinoma <i>in situ</i>, prior stage 0-II breast cancer, hereditary breast cancer syndrome, or high MD (heterogeneously/extremely dense)], with a baseline serum 25-hydroxyvitamin D [25(OH)D] ≤ 32 ng/mL. Participants were randomized to 12 months of vitamin D3 20,000 IU/week or matching placebo. The primary endpoint was change in MD from baseline to 12 mCancer prevention research (Philadelphia, Pa.)Randomized Double-Blind Placebo-Controlled Biomarker Modulation Study of Vitamin D Supplementation in Premenopausal Women at High Risk for Breast Cancer (SWOG S0812).PMC6609474N/AU10 CA180819UG1 CA233329UG1 CA190002UG1 CA189953P30 CA016672UG1 CA189974UG1 CA189830UG1 CA189960UG1CA189974U10 CA180828U10 CA180858UG1 CA189858Ford LBevers TKruper LArun BGoodman GEHershman DLMinasian LMTehranifar PKairouz SSDoster JEAnderson GLTerry MBYee MKuwajerwala NZarwan CCrew KDNeuhouser MBrown EALew DLBrown PHfalseRandomized Double-Blind Placebo-Controlled Biomarker Modulation Study of Vitamin D Supplementation in Premenopausal Women at High Risk for Breast Cancer (SWOG S0812).Observational studies have reported an inverse association between vitamin D intake and breast cancer risk. We examined whether vitamin D supplementation in high-risk premenopausal women reduces mammographic density (MD), an established breast cancer risk factor. We conducted a multicenter randomized double-blind placebo-controlled trial in premenopausal women at high risk for breast cancer [5-year risk ≥ 1.67%, lifetime risk ≥ 20%, lobular carcinoma <i>in situ</i>, prior stage 0-II breast cancer, hereditary breast cancer syndrome, or high MD (heterogeneously/extremely dense)], with a baseline serum 25-hydroxyvitamin D [25(OH)D] ≤ 32 ng/mL. Participants were randomized to 12 months of vitamin D3 20,000 IU/week or matching placebo. The primary endpoint was change in MD from baseline to 12 m2019-01-01T00:00:00Z2019 Jul2026-04-12T23:53:47.071Z2025-05-29T21:30:39.351ZS-EPMC66094743113852210.1158/1940-6207.capr-18-044410.1158/1940-6207.CAPR-18-0444