Breast cancer and soy: what you should know

Soy has received mixed reviews in recent years, leading to confusion about it and potential health benefits or risks. The link between soy and breast cancer has been particularly convoluted. Should you eat soy in large amounts to lower your risk? Or avoid it due to the potential estrogenic effects? Are the health benefits backed by research, or is it a result of slick marketing campaigns?

The data about breast cancer in Canadian women is staggering. Breast cancer is the most common cancer among women in Canada. 1 in 9 women will develop breast cancer during their lifetime, and 1 in 28 will die from this disease. Interestingly, breast cancer rates are significantly lower in Asian countries. But when Asian women come to North America, their rates increase to near that of Western women, suggesting an environmental issue, rather than race. The most obvious dietary difference between western and Asian lifestyle is soy, which has led to intense research into soy, and whether it does in fact contribute to lower rates of breast cancer in these geographical areas.

What’s so good about soy?

As far as whole foods go, soy definitely holds its nutritional weight. Soy contains protein. It is one of the few plant proteins that is a “complete” protein – which means it contains all the building blocks (amino acids) to make a protein in humans who eat it. Since evidence shows people who follow a primarily plant-based diet have a lower risk for some diseases, soy fits the bill as a healthy, protein-rich alternative to animal meats. It also is full of vitamins and minerals, and omega-3 fats (you, the good ones). The other (and arguably most important) component in soy is something called “isoflavones”.

Isoflavones have been toted as one of the components that makes a “superfood” (I term I try to avoid) so super. The controversy surrounding those in soy is due to the fact that they are “phytoestrogens” and structurally similar to the human estrogen hormone. Since estrogen levels have been linked to some cancers in humans, the question is whether these “phytoestrogens” in soy can increase the risk for estrogen-responsive cancers, or reduce the risk by preventing against hormone-dependent cancers.

What the research shows

Some research has shown a reduced risk of breast cancer with moderate intakes of soy (equivalent to about ½ cup of soy milk per day). However, not all studies have found this relation. Some research has shown that dietary patterns that are vegetable, fruit and soy based, appear to reduce the risk of breast cancer by about 30%.

But before you run out to the grocery store to stock up on tofu and soy milk, know this: the age of exposure appears to be very important in that, the earlier you start eating soy products, the greater the protective benefits. Adding soy to your diet later in life may have a much smaller benefit, or none at all. This may help explain some of the differences in findings between different studies.

The bottom line

For healthy women, soy foods are a good addition to any healthy diet that focuses on variety and moderation. If you’ve been eating soy since your early years (prior to age 10), this may have some protective benefit for you. Higher doses of soy may have estrogen-like effects, which may contribute to growth of certain breast cancers. Thus, the recommendations for women with breast cancer, and breast cancer survivors are slightly different.

For women with breast cancer, health professionals are recommending women with breast cancer “take only moderate amounts of soy foods as part of a healthy, plant-based diet. They should not ingest very high levels of soy in their diet or take concentrated sources of soy (such as supplements) containing high amounts of isoflavones” (American Cancer Society Guidelines on Nutrition and Physical Activity).

And for the breast cancer survivor, current research suggests that 3 or less servings of soy per day is safe, and does not have any special benefit nor harmful effects “as part of a healthy diet”. It is recommended that breast cancer survivors avoid high doses of soy isoflavones, such as powders and supplements.

Visit the American Cancer Society or Canadian Cancer Society for more information.