Soy foods have a lot of isoflavones, which are weak estrogen-like compounds found in plants. Because estrogen can promote the development, growth, and spread of breast cancer, doctors have worried that eating a lot of soy foods or soy isoflavones (which can be taken as a dietary supplement) might worsen the prognosis of women diagnosed with breast cancer. The study reviewed here found that Chinese women diagnosed with breast cancer who ate a diet rich in soy foods had a lower risk of dying of breast cancer and a lower risk of breast cancer coming back (recurrence) compared to women diagnosed with breast cancer who didn't eat a lot of soy.
In this study, researchers looked at the medical histories and soy consumption of 5,042 Chinese women (age 20 to 75) diagnosed with breast cancer. Over 4 years of follow-up, 444 of the women died and 534 had breast cancer come back. The researchers found:
Some women in the study were taking tamoxifen, a hormonal therapy medicine that blocks the effect of estrogen on breast cancer cells. Tamoxifen is often prescribed to lower the risk of recurrence in women diagnosed with early-stage, hormone-receptor-positive breast cancer. Surprisingly, the women not taking tamoxifen who ate the most soy seemed to have a better prognosis than women who took tamoxifen and ate the least soy.
The results suggest that a diet rich in soy doesn't worsen prognosis in women diagnosed with breast cancer and may offer some protection against recurrence.
Soybeans are the most widely used, least expensive, and least caloric way to get large amounts of protein with very little fat and no cholesterol. You can eat soybeans in many forms, including tofu, the beans themselves (also known as edamame), soy milk, miso, and soy powder. Asian women typically eat much more soy than women in Western countries (about 10 times more). Still, other research has shown that Asian women have lower rates of hormone-receptor-positive breast cancer than women in the United States.
The relationship between soy and breast cancer prognosis is complicated by other factors. Most women living in Asia depend on soy as their main source of protein. They consume only small amounts of beef, chicken, and pork -- which means less animal fat and other possibly unhealthy substances (such as growth hormones and antibiotics) in these animal protein sources. Also, compared to the average woman in the United States, the average Asian woman:
All of these other factors add up to produce a healthier lifestyle and may have contributed to the better breast cancer prognosis seen in the Chinese women in this study who ate the most soy.
In the Breastcancer.org pages on Nutrition and Breast Cancer Risk Reduction, you can learn more about foods that may keep you as healthy as you can be.
Soy consumption appears to be safe, and potentially even protective, for women with breast cancer despite fears about estrogen-like effects, according to a population-based study.
Chinese breast cancer patients who ate the most soy -- more than 15.31 g soy protein per day on average -- had a significant 29% lower risk of dying than those who consumed the least (5.31 g or less), according to Xiao Ou Shu, MD, PhD, of Vanderbilt University Medical Center in Nashville and colleagues.
A similar benefit appeared for breast cancer recurrence risk, regardless of tamoxifen use, they reported in the Dec. 9 Journal of the American Medical Association.
While American women typically eat less than one-tenth as much soy as their counterparts in China, the results should be reassuring regardless of consumption, according to an accompanying editorial.
Editorialists Rachel Ballard-Barbash, MD, MPH, of the National Cancer Institute, and Marian L. Neuhouser, PhD, of the Fred Hutchinson Cancer Research Center in Seattle, cautioned that any potential benefits cannot be extrapolated to dietary supplements containing soy.
But "patients with breast cancer can be assured that enjoying a soy latte or indulging in pad thai with tofu causes no harm and, when consumed in plentiful amounts, may reduce risk of disease recurrence," they wrote.
Concerns about soy have centered on its rich supply of phytoestrogens -- mainly isoflavones -- that act on estrogen receptors and may compete with tamoxifen.
While soy studies have been mixed, showing both estrogen-like and antiestrogenic properties, animal experiments suggested that one of the major soy isoflavones enhanced proliferation of breast cancer cells and promoted estrogen-dependent breast tumor growth.
So Shu's group analyzed the Shanghai Breast Cancer Survival Study, a population-based cohort study of 5,042 women in China ages 20 to 75.
During an average of 3.9 years of follow-up, vital statistic registry data and medical chart reviews revealed that 444 women died and 534 had a recurrence or breast cancer-related death.
Soy intake showed similar inverse associations with these negative outcomes whether consumption was measured by soy protein or soy isoflavones in foods reported on a habitual intake questionnaire.
Soy protein intake in the highest quartile, more than 15.31-mg-per-day, was associated with an all-cause mortality hazard ratio of 0.71 (95% CI 0.54 to 0.92) compared with the lowest group, 5.31-mg-per-day or less.
Similarly, women who ate the most soy isoflavones (more than 62.68 mg/day) had an all-cause mortality hazard ratio of 0.79 (95% CI 0.61 to 1.03) compared with those in the lowest quartile (20.00 mg/day or less).
For recurrence and breast cancer-specific mortality, the highest versus lowest intake quartiles had a 32% lower risk with soy protein (HR 0.68, 95% CI 0.54 to 0.87) and 23% lower risk with soy isoflavones (HR 0.77, 95% CI 0.60 to 0.98).
Benefits of soy appeared to increase with intake up to 11 g of soy protein or 40 mg of soy isoflavone intake per day, then level off or even reverse, the researchers said.
Higher soy intake tended to be associated with lower mortality and recurrence rates regardless of tamoxifen use, although point estimates were generally not significant, which the researchers attributed to small sample sizes.
Notably, the highest soy intake in women who didn't take tamoxifen was associated with better outcomes than the lowest soy intake in women who did take the drug, "suggesting that high soy food intake and tamoxifen use may have a comparable effect on breast cancer outcomes," the researchers wrote.
Likewise, the associations did not vary by menopausal status, cancer stage, or estrogen receptor status of a woman's breast cancer.
Since soy isoflavones were not more strongly related to outcomes than soy protein, other constituents, alone or in combination, might be responsible for the effects, Shu's group speculated. Another explanation is that isoflavone intake is more difficult to estimate, with larger measurement errors, they said.
Higher soy intake also accompanied other aspects of a healthy lifestyle, such as more exercise and higher vegetable and fish consumption, that the analyses may not have been able to entirely eliminate as a source of confounding, the researchers cautioned.
The editorialists additionally cautioned that traditional, whole soy foods, such as whole soy beans and tofu, are more common in Shanghai, while processed soy in supplements and meat substitutes are more common in the U.S.
"Clinicians should be aware of these dietary differences in the quantity and quality of soy foods when advising patients," Ballard-Barbash and Neuhouser wrote.
Replication of these findings with longer follow-up, greater statistical power for subanalyses, and better defined diagnosis and treatment of breast cancer will be important, they concluded.
The study was supported by grants from the U.S. Department of Defense Breast Cancer Research Program and the National Cancer Institute.
Shu reported having received research development funding from the United Soybean Board in 2005.
Ballard-Barbash and Neuhouser reported no conflicts of interest.
Primary source: Journal of the American Medical Association Source reference: Shu XO, et al "Soy food intake and breast cancer survival" JAMA 2009; 302: 2437-43.Additional source: Journal of the American Medical AssociationSource reference: Ballard-Barbash R, Neuhouser ML "Challenges in design and interpretation of observational research on health behaviors and cancer survival" JAMA 2009; 302: 2483-84.
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