Soyfood Consumption in Breast Cancer Survivors: Don't Overstate the Facts!
Soyfood Consumption in Breast Cancer Survivors: Don't Overstate the Facts!
Soyfoods are consumed by many because of cultural factors, for potential beneficial effects on overall health, and for the unproven hope that they will ease menopausal symptoms in women. As Messina and colleagues nicely note in their article, over the last couple of decades the pendulum has swung back and forth concerning the effect of soy on breast cancer risk. In the 1980s, soy products were on the National Cancer Institute’s list of potential chemoprevention agents, based largely on the lower incidence of breast cancer in populations in the Far East, populations well known to consume more soy products than do Western populations. Subsequently, however, soy products generated breast cancer–related fear, based on cell culture and xenograft data; this information is also nicely reviewed by Messina et al. The authors additionally note that, in the recent past, many patients with a new diagnosis of breast cancer decreased or eliminated their consumption of phytoestrogen-containing soyfoods. This change is based, presumably at least in part, on healthcare provider recommendations. Newer data, supporting the notion that soy products are more safe than dangerous, are also thoughtfully reviewed by Messina and colleagues, leading them to suggest that this pendulum should swing more towards an equilibrium position.
In reaching that conclusion, the authors comprehensively discuss the available basic research and clinical data regarding isoflavones in soyfoods, including literature about both their estrogenic and anti-estrogenic effects. These data basically delineate soy isoflavones as being selective estrogen receptor modulators, much in the way that tamoxifen and raloxifene (Evista) are categorized. Noting this comparison, it is worth remembering that it took tens of thousands of patients followed for many years to get a decent understanding of the effects of tamoxifen and raloxifene as chemopreventive agents. Similar data for soy products are neither currently available nor likely to be available in the future. These data, or lack thereof, should remind us that there are marked limitations in our knowledge regarding the effect of soyfoods on breast cancer–related issues.
Messina et al also review substantial epidemiologic data from observational studies regarding the potential risks and benefits of soy products. Keep in mind, however, that even more substantial amounts of data from observational studies were available regarding hormonal therapies (including estrogen alone and estrogen/progesterone combinations), and the fact that these data did not accurately predict what we learned about hormonal therapy through the Women’s Health Initiative prospective clinical trials. Results from these trials, which were not predicted by results from prior observational studies, demonstrated that these agents did not protect against cardiac vascular disease as previously thought, but, rather, increased the risk of cardiac conditions; that these agents did not protect against neurocognitive disorders, but, rather, increased the risk of those conditions; and that estrogen alone does not increase the incidence of new breast cancers.[7,8] These data should again remind us that there are marked limitations in our current knowledge regarding long-term effects of soyfoods on breast cancer–related issues.
Understanding that they should respect the limited knowledge regarding the effect of dietary soy on breast cancer–related issues, clinicians should be careful of what they proscribe for patients. Proscribing soyfood intake for patients who prefer to ingest soyfood is not justified based on the available evidence. As another recent example of our incorrect recommendations in the not-too-distant past, we proscribed upper-extremity exercise for women at risk for developing lymphedema, while new data support that we should prescribe it.
This rule about clinicians being careful of what they proscribe might take on further meaning if we look at the other side of the coin—that being, that healthcare providers should be wary of prescribing therapies that might appear to be of benefit but which have not been proven to be beneficial, noting that many such therapies are eventually proven to cause net harm. A few examples of these phenomena are not hard to detail, and include such things as arsenic treatment for cholera; high-dose chemotherapy with stem cell rescue for breast cancer; and radiation therapy for such conditions as acne and enlarged thymus glands.
To summarize the current review article, Messina et al state that “evidence to date convincingly suggests that soyfoods are not harmful to breast cancer survivors.” We, taking a slightly more conservative stance, might have changed a couple of words in that sentence so that it would read: “The evidence to date fails to support that soyfoods are harmful to breast cancer survivors.” We agree with the authors’ conclusion that patients who are already consuming soyfoods should not be discouraged from continuing to do so.
The story is different with regard to the use of soy as a pharmacologic product. At the present time, there is not good evidence to recommend pharmacologic doses of soy for any reason, including the treatment of hot flashes.
On a final note, the recommendations we make now are much in synch with those expressed in a review article about the use of soy in breast cancer survivors, written more than a decade ago. The concluding paragraphs from this review, which express sentiments that continue to be true today, are as follows:
“The available data…indicate a lack of any convincing information to substantiate either of two extreme and opposing claims, each of which has been prominently and repeatedly put forth in both the lay and scientific literature…: (1) soy is protective against breast cancer and because of this should be recommended for consumption by healthy women and breast cancer patients, and (2) soy is harmful for women with a history of or at high risk for breast cancer, and because of this should be avoided by such women.
The honest response to each of these diametrically opposed claims is that no convincing data exist to support either claim. In fact, there are strongly conflicting data regarding both. As such, if women (with or without breast cancer) enjoyed partaking of soy products, then it seems quite reasonable for them to partake of them. As with most things, moderation in intake is probably wise. In this regard, Asian soy intake may serve as a general guide for Western women.”
Financial Disclosure: The authors have no significant financial interest or other relationship with the manufacturers of any products or providers of any service mentioned in this article.
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