When I’m asked to speak about how diet and lifestyle impact breast cancer risk, whether I’m speaking to health professionals, cancer survivors or the general public, one of the most-asked questions involves the relationship of soy to breast cancer.
Is it protective? Is it a risk? Should breast cancer survivors avoid it? And what about the soy protein now added to cereals, bars, breads and meat substitutes?
The Back-and-Forth Story
Much of the positive – and negative – thoughts about soy involve phytochemicals (natural plant compounds) it contains known as isoflavones (primarily one called genistein).
♦ First it was thought that decreased rates of breast cancer in Asian women might stem from their consumption of soy’s phytoestrogens. These compounds are known as phytoestrogens because they are similar in chemical structure to human estrogen. It was thought that if the much, much weaker phytoestrogens from soy snapped into estrogen receptors on cells, then the receptors would be unavailable to the more powerful body estrogen, thus reducing development of estrogen-related cancers.
♦ Then some early lab studies suggested that genistein increased growth of estrogen receptor-positive breast cancer cells, and promoted growth of mammary cancer in rodents. Since high levels of estrogen in people link with increased risk of breast cancer, concern developed that even the weaker phytoestrogens might pose risk, rather than offer protection.
♦ But animal studies don’t provide perfect answers about cancer risks in humans. Rats and mice (the animals generally studied) metabolize isoflavones differently than humans. So the harmful effects seen in rodents related to their very high blood levels of isoflavones aren’t grounds for expectations in humans. On the other hand, some of the protective effects seen in cell studies involved concentrations of isoflavones higher than would occur from eating soy.
We need to look at a bigger picture, however. Soy’s effects seem to reach beyond estrogen-related mechanisms.
♦ Isoflavone compounds are antioxidants, which means they have potential to help prevent damage to DNA that can begin cancer development.
♦ In cell and animal studies, soy phytochemicals could inhibit growth of cancers, apparently by reducing inflammation and decreasing activation of enzymes and proteins that promote uncontrolled cell growth and stimulating self-destruction of abnormal cells.
Importance of Human Studies
Laboratory studies with cells and animals allow us to study and test hypotheses about mechanisms and how individual components of food might affect steps in the development of cancer. But – despite the headlines that sometimes appear — we must not assume that findings from laboratory studies can be directly translated to what we can expect in people.
Some population studies link soy consumption with lower breast cancer risk. These findings come mainly from Asia, where most women consume moderate amounts of soy throughout life, rather than from Western population studies.
Why It’s So Confusing
So soy foods, such as tofu, tempeh, edamame and soymilk, are safe and do not pose cancer risk. But is including soy foods as part of your healthy eating habits “neutral” in effect, or will it actually help reduce breast cancer risk? The answer, it seems, may be “it depends”….
- Research now suggests that hormone-related protection against breast cancer may relate to soy consumption before or around puberty as breast cells are in transition to their mature form.
- Soy may affect cancer risk in some of us more than in others, due to individual genetic differences in how soy compounds are metabolized. Differences in the types of gut bacteria that ferment soy compounds could also explain why some people may benefit more than others.
- Population studies often divide people studied into four or five groups based on how much of a particular food they consume. Interpreting these findings correctly makes it important to consider how much the “high” and “low” consumers are each eating. In studying soy, the amount consumed by “high” consumers in the U.S. is quite different from “high” consumption in Asia. That may also be part of why findings in these two groups may differ. (Come back for more on this in the next Smart Bytes® post!)
What About ER+ Breast Cancer Survivors?
For a while, we didn’t have any studies about soy that included large numbers of breast cancer survivors. Following the long-standing standard of medical care — first, do no harm – the message to breast cancer survivors about soy was caution, just in case it might pose risk. Women who had Estrogen Receptor-positive (ER+) cancer, especially if taking tamoxifen (a medication that works through estrogen receptors), have often been warned to be extra-cautious about any food with soy… just to be on the safe side.
Today, however, we do have research that included lots of ER+ breast cancer survivors. In the past few years, five population studies and one major analysis of several studies all found that women who’d had ER+ breast cancer showed either decreased recurrence or decreased deaths related to moderate consumption of soy foods, or no effect. None of the studies showed an increase in risk for ER+ women.
Three studies were conducted in Asia. We need to be careful about applying results to U.S. women, because often these women have been consuming soy all their lives.
♥ In a study of over 500 Chinese breast cancer survivors, women with ER+/PR+ (progesterone receptor-positive) cancer showed decreased risk of recurrence among those with greater soy food consumption. Those with ER+/PR- or ER-/PR+ showed no effect. In another analysis separating groups, women taking tamoxifen showed no link between soy consumption and recurrence; those taking an aromatase inhibitor (another medication for ER+ breast cancer) showed decreased risk of recurrence with greater soy food consumption.
♥ In another study of 5042 Chinese breast cancer survivors, among all survivors considered together, and among women with ER+ breast cancer, soy consumption was linked with decreased breast cancer recurrence or breast cancer-specific death, and decrased overall deaths.This was true regardless of whether women were taking tamoxifen or not taking tamoxifen (researchers analyzed separately to see if this made a difference).
♥ In the most recent study of 616 Chinese breast cancer survivors, greater consumption of soy and greater consumption of the isoflavone compounds it contains were both associated with decreased deaths from breast cancer. This reduction was even stronger for women with ER+ breast cancer than for the group as a whole.
Two studies of breast cancer survivors were conducted in the U.S., and thus are especially of interest to American women, who likely have not been life-long soy consumers.
♥ In a report from the famous WHEL study of more than 3000 American women breast cancer survivors, greater soy consumption had no statistically significant effect on breast cancer recurrence, but was associated with a trend for reduced overall mortality (although this was not statistically significant). Soy showed more benefit among women with ER+ or PR+ breast cancer.
♥ Among 2280 American women in the LACE study, greater isoflavone consumption was linked with decreased risk of recurrence among women who were ER+, especially those who were postmenopausal, and especially those who used tamoxifen. It’s important to note that “high” consumption was not from taking large doses of isoflavone supplements, but comparable to typical diets in Asia.
We also have a pooled analysis of several studies, which includes both Asian and U.S. women.
♥ Among over 9500 women in the After Breast Cancer Pooling Project, greater soy consumption was linked with a statistically significant decreased risk of breast cancer recurrence, as well as with a trend (not statistically significant) for decreased overall deaths and death due to breast cancer. When analyzed separately, this decrease in recurrence was statistically significant only for postmenopausal women. (There was no risk seen for premenopausal women, it’s just that decreased recurrence could have been due to chance.) And in contrast to the studies above, while greater soy consumption was linked to reduced risk of recurrence among both ER+ and ER- women, the link was only statistically significant among ER- women; in those with ER+ cancer the association was not statistically significant (again – not showing any risk, but the 20% decrease in recurrence in this group was a trend that could also have been due to chance).
Once again, it’s important to note that the decreased recurrence seen among “high” consumers does not refer to massive amounts of soy. In the U.S. studies, a few servings of soy foods per week put a woman in this group.
So what is “moderate” consumption of soy foods? And what about the soy protein being added to so many breads, bars and other foods? Check back for the next Smart Bytes® post for clarity on these important questions!
You can read an update from the American Institute for Cancer Research (AICR) about soy for breast cancer survivors. The most recent nutrition guidelines for cancer survivors from the American Cancer Society also indicate no apparent harmful effects of soy consumption on breast cancer recurrence or survival. Each breast cancer survivor should make decisions about these matters in discussion with her personal physician; these materials may be helpful to read ahead and share.
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