Soy and Breast Cancer Recurrence

It's common for women to question the recommendation to add soy to their diet when they have a history of breast cancer (or any other estrogen-related cancer). For years women were told to avoid the "phytoestrogens," aka plant estrogens, that were present in foods like soy, yams, flax seed, and even marijuana. The idea was that if a woman had an estrogen sensitive cancer, often referred to as estrogen receptor positive (ER+), then the phytoestrogen might be an agonist for the receptor. That means that the phytoestrogen could stimulate the receptor the same way that a human estrogen could.

In medicine it's fairly common for recommendations to evolve. We used to think it was ok to smoke on airplanes, and once thought high fructose corn syrup was a wonder food of modern science.

Turns out that the phytoestrogens in soy and other foods are much more likely to have a cancer inhibiting effect rather than a cancer promoting one. Take this for example: the main phytoestrogens in soy are called isoflavones, and include compounds like genistein and daidzein. In Japan women consume between 25-50mg of soy isoflavones daily. In the US, women typically consume less than 3 mg. That is a big difference. If soy was really a risk factor for breast cancer wouldn't we think that Japanese women would have high levels of breast cancer? In reality the opposite is true; in Japan, 1 out of every 38 women is diagnosed with breast cancer. In the US, 1 out of every 8 women is diagnosed.

Because laboratory studies investigating soy's effect on breast cancer initially showed mixed results, women were told to minimize soy consumption when they had been diagnosed with cancer. However, soy started creeping into the American diet at a pretty rapid rate in the early 2000's. In 1997 about 15% of the US population ate soy weekly; by 2003, that number had increased to 28%. As a result, there was some worry that women might be being exposed to a cancer risk factor.

So, a large study was undertaken to try and determine if there really was a risk. This study, named the Shanghai Breast Cancer Survival Study, recruited 5042 breast cancer survivors and tracked their diagnosis, treatments, lifestyle exposure, and disease progression between 2002 and 2009. The study ended up being published in the Journal of the American Medical Association in 2009. The findings showed that women who ate more soy foods had lower risks of dying or having their cancer come back. This was true for BOTH estrogen sensitive and estrogen insensitive (ER-) breast cancer, BOTH early and late stage disease, and didn't depend if a woman was pre- or postmenopausal.

Astonishingly, women who consumed the most soy and DID NOT take Tamoxifen had similar, or even better, survival rates compared to women who took tamoxifen but ate little soy. Tamoxifen is a drug called a selective estrogen response modifier (SERM) that blocks estrogen receptors. It is frequently used in premenopausal women diagnosed with ER+ breast cancers to limit the effect that their naturally occurring estrogens may have on cancer cells. Per the authors, "Among women whose soy food intake was in the highest quartiles, tamoxifen use did not appear to confer any additional benefit."

Another study that was published about the same time as the Shanghai study reported that women who ate the highest levels of soy- in this study measured as intakes of specific isoflavones- had an approximately 50% reduction in recurrence rates when also treated with tamoxifen.

It's thought that soy might offer it's protective effect via a couple different mechanisms. The isoflavones might compete with estrogen for receptor binding, reducing estrogen's effect. Flax seed has also shown this ability. However, the phytoestrogens might also work by affecting the estrogen receptors themselves. Estrogen isn't just estrogen, it's a family of hormones that includes estrone, estradiol, and estriol. There isn't just a single estrogen receptor, either; there are alpha receptors and beta receptors. These different estrogens and different estrogen receptors work in dizzyingly complex ways depending on tissue type, cancer type, and even sex. Broadly speaking, in breast cancer, the research is suggesting that the estrogen alpha receptors can act as promoters of breast cancer and are stimulated by human estradiol. The estrogen beta receptors, which are stimulated by estriol, and most phytoestrogens, are thought to be protective in breast cancer.

So how much soy should a woman eat? The Shanghai study found that soy offered it's protective effect up to an intake of 11 grams of soy per day, or 40mg of isoflavones. Intakes higher than this did not provide any additional protection. Below is a table showing isoflavone content in popular soy foods. As you can see it does not require a huge intake to receive the potential benefit:

It should be noted that there is one soy product that Americans consume a lot of, soy oil. Estimates put annual consumption of soy oil between 38 and 81 pounds per year. Unfortunately, soy oil contains ZERO isoflavones. Soy oil is made up of omega-6 polyunsaturated fatty acids, which when consumed in an out of balance fashion with omega-3 oils can cause problems. Omega-6 fats can be thought of as inflammatory precursors, whereas omega-3 fats have an anti-inflammatory effect. Soy oil consumption will not impart the benefits gotten through consuming other soy foods, and may even predispose to obesity, diabetes, and depression. It's hard to avoid consuming soy oil, it's everywhere, so a good move is to consume an adequate amount of omega-3's to compensate. Freshly ground flax is a good choice.

There is some concern about consuming enormous amounts of soy products besides soy oil. As the saying goes, too much of anything can be a bad thing. Based on today's research is seems that consuming a moderate amount of soy is safe and can offer protection against a recurrence of breast cancer.

If you're looking for a two-fer consider eating soybeans or edamame, as they contain a decent amount of resistant starch, which is another potent anti-cancer food as well as healthy levels of isoflavones.

Over 90% of soybeans grown in the US are now genetically modified. The most common genetic modification is herbicide tolerance. This allows farmers to spray their soy crops with herbicides that will kill the weeds, but not the soy plants. Just about everyone who is even minimally interested in health has likely heard of the herbicide glyphosate. Glyphosate used to be the main ingredient in the weedkiller Roundup. The initial promise of crops genetically modified to tolerate herbicides was reduce the use the weedkiller. That hasn't quite worked out, as the use of glyphosate has increased 15x since the 1990's. Glyphosate isn't the main ingredient in Roundup anymore, as the formula was changed when glyphosate was classified as a "probable human carcinogen" in 2015 by the International Agency for Research on Cancer. With that characterization many countries outlawed it's use. Not the US, though. Nowadays, Roundup contains easy to pronounce chemicals like triclopyr, fluazifop, diquat, and imazapic.

Aside from being being sprayed with a "probable carcinogen," GMO crops may impart other risks to human wellbeing. One of the major concerns is a disruption of our beneficial bacteria, a condition referred to as dysbiosis that is caused by herbicide residue on food. Roughly 1-3% of a person's weight is made up of bacteria, and these bacteria are intimately involved with our health. When we don't have the beneficial bacteria in our bodies we are at risk for many health issues. Probiotics, resistant starches, fermented foods, and fiber can help maintain populations of healthy bacteria. Another concern in that GMO foods can cause gastrointestinal inflammation and damage. A 2013 study found that pigs fed GMO corn and soy experienced higher rates of stomach inflammation. Other studies have associated chronic glyphosate exposure with intestinal permeabilit (this one too) and changes in neurodevelopment and increasing the risk of autism.

There are two sides of every coin, and both publications and opinions exist supporting the notion that GMO foods are safe for human consumption and pose no risk to human health. However, as with many things, like smoking on planes and high fructose corn syrup, our recommendations on consuming GMO crops is very likely to evolve with time.