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Is Soy Safe?

Updated: Feb 7

Can breast cancer patients and Survivors eat soy?

One of the questions I hear most frequently in my practice is whether soy is a friend or foe in regards to breast cancer. About 20 years ago there was some question around this subject and it became very prominent in the media to hear that soy caused cancer. Well, after many studies with many women, the data is clearly pointing to soy as a great ally for breast cancer survivors--both because it is protective against recurrence as well as being helpful for hot flashes and night sweats.

The role of estrogen and progesterone in breast cancer

For most women who have been diagnosed with breast cancer, the tumor will be “estrogen or progesterone positive”. This means that there are receptors on the tumor itself that respond to circulating hormones in the body and those hormones cause the tumor to grow. For this reason, most women with breast cancer will receive some sort of therapy that will decrease the amount of these hormones in their body. Chemotherapy itself can stop a woman’s menstrual cycles, and following surgery, radiation, and/or chemotherapy, many women will be given more drugs to decrease estrogen in their body.

Hormones and breast cancer survivors

Women who have had an estrogen or progesterone positive tumor will generally be given either tamoxifen or an aromatase inhibitor such as arimidex for 5-10 years following their breast cancer treatment. The intent of these therapies is to decrease or block estrogen from binding to any remaining cancer cells in the body and prevent their growth. These therapies are helpful for preventing recurrence of the cancer, but can cause many menopausal type symptoms. These symptoms can range from mildly irritating to profoundly problematic. They can come in the form of hot flashes, night sweats, anxiety, weight gain, heart palpitations, fatigue, skin changes and more. So how can women manage these symptoms without increasing the risk of the cancer returning? In this situation, many women turn to natural therapies to help them through this very real concern.

Soy: Friend or Foe?

Over the past 20 years that I have been seeing patients, popular opinion about soy has varied widely from a well loved food and therapeutic to a dangerous cancer promoter that must be avoided. So what’s the current state of the science on soy? Well, the good news is that soy and its isoflavones have been shown to be safe in breast cancer survivors.1 Further, consumption of soy foods has been correlated with decreased breast cancer risk and increased survival in women with breast cancer. 2,3,4

What benefits do women have from soy?

Most studies have shown at least modest benefit from soy for hot flashes, night sweats, anxiety, insomnia, and depression during menopause. 5,6 Regular intake of soy and soy products can also help prevent osteoporosis, which is of increased concern specifically for women who have had breast cancer at age 44 or lower. Additionally, the isoflavone genestein has shown to have a favorable impact on cardiovascular markers in menopausal women.7 Because many chemotherapeutics can negatively impact the heart, this may be an additional benefit.

What is the best way to consume soy?

Soy products have grown in popularity in the past 20 years and there are many options out there. I generally advise my patients to consume soy in more traditional preparations because they tend to be a little more easily tolerated. Tofu, tempeh, miso, and even soy milks are great options. Genestein, an isoflavone found in soy, also provides benefits to women with menopausal symptoms. All of the above foods contain genestein. For people who don’t tolerate or don’t like soy products, you can also find genestein encapsulated in supplement form.

  1. Donovan A McGrowder 1, Fabian G Miller 2 3, Chukwuemeka R Nwokocha 4, Medicinal Herbs Used in Traditional Management of Breast Cancer: Mechanisms of Action. Medicines (Basel).2020 Aug 14;7(8):47.

  2. Xiao Ou Shu 1, Ying Zheng, Hui Cai, et al. Soy food intake and breast cancer survival. JAMA. 2009 Dec 9;302(22):2437-43.

  3. Lenka Varinska 1, Peter Gal 2 3 4 5, Gabriela Mojzisova et al. Soy and breast cancer: focus on angiogenesis. Int J Mol Sci. 2015 May 22;16(5):11728-49.

  4. Yuxia Wei 1, Jun Lv 1 2 3, Yu Guo et al. Soy intake and breast cancer risk: a prospective study of 300,000 Chinese women and a dose-response meta-analysis. Eur J Epidemiol. 2020 Jun;35(6):567-578.

  5. Alessandra Crisafulli 1, Herbert Marini, Alessandra Bitto et al. Effects of genistein on hot flushes in early postmenopausal women: a randomized, double-blind EPT- and placebo-controlled study. Menopause. Jul-Aug 2004;11(4):400-4

  6. Prakash Thangavel,1 Abraham Puga-Olguín,2 Juan F. Rodríguez-Landa,2 and Rossana C. Zepeda. Genistein as Potential Therapeutic Candidate for Menopausal Symptoms and Other Related Diseases. Molecules. 2019 Nov; 24(21): 3892

  7. Alessandra Crisafulli 1, Domenica Altavilla, Herbert Marini et al.Effects of the phytoestrogen genistein on cardiovascular risk factors in postmenopausal women. Menopause. 2005 Mar;12(2):186-92.

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