Soya and breast cancer
times a month, compared to less than three, from childhood onwards. A fur- ther protective effect was seen in those who ate soya during adolescence and adulthood (7). The protective effect of soya may also apply to women who have had breast cancer. The Shanghai Breast Cancer Survival Study investigated the diets of over 5,000 women (aged 20 to 75) previously diagnosed with breast cancer an average of four years earlier. Results showed that those who ate more soya foods (11g of soya protein per day, equivalent to one and a half servings of tofu or soya milk) were 29 percent less likely to die from the disease and had a 32 percent lower risk of recurrence. The conclusion was that moderate soya food intake is safe and potentially beneficial for women with breast cancer (17).
Whether soya can help protect against breast cancer in post-meno- pausal women is unclear. A study of almost 2,000 breast cancer survivors (di- agnosed between 1997 and 2000 and followed for an average of six years), revealed that postmenopausal women treated with tomoxifen consuming the most soya (daidzein) had a 60 percent lower risk of recurrence (6). The authors concluded that soya isoflavones con- sumed at levels comparable to those in Asian populations may reduce the risk of cancer recurrence in women receiv- ing tamoxifen therapy and moreover, appears not to interfere with tamoxifen efficacy. More research is needed to confirm this protective effect. Although the evidence suggesting
a protective role for soya (at least in premenopausal women) is strong, it ap- pears to focus mainly on Asian women. Furthermore, a recent review reported that soya intake lowered breast cancer risk in Asian but not Western popula- tions. This may indicate that protection against breast cancer requires soya consumption at levels typical in Asian diets (13).
Isoflavone intakes vary widely
around the world from just 0.8-17.0mg per day in the UK, US, Australia and New Zealand to18-200mg per day in Japan, China and Korea (2). Vegetarians in the UK may consume slightly higher than average levels; 7.4mg per day compared to an estimated 1.2mg for omnivores (23), but these levels are still much lower than those seen in many Asian countries. The wide variation in isoflavone intakes between different popula- tions may explain why some Western studies show no protection. A review
NHDmag.com June '10 - issue 55
of eight Asian studies showed that women consuming at least 20mg per day isoflavones had a 29 percent lower risk of breast cancer compared with those eating less than 5.0mg per day. In contrast, 11 studies of women eating Western diets found no association between isoflavones and breast cancer risk. In this review, the average high- est intake was 0.8mg isoflavones per day. Therefore, women may need to consume the amount of soya in Asian diets to gain protection against breast cancer. Alternatively, the protective ef- fects may only occur when exposure to soya occurs early in life (21).
Mechanism
How soya isoflavones lower breast cancer risk has yet to be fully under- stood, some putative mechanisms have been suggested. When isoflavones bind to oestrogen receptors several re- sponses occur; synthesis of sex-hormone binding protein increases (which lowers availability of these hormones), synthesis of oestrogen is inhibited and the clear- ance of steroids from the circulation is increased (17). Additionally, isoflavones inhibit the activity of enzymes that con- vert androgens to oestrogens. They are also anti-proliferative, pro-apoptotic, anti-angiogenic, anti-oxidative and an- ti-inflammatory. Taken together, these actions indicate that soya isoflavones may be a potent agent for preventing breast cancer (13).
Conclusion
There is little evidence to suggest that the consumption of soya foods, in amounts consistent with an Asian diet, increase breast cancer risk or worsen the prognosis of breast cancer patients (18, 12). In 2006, the American Cancer Society concluded that breast cancer patients can safely consume up to three servings of traditional soya foods per day (3). However, they advise against the use of more concentrated sources of isoflavones such as powders and supplements. While more research is required to allay concerns, the existing research should provide some degree of assur- ance that isoflavone exposure at levels consistent with typical Asian soya food intakes does not result in adverse stimu- latory effects on breast tissue (12). In summary, soya intake recom- mendations for breast cancer patients are similar to those for healthy women. Overall the evidence suggests that the intake of approximately 15g (10-25g) of soya protein and 50mg (30-100mg) of isoflavones per day is safe (10). This
amount of soya protein and isoflavones is provided by approximately two serv- ings of traditional soya foods. The Vegetarian & Vegan Foundation have published a fully-referenced guide reviewing the links between soya intake and health, the Soya Story is available online from:
www.soyafacts.com.
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www.statistics.gov.uk/cci/ nugget.asp?id=575 [Accessed February 25, 2010] 15 Petrakis NL, Barnes S, King EB, Lowenstein J, Wiencke J, Lee MM, Miike R, Kirk M and Coward L 1996. Stimulatory influence of soy protein isolate on breast secretion in pre- and postmeno- pausal women. Cancer Epidemiology Biomarkers and Prevention. 5, 785–94 16 Shu XO, Jin F, Dai Q, Wen W, Potter JD, Kushi LH, Ruan Z, Gao YT and Zheng W 2001. Soyfood intake during adolescence and subsequent risk of breast cancer among Chinese women. Cancer Epidemiology Biomarkers and Prevention. 10 (5) 483-488 17 Shu et al, 2009. Soy food intake and breast cancer survival. Journal of the American Medical Association. 302 (22) 2437-2443 18 Trock BJ, Hilakivi-Clarke L, Clarke R 2006. Meta-analysis of soy intake and breast cancer risk. Journal of the National Cancer Institute. 98 (7) 459-471 19 Wu AH, Ziegler RG, Nomura AM, West DW, Kolonel LN, Horn- Ross PL, Hoover RN and Pike MC 1998. Soy intake and risk of breast cancer in Asians and Asian Americans. American Journal of Clinical Nutrition. 68 (6 Suppl) 1437S-1443S 20 Wu AH, Wan P, Hankin J, Tseng CC, Yu MC and Pike MC 2002. Adolescent and adult soy intake and risk of breast cancer in Asian- Americans. Carcinogenesis. 23 (9) 1491-1496. 21 Wu AH, Yu MC, Tseng CC and Pike MC 2008. Epidemiology of soy exposures and breast cancer risk. British Journal of Cancer. 98 (1) 9-14. 22 Ziegler RG, Hoover RN, Pike MC, Hildesheim A, Nomura AMY, West DW, Wu-Williams AH, Kolonel LN, Horn-Ross PL, Rosenthal JF, Hyer MB 1993. Migration patterns and breast cancer risk in Asian-American women. Journal of the National Cancer Institute. 85 1819-27. 23 Ritchie MR, Cummings JH, Morton MS, Michael Steel C, Bolton- Smith C, Riches AC 2006. A newly constructed and validated isoflavone database for the assessment of total genistein and daidzein intake. British Journal of Nutrition. 95 (1) 204-213.
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