Breast and Uterine Effects of Soy Isoflavones and Conjugated Equine Estrogens in Postmenopausal Female Monkeys.

Author: Charles E. Wood et al.
Publication: The Journal of Clinical Endocrinology & Metabolism Vol. 89, No. 7 3462-3468.

Hormone Replacement Therapy is used to control monopause symptoms but is known to increase the risk for breast cancer for postmenopausal women. As a consequence there has been a lot of discussion about whether high levels of dietary soy are safe for postmenopausal women and evidence about their safety has been conflicting. Oriental people who consume a lot of soy have lower rates of breast cancer but some studies have suggested that soy isoflavones can stimulate breast cancer cells grown in the laboratory.

The purpose of the study was to evaluated the long-term effects of soy isoflavones on markers of breast and uterus cancer risks. The study was carried out on ovariectomized female monkeys which were put on 3 different diets during 36 months:

• Group 1: received soy protein isolate without isoflavones
• Group 2: received soy protein isolate with 129 mg isoflavones per day, this level is much higher than people receive in soy-rich diet
• Group 3: isoflavone-depleted soy protein isolate with 0.624 mg Premarin (estrogen) per day

After the 36 months the breast density, numbers of dividing breast and uterine cells and serum estrogen levels were measured.

No significant differences in were detected in epithelial proliferation and progesterone receptor expression in the breast and uterus, between group 1 and 2. Monkeys from group 3 had increased levels of all markers.

This study suggest that high levels of soy isoflavones do not stimulate breast or uterine cancer in postmenopausal monkeys. High levels of isoflavones may contribute to an estrogen profile associated with reduced breast cancer risk. It is important to note that the research investigated the effects of isoflavones on normal breast tissue and not in breast cancer tissue.

• Group 1: received soy protein isolate without isoflavones
• Group 2: received soy protein isolate with 129 mg isoflavones per day, this level is much higher than people receive in soy-rich diet
• Group 3: isoflavone-depleted soy protein isolate with 0.624 mg Premarin (estrogen) per day

After the 36 months the breast density, numbers of dividing breast and uterine cells and serum estrogen levels were measured.

No significant differences in were detected in epithelial proliferation and progesterone receptor expression in the breast and uterus, between group 1 and 2. Monkeys from group 3 had increased levels of all markers.

This study suggest that high levels of soy isoflavones do not stimulate breast or uterine cancer in postmenopausal monkeys. High levels of isoflavones may contribute to an estrogen profile associated with reduced breast cancer risk. It is important to note that the research investigated the effects of isoflavones on normal breast tissue and not in breast cancer tissue.