Soybeans are rich in isoflavones, a type of phytoestrogen. Phytoestrogens are plant substances that are chemically similar to estrogen – they may block estrogen's natural effects or actually have weak estrogen-like effects in the body. Since estrogen exposure had been known for many years to negatively influence breast cancer risk, soy was originally thought by some to be potentially dangerous. This fear of soy was heavily promoted on the internet by certain groups and individuals and now there has been considerable evidence examining this question.
A careful look at all the data demonstrates the soy fears were unfounded. Isoflavones were actually shown to have anti-cancer effects in cell culture and animal studies (1). Also, in Asian countries where soy is a staple food, rates of breast cancer have traditionally been much lower than those in the U.S. This paradox launched much debate and hundreds of studies on the relationship between soy and breast cancer. In spite of myths propagated on the internet, the most recent and reliable clinical studies support a strong protective effect of minimally processed soy foods against breast cancer. In 2006, a meta-analysis in the Journal of the National Cancer Institute examining data from 18 studies on soy and breast cancer that were published between 1978 and 2004 concluded that soy overall has a protective effect (2). Again in 2008, another meta-analysis in the British Journal of Nutrition compiling data from 8 different studies (that were not included in the 2006 meta-analysis) also concluded that soy consumption decreases breast cancer risk. These effects were dose-dependent – a 16% reduced risk for each 10 mg of soy isoflavones consumed daily (3).
Consensus is now building that soy food intake during adolescence, a time when breast tissue is most sensitive to environmental stimuli and carcinogenesis, may reduce the risk of breast cancer later in life. Recent reports in Cancer Epidemiology and The American Journal of Clinical Nutrition found that soy consumption during childhood and teenage years reduced risk of breast cancer in adulthood by 60% and 40%, respectively (4).
Soy has protective effects even after a diagnosis of breast cancer. A new study of breast cancer survivors has shown that premenopausal breast cancer survivors who consumed more soy had a 23% reduced risk of recurrence (5).
Soy provides protection against other hormonal cancers as well. A meta-analysis of studies on soy consumption and prostate cancer found a 31% decrease in prostate cancer risk for high consumption of soy foods (6). Soy has also been shown to be protective against endometrial and ovarian cancers (7).
Soy products can be useful in moving toward a plant-centered diet with less saturated fat, less animal protein, more plant protein and more fruits and vegetables. In the U.S., the majority of our soy intake, which is very low compared to that of Asian countries, is consumed via soy-based additives or isolated soy protein in processed foods. Processed foods are certainly not health-promoting, even if they contain soy. The most healthful soy foods are those that are minimally processed – these include edamame, tofu, unsweetened soy milk, and tempeh. You should be aware that soy nuts, and other processed soy products do not retain much of the beneficial compounds and omega-3 fats that are in the natural bean. The more the food is processed, the more these beneficial compounds are destroyed. Minimally processed soy foods are a beneficial addition to a healthy diet. I do not recommend consuming large quantities of soy products in the hopes of reducing cancer risk. A healthy diet includes a variety of beans, and not a disproportionate share of calories from any one food. Anti-cancer effects are found in beans of all kinds, not only soybeans. I always recommend the consumption of a broad variety of phytochemical-rich foods to maximize one’s health. Beans are no exception – try to eat different types of beans, including soybeans.
References:
1. Sarkar FH, Li Y. Cancer Invest 2003;21:744-57.
2. Trock BJ et al. J Natl Cancer Inst. 2006 Apr 5;98(7):459-71.
3. Wu AH et al. Br J Cancer 2008;98, 9– 14
4. Korde LA et al.Cancer Epidemiology, Biomarkers & Prevention April 2009 18; 1050
5. Lee S et al. Am J Clin Nutr 2009;89:1920-1926
6. Guha N et al. Breast Cancer Res Treat. 2009 Nov;118(2):395-405.
7. Hwang YW et al. Nutr Cancer. 2009;61(5):598-606.
Myung SK et al. BJOG. 2009 Dec;116(13):1697-705.
Comments