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Soy Foods and Breast Cancer

Apr 8, 2020

Highlights

Soy foods are important dietary sources of isoflavones such as genistein, daidzein and glycitein, which act as phytoestrogens (plant based chemicals with a structure similar to estrogen). Many breast cancers are estrogen receptor (hormone receptor) positive and hence one may fear whether soy foods intake is associated with an increased risk of breast cancer. This blog summarizes different studies evaluating the association between soy intake and breast cancer. The findings of these studies indicate that consuming soy foods in moderate quantities do not increase the risk of breast cancer, but taking soy supplements may not be a safe option.



Soy foods have been a part of the traditional Asian cuisine since many years and soy products have recently gained popularity all over the world. Due to its high protein content, soy products are also used as a healthy analogue for meat and as common nutritional solutions for vegetarians. Different types of soy foods include unfermented soy foods such as whole soybeans, tofu, edamame and soy milk and fermented soy products such as soy sauce, fermented bean paste, miso, nattō, and tempeh. 

Soy Foods and Breast Cancer

Additionally, soy foods are also important dietary sources of isoflavones such as genistein, daidzein and glycitein. Isoflavones are natural plant compounds falling under a category of flavonoids that exhibit antioxidant, anticancer, antimicrobial, and anti-inflammatory properties. Isoflavones act as phytoestrogens, which are nothing but plant based chemicals with a structure similar to estrogen. The association of soy food intake with breast cancer has been rigorously studied for many years. This blog focuses on the different studies which evaluated the association of soy foods with breast cancer.

Association between Soy Foods and Breast Cancer 

Breast cancer is the second leading cause of cancer deaths in women in 2020. The incidence of breast cancer has slightly increased by 0.3% per year in recent years (American Cancer Society). It is most common in women aged between 20-59 years. In addition, breast cancer accounts for 30% of all female cancers (Cancer Statistics, 2020). Many breast cancers are estrogen receptor (hormone receptor) positive. As mentioned before, soy foods contain isoflavones that act as phytoestrogens. Hence, one may fear whether soy food intake is associated with an increased risk of breast cancer. Let us find out what the studies say!

Findings from Studies on Soy Foods and Breast Cancer 

1. Soy Intake and Breast Cancer Risk in Chinese women

A recent study published in the European Journal of Epidemiology evaluated the relationship between soy intake and risk of breast cancer incidence. The researchers used data from a large-scale prospective cohort study called the China Kadoorie Biobank (CKB) cohort study for the analysis. The study involved over 300,000 women aged between 30–79 from 10 geographically and economically diverse regions in China. These women were enrolled between 2004 and 2008, and followed-up for breast cancer incidence for approximately 10 years. Additionally, the researchers obtained details of soy consumption from food frequency questionnaires in baseline, two resurveys and twelve 24-h dietary recalls. (Wei Y et al, Eur J Epidemiol. 2019)

According to the data collected, mean soy intake of these women was 9.4 mg/day. 2289 women developed breast cancers during a follow up period of 10 years. Detailed analysis of the data found no significant association between soy intake and breast cancer incidence overall. 

Meanwhile, the researchers also searched and obtained 8 previous prospective cohort studies from public domain and carried out dose–response meta-analysis. The analysis showed that for every 10 mg/day increase in soy intake, there was a 3% reduction in breast cancer risk. (Wei Y et al, Eur J Epidemiol. 2019)

Key Take-aways :

The researchers concluded that moderate soy intake is not associated with breast cancer risk in Chinese women. They also suggested that a higher amount of soy food consumption might provide reasonable benefits of reducing breast cancer risk.

2. Soy isoflavone Intake and Menopausal Symptoms (MPS) among Chinese women with early stage breast cancer

In a recent study, the researchers investigated the association between soy isoflavone intake and menopausal symptoms (MPS) among Chinese women diagnosed with early stage breast cancer. The study was published in the Breast Cancer Research and Treatment Journal in April 2020. It used questionnaire based data from 1462 Chinese breast cancer patients. There were three follow-up time-points during the first 5 years post diagnosis. (Lei YY et al, Breast Cancer Res Treat. 2020)

Key Take-aways : 

The findings showed no association between soy isoflavone intake and menopausal symptoms among Chinese breast cancer patients.

3. Soy isoflavones and Breast Cancer in Pre- and Post-Menopausal Women from Asian and Western countries

A meta-analysis published in the PLoS One journal in 2014 included 30 observational studies involving premenopausal women and 31 studies involving postmenopausal women to explore the association of soy isoflavone intake with breast cancer. Of the studies involving premenopausal women, 17 studies were done in Asian countries and 14 were done in Western countries. Of the studies involving postmenopausal women, 18 studies were done in Asian countries and 14 were done in Western countries. (Chen M et al, PLoS One. 2014

Key Take-aways :

The researchers found that soy isoflavone intake could reduce the risk of breast cancer for both premenopausal and post-menopausal women in Asian countries. However, they didn’t find evidence suggesting the  association between soy isoflavone intake and breast cancer for premenopausal or post-menopausal women in Western countries.

4. Soy Food Intake and  Incidence of bone fracture in Breast Cancer Survivors

In a large prospective study named “The Shanghai Breast Cancer Survival Study”, the researchers investigated the incidence of bone fracture and its association with soy food intake in breast cancer survivors.  The study included data from 4139 stage 0-III breast cancer patients,1987 pre-menopausal and 2152 postmenopausal patients. Soy food intake was assessed at 6 and 18 months post diagnosis. Also, the fractures were assessed at 18 months and at 3, 5, and 10 years post diagnosis.(Zheng N et al, JNCI Cancer Spectr. 2019

Key take-aways :

Findings from the study indicate that increased consumption of soy isoflavone may reduce the risk of bone fractures in pre-menopausal patients but not in post-menopausal patients.

5. Soy isoflavones Intake and Breast Cancer Recurrence 

In a study done by Kang X et al., they analyzed the associations between soy isoflavones intake and recurrence of breast cancer and death. Study used a questionnaire-based data from 524 breast cancer patients for analysis. The study was carried out on patients who underwent surgery for breast cancer between August 2002 and July 2003. The patients also received adjuvant endocrine therapy at the Cancer Hospital of Harbin Medical University in China. Mean follow up period was 5.1 years. The study was further assessed by hormonal receptor status and endocrine therapy. (Kang X et al, CMAJ. 2010).

Key Take-aways:

Findings from the study indicated that a high intake of soy isoflavones as part of diet may reduce the risk of recurrence in post-menopausal breast cancer patients who were positive for estrogen and progesterone receptors, and those who were receiving endocrine therapy. 

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6. Dietary Soy Supplements and Breast Cancer Risk in French women

A study published recently in the The American Journal of Clinical Nutrition in 2019, investigated the association between dietary soy supplement intake and breast cancer risk. The study included data of 76,442 French women from the INSERM (French National Institutes for Health and Medical Research) Etude Epidemiologique aupres de Femmes de la Mutuelle Generale de l’Education Nationale (E3N) cohort. The women included in the study were aged more than 50 years and born between 1925 and 1950. They were followed from 2000 to 2011 with an average follow-up time of 11.2 years. In addition, the soy supplement use was assessed every 2-3 years. (Touillaud M et al, Am J Clin Nutr. 2019)

The researchers found that there was no overall association between current or past use of dietary soy supplements (containing isoflavones) and breast cancer risk. However, when they analyzed the data by estrogen receptor (ER) status, it was found that there was a lower risk of estrogen receptor positive (ER+) breast cancer and a higher risk of estrogen receptor negative (ER–) breast cancer in the current users. Data also showed that women with a family history of breast cancer were at a higher risk of ER– breast cancer. Premenopausal, recently postmenopausal women and women with no family history of breast cancer had a lower risk of ER+ breast cancer.

Key Take-aways: 

The findings of this study indicate that there are opposing associations of dietary soy supplements with ER-positive and ER-negative breast cancer risk. In addition, women who have a family history of breast cancer should be more cautious while taking dietary soy supplements. 

7. Effect of Soy Supplementation on Breast Cancer Risk Markers such as Mammographic/Breast Density

A study published in 2015 evaluated the effect of soy supplementation on mammographic/breast density in 66 previously treated breast cancer patients and 29 high-risk women. Mammographic density, also known as breast density, is the percentage of dense tissue of an entire breast. It is one of the strongest risk factors of breast cancer. The clinical study included women aged 30 to 75 years who were :

  • diagnosed with breast cancer and were either treated or not treated with the standard of care hormone therapy or an aromatase inhibitor (AI) at least 6 months earlier, with no evidence of recurrence; or

  • high-risk women with a known BRCA1/BRCA2 mutation, or a family history consistent with hereditary breast cancer.

The participants were categorized into 2 groups. The first group received soy tablets containing 50 mg isoflavones and the control group received placebo tablets containing microcrystalin cellulose. Digital mammograms and breast MRI scans were obtained at baseline (before supplementation) and 12 months after daily 50 mg soy isoflavones tablet or placebo tablet supplementation. (Wu AH et al, Cancer Prev Res (Phila), 2015). 

Key Take-aways:

The analysis found a slight decrease in mammographic density percentage (measured by the ratios of month 12 to baseline levels) in the group which received soy supplementation  as well as in the control group. However, these changes did not differ between the treatments. Similarly, the outcomes in breast cancer patients and high-risk women were also comparable. In conclusion, the researchers stated that soy isoflavone supplementation  does not impact mammographic density.

8. Adolescent and Adult Soy Food Intake and Breast Cancer Risk

In a study published in 2009, the researchers analyzed data from the Shanghai Women’s Health Study to evaluate the association of adolescent and adult soy food intake with breast cancer risk. The study included 73,223 Chinese women aged between 40–70 years who were recruited between 1996 and 2000. Questionnaire-based data was used to assess the dietary intake during adulthood and adolescence. 592 cases of breast cancer incidence were reported after a follow-up of about 7 years. (Lee SA et al, Am J Clin Nutr. 2009)

Key take-aways :

The findings of the study indicated that high soy food intake may reduce the risk of breast cancer among premenopausal women. Women who consumed a high amount of soy foods consistently during their adolescence and adulthood had a reduced breast cancer risk. However, they didn’t find any association with soy food consumption for postmenopausal breast cancer.

What should we infer from these Studies?

These studies indicate that eating soy foods in moderate amounts do not increase the risk of breast cancer. In addition, few of the studies suggest that soy foods may lower breast cancer risk, especially in Chinese/Asian women. One study also indicates that these benefits are predominant in women consuming soy foods consistently during their adolescence and adulthood. Soy foods can also reduce cholesterol levels and decrease the risk of heart diseases. However, it may not be safe to take dietary soy supplements, especially by women with a familial history of breast cancer. In summary, it is safe and healthy to take moderate quantities of soy foods as part of our diet/nutrition instead of taking supplements. Avoid soy supplement intake unless recommended by your health care providers.


Cancer patients often have to deal with different chemotherapy side effects which affect their quality of life and look out for alternative therapies for cancer.Taking the right nutrition and supplements based on scientific considerations (avoiding guesswork and random selection) is the best natural remedy for cancer and treatment related side-effects.


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