Actuality

There is essentially no human evidence indicating that traditional soyfoods increase tumor recurrence in breast cancer patients.  In fact, there is evidence suggesting consuming soyfoods may actually be of benefit to such women.

Breast cancer incidence and mortality rates in soyfood-consuming countries such as Japan are much lower than they are in Western countries [1].  Several factors likely contribute to the differing breast cancer rates of these two regions.  One may be the high soy consumption in Asia.  There is in fact an impressive body of evidence suggesting soyfoods reduce risk of breast cancer.  However, studies in support of this hypothesis indicate that to derive the proposed benefit soyfoods must be consumed during childhood and/or adolescence.  Consuming as little as one serving per day may be enough to reduce risk later in life by as much as 25 to 50 percent [2, 3].

Despite the proposed benefits there is also concern that soyfoods might be harmful to some types of breast cancer patients.  More specifically, there is concern that the isoflavones in soyfoods could stimulate the growth of estrogen-sensitive tumors – these are tumors whose growth is stimulated by the hormone estrogen [4].

Isoflavones are naturally-occurring constituents of soybeans that are commonly referred to as plant estrogens or phytoestrogens.  There are many different types of phytoestrogens in foods (such as lignans in flaxseed) but soy is definitely one of the richest sources.  However, isoflavones and estrogen are not equal, they have different chemical structures and although they share some properties in common they are also very different.  That these two molecules are different is not at all surprising.  Small differences in chemical structure can result in very different physiological effects.  Consider the example of cholesterol, which is found in animal foods, and phytosterols, which are found in plants; these two molecules have almost identical chemical structures but the former raises blood cholesterol levels and the latter decreases them [5].

The primary basis for concern about isoflavones is the finding that when isolated isoflavones and certain types of isoflavone-rich products are added to the diets of mice with existing estrogen-sensitive tumors, tumor growth is stimulated [4].  However, not only are the results from animal studies often not predictive of events in humans, but even in the animal studies, soyflour, which is a minimally processed soyfood, doesn’t stimulate tumor growth [6].

Furthermore, and most importantly, the human evidence is very supportive of safety.  Neither isoflavone supplements nor soyfoods adversely affect markers or indicators of breast cancer risk [7, 8].  Markers of breast cancer risk include breast tissue density, which is measured via mammography, breast cell proliferation, which requires that breast biopsies be taken, and blood estrogen levels.  Thus, there is essentially no clinical evidence suggesting soyfoods are likely to be harmful to breast cancer patients.  Accordingly, in 2006, the American Cancer Society stated that breast cancer patients can safely consume up to three servings of traditional soyfoods daily [9].

In fact, there is evidence suggesting that breast cancer patients may benefit by consuming soyfoods.  A recently published study in the Journal of the American Medical Association (JAMA) found that among Chinese breast cancer patients, those who consumed the most soyfoods were about 30% less likely to die from breast cancer or to experience breast cancer recurrence over a four year period compared to patients with the lowest intake of soy [10].  One other study also reported that patients with estrogen-sensitive breast cancer would benefit by consuming soy [11].   In an editorial accompanying the article in the JAMA, researchers from the National Cancer Institute and the Fred Hutchinson Cancer Research Center in Seattle stated that “… clinicians can advise their patients with breast cancer that soyfoods are safe to eat and that these foods may offer some protective benefit for long-term health [12].”

References

1. Pisani P, Bray F, Parkin DM: Estimates of the world-wide prevalence of cancer for 25 sites in the adult population. Int J Cancer  97, 72-81 (2002).
2. Messina M, Wu AH: Perspectives on the soy-breast cancer relation. Am J Clin Nutr  89, 1673S-9S (2009).
3. Messina M, Hilakivi-Clarke L: Early intake appears to be the key to the proposed protective effects of soy intake against breast cancer Nutr Cancer  61, 792-8 (2009).
4. Helferich WG, Andrade JE, Hoagland MS: Phytoestrogens and breast cancer: a complex story. Inflammopharmacology  16, 219-26 (2008).
5. Katan MB, Grundy SM, Jones P, Law M, Miettinen T, Paoletti R: Efficacy and safety of plant stanols and sterols in the management of blood cholesterol levels. Mayo Clin Proc  78, 965-78 (2003).
6. Allred CD, Twaddle NC, Allred KF, Goeppinger TS, Churchwell MI, Ju YH, et al.: Soy processing affects metabolism and disposition of dietary isoflavones in ovariectomized BALB/c mice. J Agric Food Chem  53, 8542-50 (2005).
7. Messina MJ, Wood CE: Soy isoflavones, estrogen therapy, and breast cancer risk: Analysis and commentary. Nutr J  7, 17 (2008).
8. Hooper L, Ryder JJ, Kurzer MS, Lampe JW, Messina MJ, Phipps WR, et al.: Effects of soy protein and isoflavones on circulating hormone concentrations in pre- and post-menopausal women: a systematic review and meta-analysis. Hum Reprod Update  15, 423-40 (2009).
9. Doyle C, Kushi LH, Byers T, Courneya KS, Demark-Wahnefried W, Grant B, et al.: Nutrition and physical activity during and after cancer treatment: an american cancer society guide for informed choices. CA Cancer J Clin  56, 323-53 (2006).
10. Shu XO, Zheng Y, Cai H, Gu K, Chen Z, Zheng W, et al.: Soy food intake and breast cancer survival. JAMA  302, 2437-43 (2009).
11. Guha N, Kwan ML, Quesenberry CP, Jr., Weltzien EK, Castillo AL, Caan BJ: Soy isoflavones and risk of cancer recurrence in a cohort of breast cancer survivors: the Life After Cancer Epidemiology study. Breast Cancer Res Treat  118, 395-405 (2009).
12. Ballard-Barbash R, Neuhouser ML: Challenges in design and interpretation of observational research on health behaviors and cancer survival. JAMA  302, 2483-4 (2009).