Is soy good or bad?

Soy is probably the most controversial plant food in the limelight right now. The main concern is around whether they can increase breast cancer in women, cause man boobs, and also its effect on the thyroid. 


This is particularly relevant to the allergy community because soy milk is most similar to cow’s milk in nutrition and there is a high rate of cross-reactivity if you have cow's milk protein allergy. 


I’ve never really believed all the negativity in regards to breast cancer risks because, being Chinese, I grew up on 1-2 servings of soy a day. In fact, generations of my family did. Claims in the media that most soy consumed by asians are fermented is not entirely true either – for our family it was around 10-20% fermented but tofu and soy milk featured frequently and they are not fermented.  My grandparents were vegetarians and relied on soy for their protein and calcium intake – they both lived to their 90s, and last time I checked, NONE of my immediate or extended male family members in Taiwan had man-boobs and they regularly consumed soy. 


Here’s what I found when I looked at the research:

  • 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 isoflavones). Many people are put off by the word ‘phytoestrogens’ assuming they are ‘estrognes’ and think that eating soy would give you manboobs or feminize boys, but actually phytoestrogens behave quite differently in the body compared to a woman’s natural estrogen. 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 phytoestrogen. There are no studies I’m aware of that showed that infants given soy formula showed feminization. And only ONE case report of a man who got manboobs – but that's cos he drank 3 quarts per day of soy milk.

  • There are two types of estrogen receptors in the body – alpha and beta. Our own estrogen preferentially binds to the alpha receptors but the isoflavones preferentially bind to beta (2). Breast tissue contain both alpha and beta receptors. It’s thought that the activation of alpha receptors in the breast promote tumor growth whereas the activation of betareceptors has the opposite (anti-estrogenic and therefore anti-growth) effect.This might explain why oestrodiol 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 has 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. I want to see data from research on humans, not rats.

  • So here's a human study - and a huge one at that. 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, adolescent 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.

  • Be aware of GMO soy because these have been engineered to withstand high concentrations of round up (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 a 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, tempeh. As with anything processed, the benefits seem to diminish and I believe this is where harm might creep in.

  • 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!

As for thyroid – yes I understand that 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,  be guided by your physicians on this.

So in conclusion, much more research is needed to give a definitive opinion and recommendation on soy in its cancer-protective effects, but the studies on humans that I have come across (referenced here) seem to suggest that it may have a protective effect against breast cancer and prostate cancer. According to WHO, Japan has the longest life expectancy in the world, with a recent study in The Lancet (feb 2017) showing women in South Korea projected to live the longest and beyond 90 years by 2030. Both these countries eat moderate amounts of soy in their diet (although there may be other dietary or environmental factors e.g. green tea 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.

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