Effect of a Western-Style Diet Fortified with Increased Calcium and Vitamin D on Mammary Gland of C57Bl/6 Mice

We previously reported hyperproliferation and hyperplasia in C57Bl/6 mouse mammary gland after feeding a Western-style diet (WD); these findings decreased after supplementing WD with increased calcium and vitamin D₃. We now again fortified WD with increased calcium and vitamin D₃ from two sources: (1) a food source, calcium- and vitamin D₃-enriched yogurt (WD_y diet) or (2) adding calcium and vitamin D₃ directly to WD (WD_CaD diet). After 6 months of feeding the number of mammary ducts was higher in mice consuming WD compared to WD_y (216.0 vs. 202.7, P <.05) and WD_CaD (216.0 vs. 194.9, P <.001). The percentage of small ducts increased in WD compared to AIN-76A controls (23.3% vs. 17.4%) but was lower in the WD_y (17.1%) and WD_CaD (14.8%) groups. WD mice had higher numbers of epithelial cells per duct than WD_y (33.2 vs. 27.4, P <.001) and WD_CaD (33.2 vs. 27.8, P <.001) mice, and AIN-76A-fed mice had higher numbers than WD_y (31.1 vs. 27.4, P <.005) or WD_CaD (31.1 vs. 27.8, P <.01) mice. Mitotic index was higher in WD than in WD_CaD mice (0.0020 vs. 0.0009, P <.001). Thus, small mammary gland ductules and mitosis increased after feeding WD and decreased after supplementing the diets with increased calcium and vitamin D₃, administered either in a dairy food (yogurt) or directly as calcium carbonate plus vitamin D₃ in WD, suggesting further study of these nutrients for their possible relationship to breast cancer prevention.