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Soy and the Prevention of Breast Cancer

Hollie  Black

February 22, 2010





        Soy is the colloquial name given to describe this oilseed legume that is native to Eastern Asia (Wikipedia Contributers, 2010). According to ancient Chinese legend, soy is considered one of several “sacred foods” (Wikipedia Contributers, 2010), along with rice, wheat, barley, and millet. Soy first appeared in the United States in the late 18th Century, where it was used primarily as an industrial product. Because of the affordability of soy, it was especially used during the Great Depression to regenerate the soil. Soy was not used as a food source in the U.S. until World War II, where it was the least expensive source of protein and edible oil. Today, the United States is one of the leading soybean exporters in the world, followed by Brazil and China.


Health Benefits

Recently soy has become even more popular in the U.S. because of the supposed health benefits of either eating soy products or taking soy supplements. Soy is considered a “complete protein” (Wikipedia Contributers, 2010), meaning that it contains essential amino acids that cannot be manufactured by the body and must be consumed. Soy also contains phytic and omega- 3 fatty acids and isoflavones. Phytic and omega-3 fatty acids both help to prevent heart disease. Omega- 3s contain alpha linolenic acid, which prevents blood from clotting. Soy’s isoflavone content is perhaps its most controversial ingredients. On the one hand, genistein isoflavone inhibits the formation of new blood vessels and regulated cell division, which can prevent unregulated cell growth, also known as cancer. On the other hand, soy also contains phytoestrogens, isoflavones that mimic human estrogen, which can also prevent cancer. Some studies have indicated that theses molecules can also be considered carcinogenic as well (American Cancer Society,Inc., 2008).

Epidemiology of Breast Cancer in the United States

There is an ever-increasing dichotomy between research that shows that soy can prevent certain diseases, such as breast and prostate cancer and research showing that high soy intake can in fact increase the risk for obtaining said diseases.

There were nearly 200,000 new cases of breast cancer in the United States in 2009  (American Cancer Society,Inc., 2008). Other than skin cancer, breast cancer is the most common type of cancer in women.  Currently, the mortality rate for breast cancer is around 18 per 100,000. The mortality rate of breast cancer is steadily decreasing due to early detective screenings and better treatment options. Much research is being conducted to find measures that could be taken to prevent or lower one’s risk for developing breast cancer. The intake of soy and the prevention of breast cancer is one such ongoing study.


SEER Incidence Breast Cancer

Incidence Rates by Race



All Races

123.8 per 100,000 women


127.8 per 100,000 women


117.7 per 100,000 women

Asian/Pacific Islander

89.5 per 100,000 women

American Indian/Alaska Native a

74.4 per 100,000 women

Hispanic b

88.3 per 100,000 women

(SEER Cancer Statistics Review, 2008)

From 2002-2006, the median age at diagnosis for cancer of the breast was 61 years of age3. Approximately 0.0% were diagnosed under age 20; 1.9% between 20 and 34; 10.5% between 35 and 44; 22.5% between 45 and 54; 23.7% between 55 and 64; 19.6% between 65 and 74; 16.2% between 75 and 84; and 5.5% 85+ years of age.

The age-adjusted incidence rate was 123.8 per 100,000 women per year. These rates are based on cases diagnosed in 2002-2006 from 17 SEER geographic areas. (SEER Cancer Statistics Review, 2008)

The incidence of breast cancer in Asian countries is much lower than that in the United States. Many studies have indicated that this lower incidence is inked to the dietary differences of the two cultures. Many foods in Asian countries are soy based, connoting that soy could be a possible preventative measure that has allowed the incidence of breast cancer to stay low in these countries. The possibility of soy being used as preventative medicine for breast cancer has sparked several studies in the U.S. to find the truth behind soy’s healing properties. Most women in the U.S. do not get the same high intake of soy as Asian women get from tofu, because most of the soy consumed in the U.S. is in smaller portions in additive form  (Bruce J. Trock, 2006).

Analysis of Studies

Several studies were compounded and investigated thereupon in a meta-analysis performed by several scientists from various university medical centers in Baltimore and Washington D.C. In this analysis, the results of eighteen case-control and cohort studies were examined.

There were several disparities within the studies when looking at the intake of soy because of the different measurements used and the different soy products that were used. Some women took high dose soy supplements, while others simply ate soy that is found in various foods in different amounts.

The results of this epidemiologic study meta-analysis were varied, due to the multiplicity of hypotheses that were studied (i.e. the amount of soy consumed, subject’s subsequent risk factors, and other confounding factors). In any case, some conclusions could be drawn based on some common results found within studies. Overall, a marginally small reduction in breast cancer was observed in individuals who had a high intake of soy across the board. Ten studies did show that there was a slight increase in breast cancer in some postmenopausal women, while others did not indicate whether the menopausal condition  (Bruce J. Trock, 2006). However, prepubescent exposure to soy did show a marked reduction in breast cancer in rats, which calls for more in depth study of the effects of childhood soy intake and the prevention of breast cancer. The meta-analysis concluded that soy is beneficial in small doses, but there could be potential adverse effects; so, no high dose isoflavone containing products (soy) should be administered as a breast cancer preventative measure until more studies have been conducted  (Bruce J. Trock, 2006).

An appraisal of this meta-analysis by Northwestern State University specifically looked at the credibility of studies that examined the effect of high soy intake and breast cancer prevention in premenopausal women. It was concluded that this meta-analysis did not have enough detailed information about how results were evaluated, and that no change in practice should be pursued  (Allyson Delaune, 2008). This appraisal was placed in the journal, Evidence-Based Nursing, so that other nurses could make the determination on how to go about treating patients. This appraisal was done in great detail to insure that no results were misinterpreted and that no recommendations were made without credible evidence.

In a randomized clinical trial conducted by the Laboratory of Microbial Ecology Technology Faculty of Bioscience Engineering Ghent University Hospital in Ghent, Belgium, the possible effects of soy isoflavones in normal human breast tissue were studied. In this experiment, thirty-one healthy Belgian or Dutch women (18-62 yrs), who were undergoing esthetic breast reduction were chosen to participate.  These women were randomly assigned to soymilk, soy supplement, or no soy products(control). The women who were drinking soymilk, had to drink 250mL a day. Soymilk contains more genistein, but less daidzein than soy supplements. The women assigned to soy supplement had to take one pill three times a day, and the control group just ate what they normally ate in a typical day. This treatment lasted for five days.  During the subsequent surgeries, biopsies were taken to the lab. The biopsies found that in all of the women that had been on the soy diet, marked increases in isoflavone levels were found within the breast tissue. These levels were high enough to have eventually caused health related problems (i.e. breast cancer) (Laboratory of Microbial Ecology Technology Faculty of Bioscience Engineering Ghent University Hospital Ghent Belgium, 2009)


Due to the possibility of harmful side effects of ingesting large amounts of soy based on animal and retrospective human studies, some countries have banned its use in many products. For example, Australia and New Zealand now require a doctor’s prescription in order to buy soy based baby formula(Siegel-Itzkovich, 2005). Israel’s, a prodominantly Jewish country, health committee has suggested similar measures be taken within its country, because of the large amount of soy intake. In the Judaic faith, dairy and meat products cannot be mixed, so the use of soy as a dairy substitute is commonly practiced. There is an increased need for more research on the safety of soy products over the long run (Siegel-Itzkovich, 2005).

At this point, it is just not safe to say that in taking soy will reduce one’s risk for breast cancer. Some studies have shown a marked decrease in breast cancer risk in those that consumed soy as a child, but none have concluded an absolute decrease in risk in those that took soy after childhood. Studies do point out, however, that soy in moderation is beneficial to overall health. It is not recommended that a high amount of soy be taken, due to the possible adverse side effects from the amount of isoflavones that it contains. Yet, there are several preventative measures that one could take to lower the risk for developing breast cancer. Maintaining a healthy weight, by eating fruits and vegetables, abstaining from alcohol and tobacco products, and getting enough exercise and sleep all promote a healthy lifestyle that can significantly reduce the risk of developing  cancer and other diseases like diabetes and heart disease.





Allyson Delaune, K. L. (2008). Credibility of a Meta-Analysis: Evidence-Based Practice Concerning Soy Intake and Breast Cancer Risk in Premenopausal Women. Worldviews on Evidence-Based Nursing , 6 (3), 160-166.

American Cancer Society,Inc. (2008, November 11). Soybean. Retrieved February 20, 2010, from American Cancer Society:

Bruce J. Trock, L. H.-C. (2006). Meta-Analysis of Soy Intake and Breast Cancer Risk. J. Natl. Cancer Inst. , 98 (7), 459-471.

Laboratory of Microbial Ecology Technology Faculty of Bioscience Engineering Ghent University Hospital Ghent Belgium. (2009). Disposition of soy isoflavones in normal human breast tissue. The American Journal of Clinical Nutrition .

SEER Cancer Statistics Review. (2008, November 1). SEER Stat Fact Sheets on the Incidence of Breast Cancer. Retrieved February 2010, 2010, from Surveillance and Epidemiology and End Results:

Siegel-Itzkovich, J. (2005, July 30). Health committee warns of potential dangers of soya. British Medical Journal , 254.

Viklund, A. (2009, August 22). If you want me to like it . . . Retrieved February 21, 2010, from Gryphon's Aerie, Thinking . . . trying not to fry the circuits:

Wikipedia Contributers. (2010, February 17). Soybean. Retrieved February 20, 2010, from Wikipedia, The Free Encyclopedia.:  




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