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Soy: How a Good Food Got a Bad Reputation

By Michelle Darian, MS, MPH, RD, August 3, 2020

soy tofu cancer risk When you think of soy, what is the first thing that comes to mind? Is it the nutrient dense food or the “health risks” that accompany it? In the nutrition realm, particular foods get a bad rep that they don’t necessarily earn. A single study with a controversial finding can dominate headlines—even if 100 other studies conclude the contrary! Soy foods are an unfortunate example of this. Here's how soy became linked with an increase in cancer risk, how the research has panned out since then, and how soy fits into a healthy diet. 

 

Soy's link to estrogen and other hormones

Soy is a legume that has been eaten for thousands of years in a number of forms. Soybeans contain polyphenols (a class of compounds found in plants) called isoflavones, which have a chemical structure that is similar to the hormone 17-β-estradiol, an estrogen hormone commonly called E2. Because of these structural similarities, isoflavones can actually bind to estrogen receptors and impart similar effects. And because hormones like estrogen are known to influence the progression of certain types of cancers, heart disease, osteoporosis and menopause, soy has caught the attention of researchers and consumers alike.[1] 

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Mouse studies led us astray in the relationship between cancer risk and soy intake 

Research shows that high levels of estrogen increase breast cancer risk. So soy's isoflavone content made it the subject of much speculation that high intake of soy could be linked to a similar risk. Early animal studies investigating the link between soy, isoflavones, and cancer did find an increased risk of breast cancer, but with a major caveat—they were done in mice! In many cases, mouse studies are important models for human biology. But in the case of soy, it's quite the opposite. In mice, isoflavones increase estrogen levels, increasing breast cancer risk. But in humans, the isoflavones in soy are actually able to block the more potent natural estrogen produced by the body [2]. In fact, recent research has noted that soy foods have actually been linked to a lower risk of certain cancers, likely due to this process.

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Where the research currently stands on soy and cancer

So much research has been conducted on the link between soy and cancer that the results are best interpreted through meta-analyses or systematic reviews. First, a review of 16 studies found that soy foods were significantly associated with a 29% decreased risk of prostate cancer.[3] Two meta-analyses found soy product consumption was significantly associated with a lower risk of developing GI cancer and gastric cancer.[4,5] A separate, large meta-analysis found that soy intake was protective against risk of breast cancer in Asian women, but had no effect on women in Western countries.[6] Lastly, a systematic review of the association between soy and risk of death from multiple causes found no association between consuming soy products and risk of all-cause death or risk of death from heart disease and cancer.[7] 

Key takeaways: Soy has been associated with a decreased risk of prostate, GI, and breast cancer (among other types) in some populations. However, further studies are warranted in order to provide more insight into this relationship. 

 

Soy intake is associated with a lower risk of type 2 diabetes—but the reason is unclear

A number of observational studies have shown that eating soy is associated with a lower risk of developing Type II Diabetes. However, scientists are unsure why this relationship exists. One study suggests that soy may improve the absorption of glucose, preventing large glucose spikes after meals.[8] Another potential explanation is that of substitution—replacing relatively unhealthy foods in the diet with soy. People who choose soy as their protein source tend to get less of their protein from animal sources high in saturated fats, which are known to increase the risk of diabetes. More experimental trials on the topic are warranted. 

 

Soy may help to prevent osteoporosis

Osteoporosis is the weakening of bones, and commonly occurs in post-menopausal women and some men. Estrogen levels are key for maintaining strong bones, so as women’s estrogen levels decrease during menopause, bone tissue breaks down more quickly than it’s able to build back up. But soy's isoflavones may inhibit the breakdown of these tissues and help build them up, helping to ameliorate age-related bone density loss.[9] However, further studies investigating this association are warranted. 

Current research on soy

Swapping animal proteins for soy can help to improve cholesterol and body weight

Eating soy products can help to decrease your LDL (bad) and total cholesterol and increase your HDL (good) cholesterol.[10] In one study, animal-based proteins were swapped for 30g/day of soy protein (about ⅓ to ½ of participant’s total daily protein intake). After just 12 weeks, cholesterol levels and body weight significantly decreased. Cholesterol and body weight are important to keep within normal limits, as elevated measures of both are precursors to heart disease.  

 

How to incorporate soy into a healthy diet

Decrease in disease risk aside, soy foods have properties that make them a great addition to a healthy diet! Soy now comes in many forms. You can find it in meat-alternatives like tofu and tempeh. Due to the rising prevalence of lactose intolerance, food scientists have put their heads together to create many alternatives to milk, cheese, ice cream and beyond from soy.

Soy is also a great source of protein. Unlike animal sources of protein, soy doesn’t have saturated fat, which is known to increase cholesterol levels. Edamame is a popular form of soy—1 cup of cooked edamame yields a whopping 17g of protein! You can even find soy-based protein powder now, which has been shown to increase muscle strength.[11]

Of note, we do not recommend isoflavone supplements, as there is not enough conclusive evidence that they improve health at this time.[12]

soy food sources

What you need to know about soy, cancer, and other health effects: 

  • Past research links the isoflavones in soy to an increase in cancer risk in mice, but not in humans. Humans and mice respond differently to the hormones that isoflavones act on.
  • Soy has been associated with a decrease in many types of cancer, type II diabetes, osteoporosis, high cholesterol and high body weight 
  • Soy foods are protein-rich, without the saturated fat that accompany many other protein sources 
  • Soy foods include tofu, tempeh, edamame, soy-based milk, cheese, ice cream and soy protein powder


Michelle Darian photoMichelle Darian, MS, MPH, RD
Michelle is a Nutrition Specialist at InsideTracker. As a Registered Dietitian, you’ll find Michelle analyzing the research behind recent nutrition trends, bringing actionable food and supplement recommendations to the platform. When she's not myth-busting, Michelle can be found exploring new restaurants and getting creative in her kitchen.

Resources

[1]. Pabich M, Materska M. Biological Effect of Soy Isoflavones in the Prevention of Civilization Diseases. Nutrients [Internet]. 2019 Jul 20 [cited 2020 Jul 22];11(7). Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6683102/

[2]. Simon S. Soy and Cancer Risk: Our Expert’s Advice [Internet]. Cancer.org. 2019 [cited 2020 Jul 14]. Available from: https://www.cancer.org/latest-news/soy-and-cancer-risk-our-experts-advice.html

[3]. Applegate CC, Rowles JL, Ranard KM, Jeon S, Erdman JW. Soy Consumption and the Risk of Prostate Cancer: An Updated Systematic Review and Meta-Analysis. Nutrients. 2018 Jan 4;10(1)

[4]. Lu D, Pan C, Ye C, Duan H, Xu F, Yin L, et al. Meta-analysis of Soy Consumption and Gastrointestinal Cancer Risk. Sci Rep. 2017 22;7(1):4048.

[5]. Tse G, Eslick GD. Soy and isoflavone consumption and risk of gastrointestinal cancer: a systematic review and meta-analysis. Eur J Nutr. 2016 Feb;55(1):63–73.

[6]. Chen M, Rao Y, Zheng Y, Wei S, Li Y, Guo T, et al. Association between Soy Isoflavone Intake and Breast Cancer Risk for Pre- and Post-Menopausal Women: A Meta-Analysis of Epidemiological Studies. PLoS ONE [Internet]. 2014 Feb 20 [cited 2020 Jul 22];9(2). 

[7]. Namazi N, Saneei P. Soy product consumption and the risk of all-cause, cardiovascular and cancer mortality: a systematic review and meta-analysis of cohort studies. Food Funct. 2018 May 1;9(5):2576–88.

[8]. Ademiluyi AO, Oboh G. Soybean phenolic-rich extracts inhibit key-enzymes linked to type 2 diabetes (α-amylase and α-glucosidase) and hypertension (angiotensin I converting enzyme) in vitro. Exp Toxicol Pathol Off J Ges Toxikol Pathol. 2013 Mar;65(3):305–9.

[9]. Castelo-Branco C, Cancelo Hidalgo MJ. Isoflavones: effects on bone health. Climacteric J Int Menopause Soc. 2011 Apr;14(2):204–11.

[10].  Ruscica M, Pavanello C, Gandini S, Gomaraschi M, Vitali C, Macchi C, et al. Effect of soy on metabolic syndrome and cardiovascular risk factors: a randomized controlled trial. Eur J Nutr. 2018 Mar;57(2):499–511.

[11].  Orsatti FL, Maestá N, de Oliveira EP, Nahas Neto J, Burini RC, Nunes PRP, et al. Adding Soy Protein to Milk Enhances the Effect of Resistance Training on Muscle Strength in Postmenopausal Women. J Diet Suppl. 2018 Mar 4;15(2):140–52.

[12].  Chen L-R, Ko N-Y, Chen K-H. Isoflavone Supplements for Menopausal Women: A Systematic Review. Nutrients. 2019 Nov 4;11(11).