Is Soy Bad for Women? Separating Facts From Fiction

If you're a woman, you may be wondering whether your favorite tofu dish is good for your breast health—or possibly harmful. Misinformation about soy-based food abounds, particularly online, and much of the confusion is due to the fact that soybeans contain something called isoflavones, which can act within the body similarly to certain types of estrogen. So how does eating soy impact your breast health?

The answer is a little murky, unfortunately. Much of the scientific literature shows that soybeans, a type of legume native to east Asia, benefit many aspects of health, including breast health. But, not all women should load up on soy. The recommendations are different depending upon your age, whether you have a breast cancer diagnosis, and the type of soy itself.

If this sounds complicated…well, that's because it is! Let's dive into the relationship between soy and women's health...and debunk some of the common soy myths along the way.

Why do women worry about soy?

The association between soy and health has been controversial, especially in the context of women's health. The controversy arises in part due to the activity of the isoflavones in soybeans which can exert estrogen-like activity in the body. Isoflavones are known as phytoestrogens (aka plant estrogens) which have a similar structure as human estrogen. Phytoestrogens can bind to cellular estrogen receptors and block other estrogens from binding there, creating a protective effect. Because of this, they can actually benefit women's health, especially if consumed during certain stages of the lifecycle.

We know this from looking at many studies of soy consumption and breast health. Because soy is so prevalent in Asian cultures, a large body of this research looks at the effects of consumption in these populations. In fact, people in Japan consume 30-50 mg of isoflavones per day on average compared to about 3 mg in the United States. (Due to increasing popularity of plant-based diets in the Western world, soy consumption in the United States has increased, but it's generally not the same type of soy that is commonly eaten in Asian cultures.)

How does soy impact breast health?

It was once thought that soy can increase the risk of developing breast cancer. However, this is not the case. Here's where the confusion comes in:

• Plant estrogen vs. human estrogen

  : The cells in our bodies have two types of estrogen receptors: estrogen receptor alpha, or ER-alpha, and ER-beta. Overexpression of ER-alpha is linked to cancer, while expression and activation of the ER-beta receptor appears to counteract many of the cancer-causing activities of ER-alpha. Genistein, one of the most abundant soy isoflavones, can activate ER-beta (aka the "good" estrogen receptor!).

• Part of a breast cancer prevention strategy

  : A meta-analysis found that women ingesting >15 mg of soy isoflavones daily had lower chance of being diagnosed with breast cancer. The authors concluded that the consumption of soy isoflavones can reduce the risk of breast cancer in pre-menopausal and post-menopausal women.

• High soy intake, low breast cancer risk

  : In another meta-analysis examining soy intake and breast cancer, soy isoflavone intake was associated with a 12% reduction in the risk of breast cancer between those with the highest soy intake vs. the lowest. In other words, soy intake was inversely associated with breast cancer.

• Start young with soybeans

  : In a study that investigated soy food intake in adolescence and adulthood via food frequency questionnaire in 70,578 Chinese women, aged 40-70 years, found that there was a 22% decreased risk of breast cancer in those with the highest soy intake versus lowest. The median intake among the highest soy group was 16.4 grams daily and among the lowest, the median intake was 3.5 grams daily. It may also benefit young women to start eating nutritious soy foods in teenage years. Research also finds that high soy intake during early adulthood and adolescence is associated with reduced premenopausal breast cancer risk.

What are the health benefits of soy?

Soy may be most well-known for its influence on breast health, but it actually has many other health benefits. Soy is a complete protein, meaning that soy protein offers all the essential amino acids, making it a solid option for vegan and vegetarian diets. This legume also offers a variety of vitamins and minerals including B vitamins and iron.

Soy also contains the most isoflavones out of any other foods. Isoflavones are compounds that have antioxidant properties. The three major isoflavones found in soybeans are genistein, daidzein and glycitein.

Soy isoflavones genistein and daidzein work synergistically to help maintain healthy cell function in tissues of the prostate, lung, and breast. These isoflavones also have been shown to support bone and cardiovascular health.

In addition to breast health, soy offers the following health benefits to women:

• Cholesterol
• Heart health
• Diabetes
• Menopause
• Bone health

Cholesterol

: In a meta-analysis of 43 trials, soy protein from foods, beverages, and isolates at a median dose of 25 grams/day with a median follow up of 6 weeks was found to significantly decrease LDL ("bad") cholesterol as well as total cholesterol compared with non-soy protein controls. Since 1999, soy has had an FDA-authorized claim to health fame when it comes to supporting heart health but was questioned in 2018.

Heart health

: When it comes to heart health, the above plays a role, since cholesterol and heart health are strongly related. However, soy can also help benefit the heart in other ways. A study following three large cohorts of American men and women without heart disease at the start of the study found that those who ate the highest amount of tofu and isoflavones from soy foods (not including soy milk), compared with those who ate the least, had an 18% and 13% lower risk, respectively, of developing heart disease. Consuming more tofu and isoflavones from soy foods helped keep the heart healthy!

Plus, soy is also nutritious and lower in saturated fat than many animal foods such as red meat. Since soy protein is a complete protein source, minimally processed soy could serve as a replacement during your Meatless Monday meal.

Diabetes

: A prospective study including 21,925 healthy Japanese men and women aged 40-79 years old found that among women, higher tofu intake was inversely associated with risk of type 2 diabetes; 8% for 3-4 times a week and 33% for almost daily consumption compared with those who ate tofu < 3 times a week. Interestingly, this association seems to be found in women, but not so much in men.

Menopause

: When estrogens and other hormones decline during menopause, women start to experience a variety of symptoms such as hot flashes along with more serious complications such as bone loss. If Bioidentical Hormone Replacement Therapy (BHRT) is not an option, phytoestrogens from soy may help to some extent.

Bone health

: While strong data for soy improving hot flashes is limited, researchers have found that soy isoflavones are effective in slowing down bone loss after menopause.

Is soy bad for women's health?

No. Nutritious forms of soy (see list below) offer many health benefits, and soy protein is a particularly important option for women who are vegans or vegetarians. However, people undergoing treatment for cancer or who have had certain types of cancer, and/or those with thyroid issues should consult their doctor and registered dietitian first for personalized recommendations. Soy doesn't seem to negatively affect the thyroid in healthy individuals, according to research. However, soy may contribute to existing hypothyroidism.

Can soy cause estrogen levels to rise?

Soy does not increase estrogen production in the ovaries but can provide nutrients (like isoflavones) to support healthy estrogen metabolism by interacting with cellular estrogen receptors.

Can women eat soy daily? How much soy is safe?

Yes, 1-2 servings a day of nutritious soy foods is safe and may actually help protect against breast cancer. Women with existing estrogen dependent cancers should consult a healthcare professional before beginning a soy regimen.

Which soy is best for you?

The general rule of thumb is that less processed is best. The same applies to soy foods. Unless you consider fermentation a process; in that case there is an exception to the rule! Fermented soy foods have been cultured thanks to the help of beneficial bacteria or yeast and include miso, tempeh and natto, which are among the healthiest options when choosing soy foods. Since they have not undergone actual industrial types of processing, they retain soy's nutritional content (such as isoflavones) and may actually be more easily digested and absorbed.

Here are some of the best soy foods to include in your diet:

• Edamame

• Soy sprouts

• Miso

• Tempeh

• Natto

• Whole soybeans

For context, 3 ounces of cooked tempeh contains 30 mg of isoflavones, ½ cup of boiled edamame contains 16 mg of isoflavones, and 1 patty of a soy burger will have approximately 5 grams of isoflavones. Although soy sauce is technically fermented, 1 tablespoon only offers 0.02 mg of isoflavones. Pro tip: choose tamari instead of soy sauce if you avoid gluten, since tamari is free of wheat.

Minimally processed soy products such as tofu and soy milk can be part of a healthful eating pattern as well. However, it's best to the limit ultra-processed soy foods that have made their way into our food system from store bought dressings or oils used in frying to soy derivatives found in meal replacement bars and frozen foods.

Types of soy to avoid

Not all forms of soy foods are necessarily good for us. Highly processed soy, the kind you'll find in a variety of soy products like meat alternatives, oil-based foods and snack foods marked as vegan, aren't as nutritious as whole soy foods or the fermented soy foods consumed in Asian eating patterns.

In the U.S., you'll also find soybean oil used in wide variety of applications ranging from food and non-food uses alike due to their status as a subsidized crop. That means your frozen soy-based veggie patty may not be your healthiest option as a meat alternative. On the plus side, you'll get a good amount of protein from it. On the downside, it will be low in those beneficial isoflavones, and potentially high in fillers or additives, wheat gluten, and sodium, depending on the brand.

Another concern about soy stems from the fact that the majority of American soybean crops are genetically modified and could contain traces of chemicals used to deter pests (the herbicide glyphosate).

And finally, while unsweetened soy milk that's minimally processed can be one of the healthier soy products out there, some soy milks are loaded with sugar and artificial ingredients. So as always, read your labels carefully!

Enjoy soy responsibly

If you choose healthy, whole-food forms of soy, and are not living with a breast cancer diagnosis or hypothyroidism, soy can be a wonderful, nutritious addition to your diet. Between the benefits for estrogen balance, heart health and diabetes prevention, this plant-based protein is one of the healthiest staples in a traditional Asian diet—not to mention one of the tastiest! Try adding natto, tempeh, edamame or tofu to your next meal.

References

• Baglia ML et al. "The association of soy food consumption with the risk of subtype of breast cancers defined by hormone receptor and HER2 status." Int J Cancer. August 2016. https://pubmed.ncbi.nlm.nih.gov/27038352/
• Barańska A et al. "The Role of Soy Isoflavones in the Prevention of Bone Loss in Postmenopausal Women: A Systematic Review with Meta-Analysis of Randomized Controlled Trials." J Clin Med. August 2022. https://pubmed.ncbi.nlm.nih.gov/36012916/
• Blanco Mejia S et al. "A Meta-Analysis of 46 Studies Identified by the FDA Demonstrates that Soy Protein Decreases Circulating LDL and Total Cholesterol Concentrations in Adults – Table 1." J Nutr. June 2019.  https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6543199/table/tbl1/?report=objectonly
• Boutas I et al. "Soy Isoflavones and Breast Cancer Risk: A Meta-analysis." In Vivo. April 2022. https://pubmed.ncbi.nlm.nih.gov/35241506/
• Konishi K et al. "Dietary Soy Intake Is Inversely Associated with Risk of Type 2 Diabetes in Japanese Women but Not in Men." J Nutr. July 2019. https://pubmed.ncbi.nlm.nih.gov/31079144/
• Ma, L et al. "Isoflavone Intake and the Risk of Coronary Heart Disease in US Men and Women." Circulation. March 2020. https://www.ahajournals.org/doi/full/10.1161/CIRCULATIONAHA.119.041306
• Messina, M. "Impacto f Soy Foods on the Development of Breast Cancer and the Prognosis of Breast Cancer Patients." Karger. April 2016. https://www.karger.com/Article/Fulltext/444735
• Okekunle AP et al. "Higher dietary soy intake appears inversely related to breast cancer risk independent of estrogen receptor breast cancer phenotypes." Heliyon. July 2020. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7334424/
• Ruscigno, M, MPH, RD. "Soy Under Fire." Today's Dietitian. April 2018.  https://www.todaysdietitian.com/newarchives/0418p18.shtml
• Yan F et al. "Soy Intake and Risk of Type 2 Diabetes Among Japanese Men and Women: JACC Study." Front Nutr. January 2022. https://pubmed.ncbi.nlm.nih.gov/35083266/
• "Straight Talk About Soy." Harvard.  https://www.hsph.harvard.edu/nutritionsource/soy/
• "Table 1 Trial characteristics for the studies included in the analyses of the effect of soy protein intake on circulating LDL cholesterol and TC concentrations in adults." https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6543199/table/tbl1/?report=objectonly