Vitamin D, Diet and Activity Study

Objective: To test the incremental effect of vitamin D supplementation (2000 IU/day) in 228 overweight and obese postmenopausal women with low blood vitamin D levels on response to a weight loss (diet + exercise) intervention in a double-blind placebo-controlled clinical trial. Primary Aim: • Determine the effects of 12-months vitamin D supplementation vs. placebo on weight loss in women following a weight loss diet and exercise program. Secondary Aims: * Determine the effects of 12-months vitamin D vs. placebo on blood biomarkers associated with increased breast cancer risk (insulin, glucose, CRP). * Test 12-months vitamin D effects on muscle strength as measured by 1RM leg press and bench press in women undergoing weight loss. * Test the effect of 12-month vitamin D supplementation on effects on quality of life (QOL) in women undergoing weight loss. Additional Aims: * In a 50% subset of women, test the effect of 12-month vitamin D supplementation vs. placebo on breast epithelial cell cytomorphology (quantified by the Masood cytology index) obtained through random periareolar fine needle aspiration (RPFNA). * Collect subcutaneous abdominal fat aspirations in the 50% subsample of women. OUTLINE: This is a randomized study. Participants are stratified according to body mass index (\< 30 vs ≥ 30) and RPFNA-consent. All participants will receive a 6-month weight loss intervention (prescribed weight loss goal of 10% of initial body weight) followed by 6 months' maintenance therapy. Baseline, 6-, and 12-month measurements will include weight, BMI, waist and hip circumference, serum 25-hydroxyvitamin D, fasting blood samples, and questionnaires on details of diet especially items containing vitamin D, physical activity levels, sun exposure including clothing and sunscreen habits, dietary supplement and vitamin use, and medications.