Effects of soy on the mammary glands of premenopausal female monkeys.

OBJECTIVE: To evaluate the interactive effects of dietary soy and menstrual cycle phase on breast proliferation, apoptosis, and sex steroid receptor expression in premenopausal female cynomolgus monkeys.
DESIGN: Animals were randomized to receive diets with either casein lactalbumin or soy as the protein source for 12 months. Paired follicular and luteal mammary gland biopsies were taken before and after treatment and evaluated for epithelial proliferation and area, apoptosis, and sex steroid receptor expression.
SETTING: Academic research environment. ANIMAL(S): Fifty-two adult female cynomolgus monkeys (Macaca fascicularis). INTERVENTION(S): Soy protein with the human equivalent of 129 mg/d of isoflavones. MAIN OUTCOME MEASURE(S): Breast proliferation and epithelial area. RESULT(S): Follicular and luteal assessments were taken on cycle day (+/- SE) 5.3 +/- 0.1 and 22.0 +/- 0.1, respectively. Serum E2, serum P, breast proliferation, and vaginal maturation were significantly higher in the luteal vs. follicular phase. Soy treatment resulted in significantly higher serum isoflavone concentrations compared with the casein lactalbumin group but did not significantly affect breast proliferation, epithelial area, apoptosis, or progesterone receptor expression; uterine size; or vaginal maturation in either the follicular or luteal phase. CONCLUSION(S): Results indicate that dietary levels of soy intake have no detectable hormonal effects on the premenopausal breast.