by Craig Weatherby
The evidence linking higher soy intakes to lower breast risk keeps accumulating.
In recent months and years, population studies have associated soy with reduced risk of breast cancer recurrences and deaths.
And these studies have failed to find any evidence that soy interferes with the benefits of the top breast-protective drugs: tamoxifen, anastrozole, and raloxifene (Boyapati SM et al. 2005; Guha N et al. 2009; Lee SA et al. 2009).
These drugs can slash the risk of developing post-menopausal breast cancer by half in specific target groups... though they raise women's risk of blood clots, cataracts, and uterine cancers very substantially.
And the findings from a new population study add to the existing epidemiological evidence that supports the widely presumed breast-health powers of whole soy foods.
In the new study, women who averaged 1/4 cup of soy foods per day were about one-third less likely to have died from breast cancer or suffer a cancer recurrence after four years of follow-up, when compared with women who ate less than 5.3 grams per day.
Let's take a look at the reasons why soy attracts prevention hopes, and scan the study details.
Soy and breast cancer: Brighter hopes and fading fears
Whole soy foods provide five likely anti-cancer factors: isoflavones, protease inhibitors, phytate, phytosterols, and saponins... as well as other potential anti-carcinogens such as phenolic acids and lecithin.
Soy oil has small amounts of plant-form omega-3s, which tend to retard tumor growth, but is much higher in omega-6s fats.
When consumed in the excess typical of American diets, omega-6s fats tend to promote tumor growth... and soybean oil is a top source of omega-6 fats in the American diet.
Soy isoflavones are estrogen-like compounds that bind to estrogen receptors on breast cells, thus offering the possibility of curbing the growth of estrogen-dependent breast tumors.
However, isoflavones have stimulated breast cancer cells in some rodent studies… when delivered at very high doses, equivalent to a human dose of 150 mg per day.
And a debate among nutrition advocates over soy foods' role in other health/nutrition realms continues to rage (see our “Soy courts controversy” sidebar).
Fortunately, given the popularity of soy foods and drinks, the new results from China support the hypothetical breast benefits of soy and showed no heightened risks.
Shanghai Breast Study supports soy
Researchers from Vanderbilt University Medical Center in Tennessee analyzed data from the Shanghai Breast Cancer Survival Study, which was conducted among 5,042 female breast cancer survivors aged 20 to 75 (Shu XO et al. 2009).
After about four years of follow-up, the women with higher soy food intake had lower rates of breast cancer death or recurrence.
The analysts—led by Shanghai-trained physician/scientist Xiao Ou Shu, M.D., Ph.D.—found that breast cancer recurrence and mortality rates dropped as soy protein intake increased, up to a level of 11 grams per day.
As Dr. Shu said, “…we found the higher the intake, the lower the mortality, up to 11 grams of soy protein…”, adding that after 11 grams daily the benefit leveled off but did not decline (HDN 2009).
Interestingly, the analysis detected no difference in risk between women whose soy intake was in forms high in isoflavones, the estrogen-blocking factor in soy, found mostly in whole, un-fractionated soy foods like tofu... compared to those whose soy intake was dominated more by soy-protein products with low isoflavone levels.
And, in line with prior studies, the same risk reductions were seen in women with either estrogen receptor-positive or receptor-negative breast cancer and in users as well as nonusers of tamoxifen.
In fact, there's little evidence that soy interferes with estrogen-mimicking drugs like tamoxifen, which (like soy isoflavones) block estrogen receptors in breast tissue and tumor cells, thereby curbing the growth of estrogen receptor-positive cancers.
As the Tennessee team concluded, “…we found that soy food intake is safe and was associated with lower mortality and recurrence among breast cancer patients… This study suggests that moderate soy food intake is safe and potentially beneficial for women with breast cancer” (Shu XO et al. 2009).
Researchers favor whole soy foods, not supplements
Dr. Shu notes that women in China—where breast cancer rates are lower than here—get almost all of their soy from whole foods, such as tofu, edamame (salted whole soybeans), or unsweetened soy milk, rather than the processed soy foods that many Americans eat, such as sweetened, flavored soy milk or soy-based protein bars.
In an editorial accompanying the published study, experts from the U.S. National Cancer Institute noted that the average daily soy intake for people living in China makes up 10 percent or more of their daily protein consumption.
Dr. Shu advises against loading up on soy supplements, which haven’t been proven to be beneficial for breast health … especially since it’s unclear if very high levels of processed soy protein could cause harm.
As she said, “Soy food intake has been shown to reduce the risk of breast cancer, and it may have cardiovascular benefit, so overall, whether or not you have cancer, soy could be very beneficial to you and could become an important component of a healthy diet. But try to get it in natural sources, not from processed food” (HDN 2009).
We couldn't agree more!
- Boyapati SM, Shu XO, Ruan ZX, Dai Q, Cai Q, Gao YT, Zheng W. Soyfood intake and breast cancer survival: a followup of the Shanghai Breast Cancer Study. Breast Cancer Res Treat. 2005 Jul;92(1):11-7.
- Christiane Northrup, M.D. (CN). Is Soy Still a Healthy Choice? Accessed at http://www.drnorthrup.com/members/healthwisdom/topic_details.php?id=288
- 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. 2009 Nov;118(2):395-405. Epub 2009 Feb 17.
- HealthDay News (HDN). Soy Beneficial for Breast Cancer Survivors. December 08, 2009. Accessed at http://www.womenshealth.gov/news/english/633839.htm
- Lee SA, Shu XO, Li H, Yang G, Cai H, Wen W, Ji BT, Gao J, Gao YT, Zheng W. Adolescent and adult soy food intake and breast cancer risk: results from the Shanghai Women's Health Study. Am J Clin Nutr. 2009 Jun;89(6):1920-6. Epub 2009 Apr 29.
- National Cancer Institute (NCI). Study of Tamoxifen and Raloxifene (STAR) Trial, 2007. Accessed at http://www.cancer.gov/clinicaltrials/digestpage/STAR and http://www.nsabp.pitt.edu/STAR/Index.html
- Shu XO, Zheng Y, Cai H, Gu K, Chen Z, Zheng W, Lu W. Soy food intake and breast cancer survival. JAMA. 2009 Dec 9;302(22):2437-43
- van de Velde CJ, Verma S, van Nes JG, Masterman C, Pritchard KI. Switching from tamoxifen to aromatase inhibitors for adjuvant endocrine therapy in postmenopausal patients with early breast cancer. Cancer Treat Rev. 2009 Nov 25. [Epub ahead of print]
- Weston A. Price Foundation (WAP). Soy Alert! Accessed at http://www.westonaprice.org/soy/index.html