Comment:
A common question from breast cancer survivors is whether soy is safe to eat. This post breaks down a 2012 Chinese study that explored this topic. The researchers found a positive association between high soy intake and improved survival for breast cancer patients, particularly those with ER-positive tumors.
Summary:
Clinical Bottom Line
This prospective cohort study of 616 Chinese breast cancer patients suggests a strong association between higher soy isoflavone and soy protein intake and improved survival. The authors report that an intake above 17.3 mg/day of soy isoflavone was linked to a 36-38% reduction in mortality risk. This protective association appeared strongest in women with ER-positive tumors. However, as this is an observational study, these findings cannot prove causation. The results could be influenced by unmeasured confounding factors, such as high soy intake being a marker for an overall healthier lifestyle. The authors rightly conclude that a larger study with longer follow-up is needed.
Results in Context
Participants
The study included 616 breast cancer patients from a hospital in Inner Mongolia, China, recruited between 2004 and 2006. After a median follow-up of 52.1 months, 79 breast cancer-related deaths were recorded.
Main Results
After adjusting for potential confounders (like age, TNM stage, ER status, and tamoxifen use), the study found an inverse association between soy intake and mortality.
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Soy Isoflavone: Compared to the lowest intake group (7.56 mg/day), patients with an intake of 17.32 mg/day or more had significantly lower mortality risk.
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17.32 intake group: Adjusted Hazard Ratio (HR) 0.64 (95% CI: 0.45-0.93).
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>28.83 mg/day intake group: Adjusted HR 0.62 (95% CI: 0.42-0.90).
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Definition: A Hazard Ratio (HR) indicates the relative risk of an event (in this case, death) over time. An HR of $0.62$ suggests a 38% reduction in the hazard of death for the high-intake group compared to the low-intake reference group.
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Soy Protein: High soy protein intake (>13.03) was also associated with decreased mortality (HR 0.71, 95% CI: 0.52-0.98).
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ER Status: The protective association was more pronounced in women with ER-positive breast cancer. In this subgroup, the highest soy isoflavone intake (>28.83 mg/day) was associated with an HR of 0.59 (95% CI: 0.40-0.93).
Assertive Critical Appraisal
Limitations & Bias
The primary limitation of this study is its observational design, which cannot establish causation. High soy intake may simply be a marker for other healthy lifestyle behaviors (e.g., higher vegetable and fish intake, lower red meat intake), a possibility the authors acknowledge. This introduces a significant risk of unmeasured confounding that statistical adjustment cannot fully eliminate.
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Measurement Bias: Dietary data was collected via a food-frequency questionnaire (FFQ). This method relies on patient recall and is susceptible to recall bias, which may affect the accuracy of the intake estimates.
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Statistical Power: The authors correctly identify that the study’s conclusions are limited by a “limited number of patients” ($n=616$) and a “relative short follow-up period” (median 52.1 months). With only 79 deaths, the statistical power to definitively analyze subgroups is low.
Reporting Quality Assessment (STROBE)
The study’s reporting adheres to many STROBE principles. The authors clearly describe the study population, the methods for data collection, and the statistical models used. Crucially, they explicitly list the extensive set of covariates they used in the multivariate Cox regression models to address potential confounding.
Applicability
These findings are relevant to clinicians counseling breast cancer survivors, particularly as the safety of soy is a common patient concern. The results support previous studies suggesting soy does not adversely affect prognosis. However, the study was conducted in a specific Chinese population with a substantially higher baseline soy intake than most Western populations, which may limit the generalizability of these specific dose-response findings.
Research Objective
The study’s aim was to conduct a prospective study evaluating the association between soy isoflavone food consumption and breast cancer prognosis, with special attention to Estrogen Receptor (ER) status.
Study Design
This was a prospective observational cohort study. Patients were enrolled at diagnosis, and dietary habits were assessed via a structured questionnaire. Participants were then followed prospectively to determine survival. The study used Cox regression models to calculate adjusted hazard ratios for mortality based on quartiles of soy isoflavone and soy protein intake.
Setting and Participants
The study recruited 616 female breast cancer patients admitted to the Affiliated Hospital of Inner Mongolia Medical College of China between January 2004 and January 2006. All participants were permanent residents of Hohhot and had no prior history of cancer. Data on demographics, clinical factors (TNM stage, ER status), and lifestyle factors were collected via face-to-face interviews.
Bibliographic Data
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Title: Positive Effects of Soy Isoflavone Food on Survival of Breast Cancer Patients in China
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Authors: Ya-Feng Zhang, Hong-Bin Kang, Bi-Li Li, Rui-Ming Zhang
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Journal: Asian Pacific Journal of Cancer Prevention
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Year: 2012
This AI-generated analysis is for informational and research purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of a qualified health provider with any questions you may have regarding a medical condition.
Original Article:
Full text pdf: Positive Effects of Soy Isoflavone Food on Survival of Breast Cancer Patients in China