Should breast cancer patients eliminate soy from their diet?

Author: Sandra Krstev Barać, Master of Nutrition

Soybeans and minimally processed soy products such as tofu, tempeh and edamame green beans enjoy an almost cult status among plant-based diet advocates, primarily due to their significant proportion of protein with high biological value. However, proteins are not the only feature of this low-calorie food - soy is also a source of isoflavone phytoestrogens. Isoflavones have a similar chemical structure to estrogens, which is why they can compete for receptor sites and block their activity.

It is believed that the ability of isoflavones to act as an estrogen antagonist can provide protection against hormone-induced malignancies such as breast, prostate and endometrial cancer and is a possible reason why Asian women whose diet is traditionally rich in soy are less likely to suffer from these diseases. However, in the event that a malignant disease does occur, soybeans quickly become a notorious foodstuff.

Isoflavones, in fact, can also act as mild estrogens, which is why it is often believed that patients with hormone-positive breast cancer should eliminate soy from their diet, so that its potential estrogenic effect does not lead to disease progression or interfere with therapy. Moreover, once you develop a hormone-dependent malignant disease, even if you have defeated it, the advice often follows that soy should be permanently eliminated from your diet. But is this advice justified?

Should you eliminate soy if you are on tamoxifen therapy?

Until about ten years ago, all our knowledge about the impact of soy intake on breast cancer was based on research conducted on animal models, which showed that soy can stimulate the spread of malignant cells and even interfere with the action of tamoxifen, a drug from the group of selective estrogen receptor modulators, often used in the treatment of estrogen receptor-positive breast cancer. In the absence of better knowledge, the opinion that soy is an undesirable food when it comes to breast cancer has taken root.

In the meantime, we have significantly deepened our understanding of phytoestrogens, established that humans and animals metabolize isoflavones completely differently, and learned that previous knowledge about soy is unfounded. Therefore, it is not surprising that today we have a series of studies conducted on women who have survived breast cancer, which have determined that moderate soy intake has no harmful effect and may even reduce the risk of recurrence and mortality, regardless of the type of breast cancer. In addition, it has been shown that it is not necessary to give up soy even during tamoxifen therapy and that this combination may even have a synergistic beneficial effect.

What is considered moderate soy intake?

Experts agree that moderate soy intake is safe for everyone, including those currently undergoing treatment for and those who have recovered from not only breast cancer but all estrogen-sensitive malignancies. In fact, cancer survivors are advised to adopt a mostly plant-based diet with an emphasis on vegetables, fruits, whole grains, and legumes, and soy can serve as an important plant-based source of protein.

Moderate soy intake is generally considered to be 1-2 servings of whole soy and minimally processed soy products per day, with one serving being:

· ½ cup cooked soybeans

· ½ cup cooked edamame green beans

· 1 cup soy milk

· 1/3 cup tofu.

Be careful with isoflavone intake via dietary supplements

The question arises whether dietary supplements such as products containing soy isoflavones (e.g. genistein and dadzein) are also safe for breast cancer patients? Although it is speculated that the effects of such products are not harmful, the fact is that there is a lack of quality studies that would confirm their safety in the context of breast cancer. Therefore, the opinion of many experts is that it is better to “play it safe” and avoid these products in all hormone-positive malignant diseases.

Finally, it is worth being aware that processed soy products are ubiquitous in industrial foods. Soybean oil or soy lecithin are common ingredients in sweets, crackers, salad dressings, industrial soups, mayonnaise, ... And while these products, in fact, do not contain phytoestrogens and will not worsen the progression of the disease in this sense, keep in mind that they should generally only be present occasionally on the menu of oncology patients.

Conclusion

Whether it is a question of breast cancer prevention or treatment, the fear of soy is completely unfounded. Moreover, this versatile food is desirable on the menu of oncology patients, primarily in the form of whole grains or minimally processed soy products. It is advisable to choose organically grown soy, if possible.

References:

· Kucuk O. Soy foods, isoflavones, and breast cancer. Cancer. 2017;123(11):1901-1903.

· Guha N, Kwan ML, Quesenberry CP Jr, Weltzien EK, Castillo AL, Caan BJ. Soy isoflavones and risk of cancer recurrence in a cohort of breast cancer survivors: the Life After Cancer Epidemiology study. Breast Cancer Res Treat. 2009;118(2):395-405.

· Shu XO, Zheng Y, Cai H, et al. Soy food intake and breast cancer survival. JAMA. 2009;302(22):2437-2443.

· Nachvak SM, Moradi S, Anjom-Shoae J, et al. Soy, Soy Isoflavones, and Protein Intake in Relation to Mortality from All Causes, Cancers, and Cardiovascular Diseases: A Systematic Review and Dose-Response Meta-Analysis of Prospective Cohort Studies. J Acad Nutr Diet. 2019;119(9):1483-1500.e17.

· Qiu S, Jiang C. Soy and isoflavones consumption and breast cancer survival and recurrence: a systematic review and meta-analysis. Eur J Nutr. 2019;58(8):3079-3090.

· Nechuta SJ, Caan BJ, Chen WY, et al. Soy food intake after diagnosis of breast cancer and survival: an in-depth analysis of combined evidence from cohort studies of US and Chinese women. Am J Clin Nutr. 2012;96(1):123-132.

· Zhang FF, Haslam DE, Terry MB, et al. Dietary isoflavone intake and all-cause mortality in breast cancer survivors: The Breast Cancer Family Registry. Cancer. 2017;123(11):2070-2079.

· Caan BJ, Natarajan L, Parker B, Gold EB, Thomson C, Newman V, Rock CL, Pu M, Al-Delaimy W, Pierce JP. Soy food consumption and breast cancer prognosis. Cancer Epidemiol Biomarkers Prev. 2011;20:854-58.

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