Vitamin D and Mammographic Breast Density

OBJECTIVES Primary: \- to determine whether adding oral supplementation with vitamin D3 (cholecalciferol) at doses of 1,000, 2,000 and 3,000 IU/d to baseline total vitamin D intake over a period of 12 months reduces mammographic breast density among premenopausal women at moderate to high risk of breast cancer who reside in the Quebec City area, Canada. Secondary: * to assess the net increase in 25(OH)D levels resulting from this vitamin D3 supplementation; * to document the compliance with, and safety of, supplementation with tested doses of vitamin D3 over one year. OUTLINE This study is a double-blind, placebo-controlled, randomized trial among premenopausal women who live in the Quebec City area and have ≥ 20% breast density at baseline. The intervention consists of the addition, over baseline total vitamin D intake from food and supplements, of three different doses of vitamin D3 (1000, 2000 or 3000 IU/day) for a period of 12 months. There are four arms in the trial: * Arm 1: 1,000 IU vitamin D3/d (1 tablet vitamin D 1000 IU + 2 tablets placebo); * Arm 2: 2,000 IU vitamin D3/d (2 tablets vitamin D 1000 IU + 1 tablet placebo); * Arm 3: 3,000 IU vitamin D3/d (3 tablets vitamin D 1000 IU); * Arm 4: placebo (3 tablets placebo/d). Vitamin D tablets (1000 IU tablets) and the placebo tablets are undistinguishable in size, shape, color and taste, and are manufactured by the same company.