Research on soy along with the growing interest in potential health benefits of these foods focuses largely on their phytoestrogen estrogen content. These plant estrogens, called isoflavones, have been shown to reduce menopausal symptoms such as hot flashes, and may also reduce heart disease risk by improving health of the arteries.
But, because they share some common properties with the hormone estrogen, there have long been questions about whether isoflavones might cause some of the same problems seen with estrogen therapy in postmenopausal women. Two studies, one from Brazil and the other from the United States, provide assurance in this regard.
In the Brazilian study, researchers randomly assigned 80 postmenopausal women to consume 100 milligrams of isoflavones per day—the amount provided by about four servings of soyfoods—or a placebo. At the beginning of the study and then 10 months later, the investigators used mammograms to measure breast tissue density.
Higher breast tissue density is associated with an increased risk for cancer. Consequently, factors that increase density, such as hormone therapy, are thought to also increase breast cancer risk. In this study, after 10 months, there was no difference in breast tissue density between groups. This finding is consistent with previously published research and it supports the safety of isoflavones and soyfoods.
To obtain additional insight into the effects of isoflavones on breast tissue, this study was also the first to evaluate breast parenchyma, which refers to the cells in the breast that make up the milk ducts and the glands that produce the milk. As for breast tissue density, isoflavones had no effect on breast parenchyma in this study.
In the second study, which was done at the University of Southern California, Los Angeles, the investigators examined the endometrium, which is the lining of the uterus. The hormone estrogen causes cells in the endometrium to proliferate, and as a result, markedly increases risk of endometrial cancer. Consequently, menopausal women who wish to use hormone replacement therapy must take a combination of two hormones—progesterone plus estrogen. The progesterone counters the harmful effects of estrogen. (This isn’t a concern for women who have had a hysterectomy, however, since they don’t have any endometrial tissue.)
In this randomized, double-blind, placebo-controlled trial, 224 postmenopausal women aged 45 to 92 years were divided into two groups. Half of the women took 25 grams of soy protein that provided approximately 100 milligrams of isoflavones, and the other half took 25 grams of milk protein. The size and duration of this study, which lasted for three years, make it especially important. The effects on the endometrium were assessed using ultrasound to measure the endometrial thickness. Generally, an endometrial thickness under 5 millimeters is considered normal. As was the case for breast tissue density in the Brazilian study, isoflavones had no effect on the endometrium. This study provides additional support for the safety of these soybean constituents.
These two studies illustrated that soybean phytoestrogens are different from the hormone estrogen and that soyfoods can safely be consumed.
Delmanto et al., Menopause 2013; Quaas et al., Menopause, 2013