Is soy good or bad?

I don't think there is any food more misunderstood than soy. 

The bad press includes: 

  1. it contributes to 'estrogen dominance' 
  2. it causes man boobs
  3. it causes breast cancer
  4. it's bad for your thyroid
  5. it should be avoided in PCOS or if you have fibroid

If you are confused about soy, I don't blame you! Let me first acknowledge that soy intolerance + allergy is common. It is up there with milk, corn, and nuts. So if you have an intolerance or allergy, you SHOULD avoid soy. 

But how about if you don't? Should you avoid it because it may cause the above issues? 

My North Star is science, and whilst I am fully aware not everything can be explained by science yet, I use available research and my medical knowledge to form my opinion. 

So... what does the science say about soy?
  • Asian women are up to 3 times less likely to develop breast cancer than North American women (WHO 2012) – this is not down to genetics because studies (1) have shown that this risk increases when they move to the west. Asian-American women born in the West had a breast cancer risk 60% higher than Asian-American women born in the East. This suggests that environmental factors in the west drive up the breast cancer risks in Asian women when they immigrate. One hypothesis is that their diet changes to adopt a more westernized diet (and one of the beneficial foods hypothesized to reduce risk is soy), but it could also due to other environmental factors. 

  • Soy contains phytoestrogens (aka plant estrogens). Many people are put off by the word ‘phytoestrogens’ assuming they caused estrogen effects in our body, but actually, phytoestrogens behave VERY differently in the body compared to a woman’s natural estrogen. Firstly, phytoestrogens are weaker. And if you are worried about estrogen then animal products contain much more ACTUAL estrogen, which is several thousand-fold more active in the human body than phytoestrogens. Much of the internet claims that soy causes manboobs came from ONE case report of a man who consumed 3 quarts per day of soy milk per day. Context matters! 

  • There are two types of estrogen receptors in the body – alpha and beta. Our own endogenous estrogen and synthetic estrogen from pills preferentially bind to the alpha receptors but the phytoestrogen from soy preferentially binds to beta (2). Breast tissue contains both alpha and beta receptors. It’s thought that the activation of alpha receptors in the breast promotes tumor growth whereas the activation of beta-receptors has the opposite (anti-estrogenic and therefore anti-growth) effect. This might explain why estradiol in pills like birth control and HRT have been associated with an increased risk of breast cancer (because they activate the alpha receptors and are far more active as a compound compared to phytoestrogen),  whereas phytoestrogens may reduce the risk of breast cancer (3,4,5) – because they activate the beta receptors. By the same token, the predominant estrogen receptor in bone is beta – and that might be why phytoestrogens have been associated with increased bone density and lower fracture risk too (6).

  • Recent claims of harm have mainly been based on studies in mice showing genistein (one of the isoflavones in soy) stimulated the growth of mammary tumors in mice. But we are not mice - they don't have the same estrogen receptors as us. 

  • The Shanghai Women’s health study looked at 73,000 women and followed them up for around 7 years. It showed that soy intake during childhood, adolescence, and adult life reduced the risk of breast cancer – the benefit being greatest during adolescence (about 40% risk reduction (7)) whereas soy intake primarily consumed only as an adult carried a risk reduction of around 25% (8).

  •  Research has also shown that in women with breast cancer, soy might reduce the recurrence of cancer and improve survival (9). One analysis combined data from three large, long-running studies of survivors from both Asian and Western countries. It found that women who ate at least 10 mg of soy per day after a breast cancer diagnosis had a 25% lower risk of recurrence compared to those eating less than 4 mg per day (10). Further studies are still needed though as it may be that women who consumed large amounts of soy had healthier lifestyle (e.g. ate less meat). So oncologists are not recommending soy consumption as a way to lower recurrent of breast cancer, but equally soy consumption is not discouraged (with the exception of soy supplements/protein extracts) by most oncologists. Every case is different, so if you have been informed differently, do follow your physician’s advice.

  • Should you avoid soy if you have PCOS? A randomized controlled trial (RCT) published in 2018 showed soy consumption was associated with a significant decrease in body weight, waist circumference, blood sugar, and triglycerides; it even helped increase glutathione (the master detoxifier in our body, helping our body detoxify).
  • Be aware of GMO soy because these have been engineered to withstand high concentrations of Roundup (sprayed during the farming process to reduce weed growth amongst crops) so these GMO soy has been found to contain high pesticides, and pesticides have been linked to higher cancer risk. (If you are anti-soy on that basis, consider the conventionally farmed livestock you are eating, a lot of which have been fed GMO Soy and the pesticides stay and become concentrated in their body)

  • I also don’t believe in soy supplements or protein powders. I believe that the benefits come from whole soy foods e.g. edamame, tofu, and tempeh. As with anything ultra-processed, the benefits seem to diminish and I believe this is where harm might creep in with the likes of soy protein isolates, supplements etc. 

  • It is a MYTH that only fermented soy is consumed in Asia. Tofu and soy milk are common and form the bulk of consumption for most Chinese, Japanese and Korean, with only a smaller portion being fermented e.g. miso. 
  • Did you know that how much benefit is conferred by soy foods is partly down to our gut bugs? Our gut microbiome converts some of these isoflavones into active compounds for us. e.g. dadzein is converted to equol, and equol is what confers the health benefits. This conversion may also be affected by our genetics. So this may be another reason why research on the benefits of soy is so confusing - it depends on what population you study! Because soy has traditionally been in the Asian diet, more Asians have the gut bugs that convert dadzein to equol. Could regular soy conversion nurture these gut bugs? It's certainly possible! We know our diet has a big role to play in shaping our gut microbiome.

As for thyroid – this is nuanced. Soy DOES contain goitrogenic compounds (as do cruciferous and flaxseeds) and so can cause thyroid issues in those who have iodine deficiency or have borderline low iodine, so for those with thyroid issues or subclinical hypothyroidism, ask your doctor about your iodine status.

If you observe cultures that consume a diet rich in soy, you'll notice that most also consume a lot of seaweed (e.g. Chinese, Japanese, Korean) - seaweed is rich in iodine. 

The other thing to remember is that, if you take thyroid medications, this should be taken AWAY from soy consumption (ideally at least 4 hours). 


The research so far suggests that soy confers health benefits including bone, breast, and cardiovascular health. If you have an intolerance or allergy to soy, avoid it, and if you have thyroid conditions, ask your doctor about your iodine status and make sure your thyroid medication is at least 4 hours away from soy consumption. 

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(1) Ziegler RG, Hoover RN, Pike MC et al Migration patterns and breast cancer risk in Asian-American women.J Natl Cancer Inst. 1993 Nov 17;85(22):1819-27

(2) Mueller SO, Simon S, Chae K, Metzler M, Korach KS. Phytoestrogens and their human metabolites show distinct agonistic and antagonistic properties on estrogen receptor alpha (ERalpha) and ERbeta in human cells. Toxicol Sci. 2004 Jul;80(1):14-25. Epub 2004 Apr 14.

(3) Contemp Oncol (Pozn). Diet and risk of breast cancer.2016;20(1):13-9. doi: 10.5114/wo.2014.40560. Epub 2016 Mar 16.

(4) Trock BJ, Hilakivi-Clarke L, Clarke R. Meta-analysis of soy intake and breast cancer risk. J Natl Cancer Inst. 98(7):459-71, 2006

(5) Wu AH, Yu MC, Tseng CC, Pike MC. Epidemiology of soy exposures and breast cancer risk. Br J Cancer. 98(1):9-14, 2008.

(6) Zhang X, Shu X-O, Li H, et al. Prospective cohort study of soy food consumption and risk of bone fracture among postmenopausal women. Arch Intern Med 2005; 165:1890-1895

(7) Sang_ah Lee, Xiao_ou Shu, et al Adolescent and adult soy food intake and breast cancer risk: results from the Shanghai Women’s Health Study1,2,3,4  Am J Clin Nutr. 2009 Jun; 89(6): 1920–1926.

(8) Korde LA, Wu AH, Fears T et al. Childhood soy intake and breast cancer risk in Asian American woen. Cancer Epidemiol Biomarkers Prev 2009; 18 (4):1050-9

(9) Chi F, Wu R, Zeng YC et al. Post-diagnosis soy food intake and breast caner survival: a meta-analysis of cohort sutides. Asian Pac J Cancer Prev. 2013; 14 (4): 2407-12

(10) Nechuta SJ, Caan BJ, Chen WY, et al. Soy food intake after diagnosis of breast cancer and survival: an in-depth analysis of combined evidence from cohort studies of US and Chinese women. Am J Clin Nutr. 96(1):123-32, 2012.