Soyfoods & Cancer

Over 11 million Americans today are cancer survivors, and about 1.5 million Americans are expected to be diagnosed with cancer this year.¹Since diet is thought to play an important role in the prevention of cancer, the medical and nutrition communities have taken a keen interest in the potential role of soyfoods in reducing cancer risk – especially the risk of breast and prostate cancer.

The US National Cancer Institute first began seriously investigating the potential for soy to reduce cancer risk nearly 20 years ago.² Interest in this area was stimulated in part by the observation that soyfood-consuming countries have relatively low breast and prostate cancer rates.³ Researchers also recognized that soybeans contain several compounds that may have chemopreventive (cancer-protective) qualities.² In the past two decades, most research has centered on one particular group of naturally-present chemicals, the isoflavones.⁴Isoflavones are essentially unique to the soybean.⁵ Consequently, if you consume soyfoods your diet is high in isoflavones, but if you don't (and don't take supplements), your diet is nearly devoid of them.

Soyfoods & Breast Cancer

Current views on soy and breast cancer stem from several comprehensive statistical analyses of epidemiologic studies. Epidemiology is the science of studying populations in order to determine the frequency and distribution of disease and measured risks. In the epidemiologic soy studies, investigators ascertained the quantity of soyfoods consumed by study participants to determine if high-soy consumers were more or less likely to have (in case-control studies), or to develop (in prospective studies), cancer.

In the most recent analysis, which was conducted by Anna Wu and colleagues from the University of Southern California, high-soy consumers were found to be about 30% less likely to report having breast cancer than Asian women who consumed relatively little soy.⁶However, in a separate large study in the United Kingdom, there was no evidence that soy intake was protective.⁷ This second study purposely included a high percentage of vegetarians, who typically consume more soy than non-vegetarians.

What might account for the protective effect of soyfood intake in the Asian studies but not in the UK study? One explanation is that soyfoods are protective against breast cancer only when consumed early in life – during childhood and/or adolescence.⁸ In the UK study, it is likely the vegetarians adopted their dietary behavior as adults, and may well have consumed little soy prior. In contrast, because soyfoods are part of the traditional Asian diet, it is likely that Asians who consume soy as adults also did so as children.

The notion that early soy intake is protective against breast cancer has several lines of support. For example, studies show that when rats are exposed to isoflavones for just brief periods of time when young, and then given a mammary or breast carcinogen, tumor development is dramatically inhibited.^9,10 More importantly, all four of the epidemiologic studies that investigated the relationship between early soy intake and later risk of developing breast cancer found protective effects,^11-14 with risk reduction ranging from 28 to 60%. It is worth noting that in these studies, "high soy intake" was typically defined as only about 1 or 1½ servings of soyfoods per day.

Finally, the hypothesis that early soy intake is protective against breast cancer is consistent with studies showing that adult soy intake alone doesn't affect markers or indicators of breast cancer risk.¹⁵ Because cancer is a difficult disease to study in humans, clinical studies generally examine the impact of an intervention, such as a dietary change, on indicators of cancer risk rather than cancer incidence per se. This is similar to the way in which we study the effects of diet on blood cholesterol levels, rather than on the incidence of heart attacks, as a means of assessing changes in heart disease risk. The fact that adult soy intake doesn't affect markers of breast cancer risk strongly suggests that if soy is indeed protective against breast cancer, benefits are derived from consumption early in life.

Soyfoods & Prostate Cancer

As in the case of breast cancer, rates of prostate cancer are relatively low in soyfood-consuming countries.³ There is also evidence that in these countries, prostate cancer doesn't progress as readily from the beginning stages of the disease, which is not life-threatening, to the stage at which the disease is capable of spreading or metastasizing to other tissues. At that point, prostate cancer is quite lethal. Since prostate tumors are generally slow-growing and are typically diagnosed at a relatively late age, modestly delaying the development of the disease and/or slowing the growth of prostate tumors can dramatically reduce prostate cancer mortality. In other words, if the progress of the disease can be sufficiently slowed, men will die with their tumor rather than because of their tumor.

In a recent comprehensive statistical analysis of epidemiologic studies, researchers from the US Department of Agriculture and Washington University in St Louis found that among studies involving Asian men, individuals who consumed the most soy were almost 50% less likely to have prostate cancer than those who consumed relatively little soy.¹⁶This suggested protective effect of soyfood intake is generally consistent with results from animal studies.¹⁷

In addition to possibly preventing the development of prostate cancer, soy consumption may also help prevent its metastasis, according to both animal and human evidence. For example, a recent study published in the Journal of the National Cancer Institute reported that the level of an enzyme that allows cells to invade tissues was markedly reduced in prostate cancer patients given the soybean isoflavone genistein.¹⁸ In a separate animal study, adding isoflavones to the diet in mice was found to inhibit prostate tumor metastasis by 96%.¹⁹ Finally, in men with prostate cancer whose treatment has been unsuccessful, preliminary evidence suggests that soy isoflavones may be of benefit. Some studies, but not all, show that isoflavones slow the rise in prostate specific antigen (PSA) levels.²⁰ PSA is protein produced by the prostate that can be measured in the blood, and is used as a marker of prostate cancer.

Summary and Conclusions

Intriguing evidence indicates that consuming modest quantities (1 – 1.5 servings) of soyfoods daily during childhood and/or adolescence markedly reduces breast cancer risk later in life. Similarly, there is evidence suggesting that soyfoods may reduce the risk of developing prostate cancer, and possibly even slow the spread of prostate tumors. Even though we cannot reach definitive conclusions about the chemopreventive effects of soyfoods at this point, soyfoods can certainly be encouraged as part of a healthy diet because they are low in saturated fat and good sources of high-quality protein and a variety of other nutrients.

1. American Cancer Society. Cancer Facts and Figures 2009. Atlanta, GA; 2009.
2. Messina M, Barnes S. The role of soy products in reducing risk of cancer. J Natl Cancer Inst 1991;83:541-6.
3. Pisani P, Bray F, Parkin DM. Estimates of the world-wide prevalence of cancer for 25 sites in the adult population. Int J Cancer 2002;97:72-81.
4. Sarkar FH, Li Y. Soy isoflavones and cancer prevention. Cancer Invest 2003;21:744-57.
5. Franke AA, Custer LJ, Wang W, Shi CY. HPLC analysis of isoflavonoids and other phenolic agents from foods and from human fluids. Proc Soc Exp Biol Med 1998;217:263-73.
6. Wu AH, Yu MC, Tseng CC, Pike MC. Epidemiology of soy exposures and breast cancer risk. Br J Cancer 2008;98:9-14.
7. Travis RC, Allen NE, Appleby PN, Spencer EA, Roddam AW, Key TJ. A prospective study of vegetarianism and isoflavone intake in relation to breast cancer risk in British women. Int J Cancer 2008;122:705-10.
8. Messina M, Wu AH. Perspectives on the soy-breast cancer relation. Am J Clin Nutr 2009;89:1673S-9S.
9. Lamartiniere CA, Zhao YX, Fritz WA. Genistein: mammary cancer chemoprevention, in vivo mechanisms of action, potential for toxicity and bioavailability in rats. J Women's Cancer 2000;2:11-9.
10. Peng JH, Zhang F, Zhang HX, Fan HY. Prepubertal octylphenol exposure up-regulate BRCA1 expression, down-regulate ERalpha expression and reduce rat mammary tumorigenesis. Cancer Epidemiol 2009;33:51-5.
11. Wu AH, Yu MC, Tseng CC, Stanczyk FZ, Pike MC. Dietary patterns and breast cancer risk in Asian American women. Am J Clin Nutr 2009.
12. Shu XO, Jin F, Dai Q, Wen W, Potter JD, Kushi LH, et al. Soyfood Intake during Adolescence and Subsequent Risk of Breast Cancer among Chinese Women. Cancer Epidemiol Biomarkers Prev 2001;10:483-8.
13. Korde LA, Wu AH, Fears T, Nomura AM, West DW, Kolonel L, et al. Childhood soy intake and breast cancer risk in Asian American women. Cancer Epid, Biomarkers and Prev 2009;18:1-9.
14. Lee SA, Shu XO, Li H, Yang G, Cai H, Wen W, et al. Adolescent and adult soy food intake and breast cancer risk: results from the Shanghai Women's Health Study. Am J Clin Nutr 2009;89:1920-6.
15. Messina MJ, Wood CE. Soy isoflavones, estrogen therapy, and breast cancer risk: Analysis and commentary. Nutr J 2008;7:17.
16. Yan L, Spitznagel EL. Soy consumption and prostate cancer risk in men: a revisit of a meta-analysis. Am J Clin Nutr 2009.
17. Wang J, Eltoum IE, Carpenter M, Lamartiniere CA. Genistein mechanisms and timing of prostate cancer chemoprevention in lobund-wistar rats. Asian Pac J Cancer Prev 2009;10:143-50.
18. Xu L, Ding Y, Catalona WJ, Yang XJ, Anderson WF, Jovanovic B, et al. MEK4 function, genistein treatment, and invasion of human prostate cancer cells. J Natl Cancer Inst 2009;101:1141-55.
19. Lakshman M, Xu L, Ananthanarayanan V, Cooper J, Takimoto CH, Helenowski I, et al. Dietary genistein inhibits metastasis of human prostate cancer in mice. Cancer Res 2008;68:2024-32.
20. Messina M, Kucuk O, Lampe JW. An overview of the health effects of isoflavones with an emphasis on prostate cancer risk and prostate-specific antigen levels. J AOAC Int 2006;89:1121-34.