Understanding Soy + Isoflavones

Soy is a proven high-quality source of protein, as well as a  complete protein, and packed with vitamins, minerals, fiber, and heart-healthy fats. Proposed health benefits that have clinical evidence to back it up include decreasing insulin resistance, blood pressure, and inflammation and reducing the risk of coronary heart disease, osteoporosis, and some forms of cancer. Soy proteins have been shown to reduce triglycerides and LDL-cholesterol, while increasing HDL-cholesterol (the good cholesterol). Lastly, soy foods have been shown to reduce the symptoms of menopause including hot flashes.

Isoflavonesare the component of soy that has intrigued the public the most. Isoflavones are polyphenols with estrogenic properties. Among many other health effects that I will touch on further in this post, isoflavones help fight against oxidative stress which helps reduce inflammation in the body  aka lowering your risk for chronic diseases.

It is estimated that 2000+ soy-related peer-reviewed articles are published annually and tons of research studies are in the works to determine the benefits of soyfoods and isoflavones. So I just wanted to dive into the current evidence we have on soyfoods to clear up confusion to some myths about soy and give everyone a little 101 education on soy.

Nutrition Comparison:

Compared to other legumes (chickpeas, lentils, pinto beans, etc) , soybeans are lower in carbohydrate, slightly higher in calories, higher in grams of fat (but the good fat!), slightly lower in fiber content and higher in calcium, iron, B2, B6, Magnesium, and potassium. 1

1 Cup Soybeans
296 Calories
17.9g Fat ( 11.3 grams of total fat content = omega-3 and omega-6 polyunsaturated fats)
14.4g Carbohydrates
10.3 g Fiber
175mg Calcium
8.84mg Iron
886mg Potassium
148mg Magnesium

Calcium absorption from soymilk is comparable to cows milk, making it a great alternative for those on a vegan and vegetarian diet

  • Iron absorption from soybeans is greater than most other plant sources due to iron being in the ferritin form.
  • Soy  and soy foods are common nutritional solutions for vegans and vegetarians, due to their high protein content and versatility in the production of meat analogues and meat and dairy substitutes.

Disclaimer: Due to some medications interacting with isoflavones found in soyfoods, I always recommend consulting with MD prior to the addition of large amounts of soyfoods in your diet as well as not recommending soy supplements or isoflavone supplements.  I merely aim to give a background on the research proven health benefits of soyfoods when added in moderationto your diet. I do not promote the addition of large amounts of any one-specific nutrient into the diet. Other note, the health benefits related to soyfoods are related to natural whole food sources of soyfood in the diet like soybeans, miso, tempeh, tofu, etc. The whole food source contributes to the health benefits. The health claims cannot be applied to supplements that don’t have the same biological content as the whole food source.


Common Soyfoods
Soy Milk
Soy Sauce
Textured Vegetable Protein
Soy Protein Isolate
Soy Flour
Soybean oil
*the more natural sources of soy in the diet contain the highest amount of isoflavones, up to 80% of isoflavones are removed in the refined soy products 

Debunking Some Myths:

  1. Consuming soy increases your risk for breast cancer. Due to soybeans containing isoflavones which are phytoestrogens that exert estrogen-like effects, concerns are that consumption of soyfoods will increase the risk for breast cancer (especially estrogen-receptor positive breast cancer). “1 According to Marji McCullough, ScD, RD, strategic director of nutritional epidemiology for the American Cancer Society, epidemiological studies that followed large populations of healthy women for many, many years either have shown no association between soy and breast cancer risk or they have actually shown a beneficial protection association with consuming soy.”1 Eating soy lowered the risk of breast cancer recurrence in those who were breast cancer survivors as well, even those with estrogen receptor-positive tumors. The reasoning behind this per Margi McCullough  is that isoflavones act like estrogen but they also have antiestrogen properties which block the potent natural estrogens from binding to the estrogen receptors. Other differences are that estrogen stimulates the vaginal maturation index as well as increases c-reactive protein which isoflavones do not. Add this to the fact that we already discussed that isoflavones have antioxidant and anti-inflammatory properties which aid in decreasing cancer risk and cancer growth. Multiple organizations have released statements concluding that isoflavones do not increase the risk of breast or endometrial cancer including the North American Menopause Society and the European Food Safety Authority. It is widely recognized that breast cancer incidence rates in soy food-consuming countries are much lower than western countries, the timing of when and how much soy food consumed throughout the lifetime has been shown to have an effect on if a benefit is seen or not, the jury is still out on this and further research still needs to be done (example: if consuming soy intake consistently throughout the lifetime has greater benefits vs. if soyfoods were only consumed later in life or are both mutually beneficial). 2
  2. Soy causes infertility issues in men.All studies that pointed to this were done with rodents. It has been shown that rodents metabolize soy isoflavones differently than humans, making these studies not applicable to humans. There is a huge lack of evidence to back up this claim and no restrictions have been put on isoflavone consumption in males. “A cross-sectional study was done involving 184 men from couples undergoing in vitro fertilization which showed that male partners intake of soyfoods and soy isoflavones were unrelated to fertilization rates, proportions of poor quality embyro’s, implantation, or live births.”2
  3. Developmental effects in children.Minimal research involving soy consumption in children is available. With the current information available on the topic (the small amount there is), the evidence suggest that neither soy or isoflavone intake affects hormone levels. In the USA and in European Union Countries, infant soy formulas are considered safe but human milk (obviously) is still the best nutrition choice for infants.



Recommendations Based on the Research:

  • Evidence indicates soyfoods can be safely consumed by all individuals except those who are allergic to soy protein.
  • 25g/day of soy protein a day is recommended a day for cholesterol reduction. *recommendation established by the US FDA.
  • Replacing less healthy foods with two to four servings of soy protein per day is recommended to lower the risk of chronic disease.
  • Substituting meat-based meals with soy-based meals will greatly decrease saturated fat intake.

The labeling of soy foods with isoflavone concentrations is being discussed as helpful information for consumers, along with shared guidelines for soy intake in order to obtain health benefits and warnings for upper level of intake, especially for long-term consumption. This means, we are only going to get more educated on the health benefits of soy!

Please send any questions you may have, I hope you enjoyed reading!

-Cinthia Scott, RD, LD, CNSC


  1. Nutrients. 2016 Nov 24;8(12). pii: E754.
  2. http://www.todaysdietitian.com/newarchives/040114p52.shtml
  3. Nutrients. 2018 Jan 5;10(1). pii: E43. doi: 10.3390/nu10010043.
  4. Nutr Hosp. 2017 Oct 15;34(Suppl 4):36-40. doi: 10.20960/nh.1569.
  5. Medicines 20174(2), 18; doi:10.3390/medicines4020018
  6. EFSA Panel on Food Additives and Nutrient Sources added to Food (ANS). Risk assessment for peri- and post-menopausal women taking food supplements containing isolated isoflavones: Safety of isoflavones from food supplements in menopausal women. EFSA J. 201513, 4246. [Google Scholar] [CrossRef]
  7. Messina, M. Post-Diagnosis Soy Isoflavone Intake Is Not Harmful to Women with Breast Cancer. Breast Dis. 201526, 193–197. [Google Scholar] [CrossRef]
  8. Bai, W.; Wang, C.; Ren, C. Intakes of total and individual flavonoids by US adults. Int. J. Food Sci. Nutr. 201465, 9–20. [Google Scholar] [CrossRef] [PubMed]
  9. Zaheer, K.; Humayoun Akhtar, M. An updated review of dietary isoflavones: Nutrition, processing, bioavailability and impacts on human health. Crit. Rev. Food Sci. Nutr. 201757, 1280–1293. [Google Scholar] [CrossRef] [PubMed]

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