Vitamin D in Postmenopausal Women at High Risk for Breast Cancer

Participants will receive oral vitamin D3, cholecalciferol 20,000 IU or 30,000 IU weekly. Before beginning the intervention, the participants will have a general physical exam including a clinical breast exam and anthropometric measures, a morning blood draw (for analysis of 25(OH)D, 1,25(OH)D, parathyroid hormone (PTH), insulin-like growth factor (IGF)-I, insulin-like growth factor-binding protein 3 (IGFBP-3), estradiol, estrone, testosterone, and sex hormone-binding globulin (SHBG) at baseline, 6 months, and 12 months) and a bilateral mammogram (at baseline and 12 months). Participants will also complete a baseline questionnaire, collecting data on demographic information and breast cancer risk factors. In addition, we will be collecting data on vitamin D sources from diet and sunlight exposure using a validated questionnaire administered at baseline and 12 months. For follow-up visits, the participant will be seen at the study site at months 3, 6, 9, and 12. During these visits, drug will be dispensed, adherence ascertained, and adverse events will also be assessed. In addition, blood (serum calcium, albumin, creatinine) and urine (urine calcium, creatinine) will be collected to monitor for toxicity. A 24-hour urine collection will be conducted at baseline and 12 months to assess for hypercalciuria. After the 1 year intervention, all participants will have a complete physical exam including clinical breast exam, a bilateral mammogram, blood draw, and complete a follow-up questionnaire.