Even though the findings by Shu et al suggest that consumption of soy foods among breast cancer patients is probably safe, studies in larger cohorts are required to understand the effects of these foods among diverse clinical subgroups of breast cancer patients and survivors. In the meantime, clinicians can advise their patients with breast cancer that soy foods are safe to eat and that these foods may offer some protective benefit for long-term health. Moreover, the potential benefits are confined to soy foods, and inferences should not be made about the risks or benefits of soy-containing dietary supplements. 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.
The study at issue looked at the relationship between soy consumption and breast cancer survival in 5042 women diagnosed with breast cancer in China. Their big result is that 4-year mortality rates are about 3% lower in women who consumed the most soy, compared with those who consumed the least.
What's interesting about the question that motivated this study is that the authors weren't just asking, 'does soy help prevent breast cancer recurrence?' Like many bioactive compounds in food, the question has a potential negative side - soy has components that could potentially cause harm to breast cancer patients:
Soy foods are rich in isoflavones, a major group of phytoestrogens that have been hypothesized to reduce the risk of breast cancer. However, the estrogen-like effect of isoflavones and the potential interaction between isoflavones and tamoxifen have led to concern about soy food consumption among breast cancer patients.
The question is worth keeping in mind when people give you dietary advice that is supposed to help treat or prevent disease. The in vivo effects of anti-oxidants, isoflavones, and what not are not always clear, and there is often a legitimate concern about the potential for harm if you take them in some highly concentrated form like a pill.
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