Soy and Breast Cancer: What Does the Science Say?

IMPORTANT: Please discuss the information below with your oncological and nutritional team prior to making changes to your diet! They will be able to provide you with the proper guidelines for your situation.

One common area of contention within the context of hormone-positive breast cancer is the effect of soy consumption on cancer risk. There has been some back-and-forth on this topic, and “to soy or not to soy?” is a frequently-heard question coming from newly-diagnosed cancer patients.

It was a concern for me. I became vegetarian at age 18 and consumed a soy-heavy diet until my mid-40s, at which point, partly spooked by warnings about soy, I backed off. As recent research shows, I needn’t have.

For a little background, the main concern for breast cancer patients is the presence of phytoestrogens in soy, known as isoflavones, and how they function in the human body. They have a mild estrogenic effect, which is why many women use them in supplement form to ameliorate the uncomfortable effects of menopause. In that sense, they are acting like estrogen, although it’s important to stress that they are not estrogen.

Soy (here, both tofu and edamame) is a staple in Asian and vegetarian cuisines, and is the only plant-based protein that is a complete protein.

But given this similarity to estrogen, does soy increase the risk of breast cancer and breast cancer recurrence? In short, studies show that if you grew up eating soy and eat it daily, as is the case in many Asian countries where soy products are dietary staples, soy has a significant protective effective against breast cancer. Results of these studies have been inconclusive in Western populations, however this seems to be due to differences in diet: not only do Westerners eat considerably less soy compared to Asians, they also don’t eat it throughout all stages of their lives.

Is there a difference in how these diverse cultures handle isoflavones? It appears that a major isoflavone-derived metabolite, equol, has well-documented antioxidant and estrogen-like actions and seems to be associated with numerous positive outcomes, but only about 30-50% of the human population has the gut microbiota to derive it from the diet. There is a need for more research on how this conversion takes place and under what conditions.

But most importantly, as stated by the American Institute for Cancer Research, “Population studies don’t link soy consumption with an increased risk of any cancer.” While the childhood and adolescent consumption of soy is what seems to offer the most long-term benefits, for those who increased their intake at a later age or don’t eat it regularly, the current view is that even if eating soy doesn’t significantly reduce your risk of cancer, there is no definitive evidence that it will make your risk worse.

For me, that means that I will continue using soy as an important protein source in my diet.

Take note:

As with other foods, unprocessed and minimally-processed soy is still the healthiest option.
  • Overdoing anything is not good, so don’t overload on overly processed soy supplements in the hopes of preventing cancer development and/or recurrence — particularly if you’re postmenopausal and not a life-long soy eater. Having said that, there is ample room for minimally-processed soy foods (tofu, edamame, tempeh, miso) in a healthy plant-based diet, and that will definitely benefit you.
  • No single thing will prevent cancer 100%, so you’d be well-served to consider your lifestyle as a whole. As a matter of fact, Zhang et al. (2017, Cancer) noted that “[w]omen who consumed high levels of dietary isoflavone were more likely to be Asian Americans, young, premenopausal, physically active, more educated, not overweight or obese, never smokers, and drank either no alcohol or <7 drinks per week.” [Emphasis mine.] That means protection came not only from soy; the women were also engaging in other behaviors associated with a lower risk of breast cancer. Bottom line, lifestyle matters!

Finally, this is only a brief summary of what I found. Soy is a topic that I’ll be keeping my eye on and will report back as newer studies are published.

In the meantime, here are three excellent reader-friendly websites for more information:

1. American Institute for Cancer Research website, “Soy: Intake Does Not Increase Risk for Breast Cancer Survivors – https://www.aicr.org/cancer-prevention/food-facts/soy/

2. Harvard School of Public Health website, “Straight Talk About Soy” – https://www.hsph.harvard.edu/nutritionsource/soy/

3. Oncology Nutrition website, “Soy and Breast Cancer” – https://www.oncologynutrition.org/erfc/healthy-nutrition-now/foods/soy-and-breast-cancer

References for this post (all articles are available online free of charge):

Baglia ML, Zheng W, Li H, Yang G, Gao J, Gao Y-T, Shu X-O (2016) The association of soy food consumption with the risk of subtype of breast cancers defined by hormone receptor and HER2 status. Int J Cancer. 139: 742–748. https://doi.org/10.1002/ijc.30117

Kucuk O (2017) Soy foods, isoflavones, and breast cancer. Cancer. 123: 1901-1903. https://doi.org/10.1002/cncr.30614

Lee SA, Shu XO, Li H, Yang G, Cai H, Wen W, Ji BT, Gao J, Gao YT, Zheng W (2009) Adolescent and adult soy food intake and breast cancer risk: results from the Shanghai Women’s Health Study. Am J Clin Nutr. 89: 1920-1926. https://doi.org/10.3945/ajcn.2008.27361

Mayo B, Vázquez L, Flórez AB (2019) Equol: A bacterial metabolite from the daidzein isoflavone and its presumed beneficial health effects. Nutrients. 11: 2231. https://doi.org/10.3390/nu11092231

Messina M, Rogero MM, Fisberg M, Waitzberg D (2017) Health impact of childhood and adolescent soy consumption. Nutr Rev. 75: 500–515. https://doi.org/10.1093/nutrit/nux016

Nagata C (2010) Factors to consider in the association between soy isoflavone intake and breast cancer risk. J Epidemiol. 20: 83-89. https://doi.org/10.2188/jea.je20090181

Patisaul HB, Jefferson W (2010) The pros and cons of phytoestrogens. Front Neuroendocrinol. 31: 400–419. https://doi.org/10.1016/j.yfrne.2010.03.003

Wu AH, Yu MC, Tseng C-C, Pike MC (2008) Epidemiology of soy exposures and breast cancer risk. Br J Cancer. 98: 9-14. https://doi.org/10.1038/sj.bjc.6604145

Zhang FF, Haslam DE, Terry MB, Knight JA, Andrulis IL, Daly MB, Buys SS, John EM (2017) Dietary isoflavone intake and all‐cause mortality in breast cancer survivors: The Breast Cancer Family Registry. Cancer. 123: 2070-2079. https://doi.org/10.1002/cncr.30615

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When I asked my oncologist about soy, he shrugged and said, “Yes, it’s true that soy foods contain plant estrogens…but you’re not a plant.”

Author: franticshanti

Why so serious?

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