Phytoestrogens are substances found in plants (phyto-) that have estrogen-like properties. Phytoesterogens latch onto the receptor sites on cells meant for estrogen. This results in a partial stimulation of those receptors, less intense than if true estrogen had attached, but stronger than if nothing had stimulated those receptors at all. This leads to a dual result. In women who have considerable estrogen, high consumption of phytoestrogens tends to occupy the receptor sites and keep real estrogen away. The net effect is to reduce the overall action of estrogen. Since estrogen promotes a number of forms of cancer (such as breast cancer), this may reduce cancer risk. However, in women who have little estrogen (after menopause, for example), consumption of large amounts of phytoestrogens will tend to mimic the effect of the missing estrogen, reducing symptoms such as hot flashes.
Phytoestrogens are found in numerous foods, most prominently soybeans (and many other types of beans), red clover, flaxseed, and whole grains. The phytoestrogens found in these foods include isoflavones and lignans.1 Herbs with high phytoestrogen content include alfalfa, hops, licorice, thyme, and verbena.1,2 Certain herbs that are often claimed to contain phytoestrogens actually do not. These include wild yam, saw palmetto, and chasteberry. Dong quai, ginseng, and black cohosh do not appear to contain phytoestrogens in the ordinary sense, but might contain substances that mimic some of the actions of estrogen under certain circumstances.
Observational studies hint that consuming high amounts of foods rich in phytoestrogenscan decrease risk of various forms of cancer, including breast, ovarian, uterine, and prostate cancer, as well as possibly help prevent heart disease and osteoporosis.1,3-5 However, observational studies are notoriously unreliable for proving a treatment effective. (Only double-blind, placebo-controlled studies can do that. For information on why such studies are essential, see Why Does This Database Rely on Double-blind Studies?). Observational studies only show association, not cause and effect. It is quite possible that people who consume high amounts of phytoestrogens are different in other ways from people who do not. For example, the Japanese, who consume a large amount of phytoestrogens, have much lower rates of breast cancer than Americans, who do not. However, there are a great many other differences in lifestyle and diet between Japanese and Americans, and these may play a more important role in breast cancer incidence than do phytoestrogens. With observational studies, it is simply not possible to sort out which factors are most important.
To see that this is not just an academic concern, consider the mistakes made regarding estrogen. Numerous observational studies indicated that treatment with estrogen as part of hormone replacement therapy could dramatically reduce risk of heart disease. However, when double-blind studies were performed, it became clear that hormone replacement therapy actually increases heart disease risk.
Thus, all conclusions about phytoestrogens based on observational studies are suspect. Unfortunately, it is not possible to perform double-blind studies on food. For this reason, the only solid information we have regards specific phytoestrogens that can be controlled in a double-blind format. To learn more, see the following articles:
1. Murkies AL, Wilcox G, Davis SR. Clinical review 92: phytoestrogens. J Clin Endocrinol Metab. 1998;83:297–303.
2. Zava DT, Dollbaum CM, Blen M. Estrogen and progestin bioactivity of foods, herbs, and spices. Proc Soc ExpBiol Med. 1998;217:369–378.
3. Horn-Ross PL, John EM, Canchola AJ, et al. Phytoestrogen intake and endometrial cancer risk. J Natl Cancer Inst. 2003;95:1158–64.
4. Adlercreutz H, Mazur W, Bartels P, et al. Phytoestrogens and prostate disease. J Nutr. 2000;130:658S–659S.
5. Kurzer MS, Xu X. Dietary phytoestrogens. Annu Rev Nutr. 1997;17:353–381.