By Lynne Hebert

tempeh skewers


Soy can be a healthful component of the diet in moderation, and there is not sufficient evidence to support a link to Cancer, Cancer prevention, Thyroid disorders, or fertility complications in humans when consuming soy in the diet. However, soy should not be completely free of concern, because the available research presents inklings of potential health hazards when adding soy to the diet.


When driving across the United States, through the midwestern region — there is seemingly infinite amounts of two plant varieties. There is certainly no lack of corn stalks, and a shorter, greener, shrub-like plant. The vast acres of green rows are soybeans, used for anything from oil, to animal feed, to tofu. Soybeans grown in the United States account for 89.5 million acres of farmland as of 2020 (1). Within an estimated 915 million acres of U.S. farmland, soy accounts for about 10% of all crops grown in the country.

Farmers have become very efficient at growing soy. The amount of land needed to grow one bushel of soybeans has decreased by 22% since 1997 (1). This is due to innovations in conservation tillage, a farming method of decreasing the amount the soil is stirred. This method has cut soil erosion by 47%, and cut greenhouse gases by 38% per bushel (1).

Soy is a very versatile food that is used in a number of ways in human and animal diets around the world. Around 70% of soy grown in the United States is used as animal feed (1), however it exists vastly in the human food system as well. Processed foods containing food additives often contain soy for its emulsifying and preserving properties. As a vegetarian source of protein, soy milk and fermented soy foods such as tofu and tempeh are used widely as a dietary staples for vegetarians, and anyone who wants to enjoy them. It can be used as a primary meat replacement due to its high protein content containing all nine essential amino acids, and a high micronutrient content. Cultures around the world have their own ways to utilize the vast possibilities of soy foods. Soy is a staple in Asian and South African cuisine in the form of soups and fermented foods.

However, the amount in which humans use soy may come with a caveat, according to emerging research. Soybeans are the most potent source of isoflavones amongst all foods on the market today. Isoflavones are a plant compound that serves as a natural fungicide to protect the soy plant as it grows, but isoflavones have the capacity to alter epigenetics in plants, animals, and fungi alike. In humans, isoflavones mimic the estradiol compound in the body, a precursor to the estrogen hormone, and can raise over-all serum estrogen in the body. Elevated estrogen levels can be significant on a physiological level. Elevated estrogen can do anything from alleviate hot flashes in menopausal women, to potentially interacting with thyroid hormones, to perhaps complicating fertility and healthy cell growth (2). Elevated serum estrogen can be a cause of breast cancer among women as well.

Inklings of evidence, research, and even rumors have circulated the media around the affects of soy on human health and hormone levels. Due to soy being a vegetarian source of Omega 3 and Omega 6 fatty acids, which serve as potent anti-inflammatories, eating soy has even been recognized as a preventative measure for returning breast cancer among survivors (2).

Conflicting messages have reached the cancer community regarding soy consumption. However, there is not enough fortified research yet to support a link between soy consumption and cancer or cancer remission.

Counter Argument

According to a handful of studies, soy consumption may have a link to raising serum estrogen in humans (3), and potentially causing thyroid disorders (4). Animal studies have also suggested potential male infertility and breast cancer associated with soy consumption.

A randomized control trial performed in Japan in 2014 utilized a cohort of 35 Japanese men. The test group consumed one cup of dairy each day for three months, and then replaced the one glass of dairy with one glass of soy milk for eight weeks. When the test group added soy milk to the diet by just one glass each day, their serum estrogen levels showed a statistically significant increase over the eight weeks. The control group, which continued with dairy, showed no increase in serum estrogen. This may suggest that soy consumption, even in small amounts, has the capacity to raise serum estrogen levels among men over time (3).

In another human study performed in Japan, one Japanese woman at 85 years old was given a soy supplement each day for 23 weeks. Over the course of the 23 weeks, she developed severe hypothyroidism and goiter. When she stopped taking the soy supplement, she went into remission and her thyroid hormones normalized. This could have many possible conflicting factors, but it may suggest that soy supplements may be a cause of hypothyroidism and goiter (4).

In the United States in 2010, an animal study was performed on rats consuming a soy-based diet. The male rats in the study became infertile over a number of weeks when fed a soy-based diet.

The male rats being fed control diets of corn-based feed, vegetable-based feed, alfalfa, and oat- based feed did not show any physiological changes over the same time period (5).

These studies may suggest that when broken down by animal cells through digestion, soy can suggest a link to cancer, hypothyroidism, and infertility. However, these studies were done on animal samples, or were done only in Japan on people of Japanese heritage. Animals digest and process soy a lot differently than humans do (6). Studies also suggest that people of different ethnicities digest and absorb soy in different ways. Native Hawaiians digest and absorb isoflavones at different levels than people of European descent, or Chinese descent, or South Japanese descent (7). Some populations are more sensitive to the biochemical effects of soy and isoflavones than others (7).

One study suggests that breast cancer risk may increase with soy consumption throughout life due to soy being part of the cultural diet, meaning that Asian women have higher rates of breast cancer due to their soy intake (8). In a prospective cohort study done in Hawaii, both diet and mammography activity was monitored among 514 women, one half were post-monopausal, and all were diverse in ethnicity. Japanese, Chinese, Caucasian American, and Native Hawaiian women were used in this study in equal numbers of participants. When monitoring diet, the research concluded that Asian women consumed significantly more soy foods, but statistically the same amount of meat as Caucasian American women. Mammograms suggested that Asian women showed higher probability of breast cancer markers including breast density. Caucasian American and Native Hawaiian women showed a positive association with mammogram density related to increased soy intake (8). This indicates a higher risk associated with increased intake of soy foods (8).

Although, according to a separate study performed on a cohort of 220 women of Native Hawaiian, Asian, and Caucasian American ethnicities, isoflavones may be more impactful in some populations than others (7). The multiethnic test group was instructed to consume two servings of soy foods per day, and the control group was instructed to consume less than three servings of soy foods per week. According to blood lab results and mammograms, soy intake had no significant difference in the biomarkers for breast cancer across all groups. What made the difference in risk was ethnicity. When consuming a diet higher in soy foods, the test group, across all ethnicities, showed an increase in isoflavones in their urine samples. Japanese women had significantly higher isoflavones in the urine samples than women of other ethnic groups. The women of Asian descent showed statistically greater biomarkers for breast cancer, regardless of soy intake (7). These women showed much higher levels of the IGF-1 protein,which is a biomarker for breast cancer (7). This may be due to the fact that the Asian participants in the study had significantly lower immune-related compounds in the blood such as C-reactive Protein and NAF1. Fertility was also studied by testing sex hormones in the luteal phase of menstruation, and fertility was statistically the same across all groups, regardless of soy intake (7).

Overall, these studies are very limited. Sample sizes are small, other conflicting factors can be at play, and the studies are short-term. There are not enough studies to suggest a real correlation between these health conditions and soy consumption.


Not enough evidence exists to suggest a link between soy consumption and hormone changes in the body such as increased serum estrogen, breast cancer risk, thyroid hormone imbalances, and infertility. As far as the current literature suggests, soy can be a healthy component to the diet, and an appropriate meat alternative, especially in replacement of processed red meats.


The Academy of Nutrition and Dietetics suggests that consuming soy foods may actually decrease the risk of returning breast cancer among cancer survivors (9). This may be due to the fact that the phytoestrogens in soybeans (isoflavones) may counteract other, more powerful estrogens in the body. Phytoestrogens have the ability to bind to estrogen receptors in the body, reducing the need for estrogen synthesis and increasing synthesis of steroids (9). Soy does not physically contain estrogen. Phytoestrogens do not perform the same way in the human metabolism as hormone estrogens created internally (9).

A study done by the Academy of Nutrition and Dietetics suggests that women who consume more soy foods in the diet also tend to consume more fruits and vegetables, and less fried food and processed meats (9). This is a protective factor in and of itself, but it is not directly related to the actual consumption of soy.

The Academy elaborates that soy foods have a different effect than soy supplements, which are far more potent (9). Soy supplements are common in Asian cultures, typically as a remedy for hot flashes and other menopausal effects. Soy supplements are very rich in isoflavones, and it may be a good idea to avoid them, according to members of the Oncology board of the Academy (9).

The more the soybean is processed, the less isoflavones it contains. Tofu and Tempeh are very processed, and only contain trace amounts of isoflavones. This appears as 2mg of isoflavones compared to around 14mg in the raw soybean. Soy milk contains about 5mg (2).

A prospectove cohort study was performed in the United States with a sample size of 524 cancer survivors. The study continued for 5.1 years, and both rates of cancer recurrence and death were measured. Diet, particularly quantity of soy in the diet was compared with the rates of cancer recurrence and death. Data showed that within a five-year window, rates of returning cancer were lower among participants consuming more soy foods (10). The rate o recurrence was significantly lower among the highest quartile of soy consumption among post-menopausal women (HR = 0.67, 95% CI 0.54-0.85, p for trend = 0.02) (10). This result was stated with a 95% confidence interval, meaning this result would likely repeat among 95% of the population. A Chinese study performed in 2008 revealed practically the same result on a much larger scale.

5,921 breast cancer survivors at a variety of cancer stages were observed for 3.9 years, and rates of mortality and cancer recurrence were observed in relation to diet. With no difference in menopausal status, women who consumed more soy foods in the diet had lower rates of cancer recurrence, and lower rates of mortality (11).

Both of these studies may suggest that soy foods could be protective against breast cancer recurrence. However, the literature is still conflicting.


Soy has also been studied for its effects on Thyroid hormones, potentially causing hypothyroidism. However, not enough research exists to show a correlation.

According to a review of fourteen trials on soy consumption and thyroid hormones — with only one exception, the consumption of soy foods had no correlation with hypothyroidism at all (12). Iodine intake was a stronger indicator of thyroid disorders (12).

Another systematic review studied the effects of boy soy supplements and soy foods in relation to thyroid function. There was a significant decrease in Thyroid Hormone among participants using a soy supplement, but clinically no change in Thyroid Hormone when consuming soy foods (13).

One study was performed on fifty-two subjects with subclinical hypothyroidism. A portion of the test group was given a 16mg isoflavone supplement each day derived from soy. A second portion of the group took a 2mg isoflavone supplement, which was intended to mimic the Western diet with moderate soy consumption. When given the high-dose isoflavone supplement, 11.3% of subjects developed overt hypothyroidism over a six-month period (14). This was a three-fold increase compared to those consuming the Western diet with 2mg daily isoflavones (soy in moderation) (14). Isoflavones in high doses may have a link to hypothyroidism, but not in the amounts that the average American consumes, according to this study (14).


The phytoestrogens in soy may also have links to fertility and infertility among men and women. According to a review conducted in 2010, the current data available from nine clinical trials revealed that there was no correlation between soy isoflavone exposure and serum estrogen levels in men (15).

Soy isoflavones also had no effect on sperm or semen quality, according to a randomized crossover study on 32 healthy and reproductive-aged men in the United States in 2010 (16). This study was done in 2010, and one third of the participants consumed a daily milk protein isolate, while the other two thirds of participants consumed either a soy protein isolate or a high- isoflavone soy protein isolate. Both urine and semen samples were gathered over a number of weeks, and results showed no statistically significant difference between any of the groups in terms of sperm count and sperm quality (16). This may suggest that among healthy men, soy consumption does not affect fertility, but the sample size is small.

According to a meta-analysis done by a research Dietitian in 2017, soy foods may be beneficial for women undergoing assisted reproductive therapy (17). Rate of fertilization and clinical pregnancies are higher among women who consume more soy foods (18). This Dietitian also concluded in their research that soy foods may have less of an impact on insulin resistance than animal proteins (18). Insulin resistance is a primary risk factor for preeclampsia and ovulation issues, which are both common causes of infertility.

One of the most influential studies in healthcare today is the Nurse’s Health Study performed in Massachusetts. One of the health parameters examined among the nurses was ovulatory fertility complications (19). The nurse’s health study used food frequency questionnaires, and found that just adding one serving of soy food per day (including tempeh, tofu, and edamame) actually correlates with a decreased risk of ovulatory infertility (18). The sample size for this study was 116,000 female nurses, so the data is recognized as very applicable to the general population (19).


This review of research concludes that a lack of evidence exists to suggest a correlation between soy consumption and hormone-related health concerns. Although, some studies suggest that further research may prove this wrong. The literature is still out on soy.

In terms of being causal to breast cancer, soy foods were not concluded to be statistically significant in impact, but the research does show some correlation between mammography activity and soy consumption — but not enough to state as a supported hypothesis. Soy supplements may be of concern, and research is still ongoing about how isoflavones can affect people of differing ethnic makeup.

In relation to thyroid complications, particularly hypothyroidism, one small study concluded that subclinical hypothyroidism may potentially convert to hypothyroidism in relation to increased soy consumption, but this study is very small and limited. Fourteen other reviews suggest no correlation at all. Again, soy supplements may be of concern to thyroid function.

In terms of fertility, no effect was seen on male fertility when consuming more soy foods. Female fertility may actually benefit from the consumption of soy foods, according to a handful of studies.

Plan of Action

As Dietitians using an evidence-based practice, it is important not to state opinions to clients on soy foods based on one study or one news article. A lot of fear is circulating around soy in the diet, especially among post-menopausal women and Cancer survivors. From a broad perspective, food fear is never something a Dietitian wants to encourage. All foods have a place in the diet in moderation. Soy is very rich in nutrients, and it may provide positive anti-inflammatory benefits from Omega 3 and Omega 6 fatty acids primarily. Soy cannot be completely ruled out just yet, but it is important to connect people with the evidence that exists so far, and inform clients that


research is still being done on this subject. This research picks my brain about why foods exist in the world’s food shed so abundantly that cannot be consumed in their raw form, such as oil derived from rapeseeds that are toxic to humans due to containing erucic acid, as just one example. Also, I wonder what the health effects are of other genetic herbicides and fungicides in plants. This research shows that a natural fungicide found in a plant may have health impacts — what about plants genetically modified to contain fungicides and herbicides? What are the health effects of consuming those compounds? On the subject of soy, 90% of all soy grown in the United States is genetically modified to contain a genetic herbicide (1). In 2010, only 10% of soy plants in the U.S. had this herbicide component (1). Does this herbicide pose other health effects when consumed that may not have been researched yet?




  1. United Soybean Board. U.S. farmers excel at growing soybeans. Retrieved from: Retrieved on: September 25, 2020.


  1. Harvard Health. Straight Talk About Soy Retrieved on: September 25, 2020.


  1. Nagata, , Takatsuka, N., Shimizu, H. The effect of soymilk consumption on serum estrogen and androgen concentrations in Japanese men. Cancer Epidemiol Biomarkers. March 1, 2001 10(3) 179-184.


  1. Nakamura Y, Ohsawa I, Goto Y, Tsuji M, Oguchi T, Sato N, Kiuchi Y, Fukumura M, Inagaki M, Gotoh H. Soy isoflavones inducing overt hypothyroidism in a patient with chronic lymphocytic thyroiditis: a case report. J Med Case Rep. September 5, 2017 11(1).


  1. Bae M, Woo M, Kusuma IW, Arung ET, Yang CH, Kim YU. Inhibitory effects of isoflavonoids on rat prostate testosterone 5α-reductase. J Acupunct Meridian Stud. December 5, 2012 (6) 319-22.


  1. Simon, S. of The American Cancer Soy and cancer risk: Our expert’s advice. April 29, 2019. Retrieved from: Retrieved on: September 25, 2020.
  1. Maskarinec, G., Meng, L. An investigation of soy intake and mammographic characteristics in Hawaii. Breast Cancer Res 3, 134 (2001)
  1. Maskarinec G, Ju D, Morimoto Y, Franke AA, Stanczyk FZ. Soy food intake and biomarkers of breast cancer risk: Possible difference in Asian women?. Nutr Cancer. 2017. 69(1)146-153.
  1. The Academy of Nutrition and Dietetics, Oncology Nutrition Practice Group. Soy and breast April 2013. 
  1. Shu XO, Zheng Y, Cai H, et al. Soy food intake and breast cancer survival. JAMA. 2009. 302(22) 2437-2443.
  1. Kang X, Zhang Q, Wang S, Huang X, Jin S. Effect of soy isoflavones on breast cancer recurrence and death for patients receiving adjuvant endocrine CMAJ. November 23, 2010 182(17). 
  1. Otun, , Sahebkar, A., Östlundh, L. et al. Systematic review and meta-analysis on the effect of soy on thyroid Function. Sci Rep 2019. 17(9) 39-64.
  1. Messina M, Redmond G. Effects of soy protein and soybean isoflavones on thyroid function in healthy adults and hypothyroid patients: a review of the relevant literature. Thyroid. 2006 Mar; 16(3) 249-58.
  1. Bruce, B., Messina, M. & Spiller, G. Isoflavone supplements do not affect thyroid function in iodine-replete postmenopausal women. J Med Food 6, 309–316.
  1. Messina M. Soybean isoflavone exposure does not have feminizing effects on men: a critical examination of the clinical evidence. Fertil Steril. 2010. 93(7) 2095-2104.
  1. Beaton LK, McVeigh BL, Dillingham BL, Lampe JW, Duncan AM. Soy protein isolates of varying isoflavone content do not adversely affect semen quality in healthy young men. Fertil Stertil. 2010. 94(5) 1717-1722.
  1. Anegas JC, Afeiche MC, Gaskins AJ, et Soy food intake and treatment outcomes of women undergoing assisted reproductive technology. Fertil Steril. 2015. 103(3) 749-755.
  1. Shaw, The relationship between soy and fertility: A meta-analysis. April 14, 2017. retrieved from: Retrieved on: September 25, 2020.
  1. Nurses’ Health Study Retrieved from: Retrieved on: September 25, 2020.