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Frustrating myths questioning the safety of soy still persist to this day, despite the fact that they were debunked many years ago. The hypotheses that soy causes breast cancer and interferes with male hormones originated mainly from rodent studies, and a very unusual case of gynecomastia in one man who was consuming 12 servings of soy a day (you probably don’t need to be told that this is an excessive amount) – neither of which which can be considered a reliable basis for setting nutrition recommendations.
Clinical studies[1] show no effects of soy on reproductive hormones in men, and countries that consume the highest amounts of soy products actually have the lowest incidences of breast cancer[2], so blaming soy for causing cancerous breast tumors seems to be a little illogical, even if we don’t bother to look at the science.
But we’re going to do that anyway…
Soy, like many other plant foods, contains a compound called isoflavones. Soy isoflavones have a similar (but not identical) structure to human oestrogen, so are classified as phytoestrogens. However, they behave very differently in the body – they actually have the exact opposite effect, and are anti-oestrogenic[3].
An umbrella review[4] in 2019 found that soy is actually beneficial to health and reduces the risk of heart attack, stroke, death from cardiovascular disease, type 2 diabetes, and several cancers including breast, prostate, ovarian, lung and colorectal. And, the most comprehensive review[5] of the scientific literature on soy consumption in humans to date, confirms that soy is safe when consumed in the amounts found in traditional Asian diets, i.e. two to three servings a day.
Is soy safe for me?
For the majority of people, soy is perfectly safe, and as we have seen above, it appears to provide many health benefits.
You will often see me recommend tofu, tempeh, edamame and soya milk as they are a great source of plant-based protein, low in saturated fat, free from cholesterol, and high in unsaturated fats. What’s more, soy products are rich in non-haem iron, calcium, zinc and selenium which are all nutrients of interest for people on a plant-based diet.
One word of caution is for people taking thyroid hormone medication used to treat hypothyroidism. While soy doesn’t interfere with thyroid function, it can interfere with the absorption of the drug thyroxine. Some doctors recommend avoiding soy if you are taking this medication, but the position of the British Dietetic Association and the British Thyroid Foundation is that soy foods don’t need to be avoided entirely, just consumed at least four hours before/after the medication.
Please speak with your GP/primary care doctor before making changes to your diet if you have a thyroid condition as your medication may need to be adjusted.
Soy is also a known allergen, and although soy allergy is rare, reactions can be serious.
However, if you don’t have a thyroid disorder or soy allergy, soy is a safe and nutritious addition to your diet.
References
- Hamilton-Reeves JM, Vazquez G, Duval SJ, Phipps WR, Kurzer MS, Messina MJ. Clinical studies show no effects of soy protein or isoflavones on reproductive hormones in men: results of a meta-analysis. Fertil Steril. 2010 Aug;94(3):997-1007. doi: 10.1016/j.fertnstert.2009.04.038. Epub 2009 Jun 12. PMID: 19524224.
- Hilakivi-Clarke L, Andrade JE, Helferich W. Is soy consumption good or bad for the breast? J Nutr. 2010 Dec;140(12):2326S-2334S. doi: 10.3945/jn.110.124230. Epub 2010 Oct 27. PMID: 20980638; PMCID: PMC2981011.
- Hooper L, Ryder JJ, Kurzer MS, Lampe JW, Messina MJ, Phipps WR, Cassidy A. Effects of soy protein and isoflavones on circulating hormone concentrations in pre- and post-menopausal women: a systematic review and meta-analysis. Hum Reprod Update. 2009 Jul-Aug;15(4):423-40. doi: 10.1093/humupd/dmp010. Epub 2009 Mar 19. PMID: 19299447; PMCID: PMC2691652.
- Li N, Wu X, Zhuang W, Xia L, Chen Y, Zhao R, Yi M, Wan Q, Du L, Zhou Y. Soy and Isoflavone Consumption and Multiple Health Outcomes: Umbrella Review of Systematic Reviews and Meta-Analyses of Observational Studies and Randomized Trials in Humans. Mol Nutr Food Res. 2020 Feb;64(4):e1900751. doi: 10.1002/mnfr.201900751. Epub 2019 Oct 14. PMID: 31584249.
- (2021) Neither soyfoods nor isoflavones warrant classification as endocrine disruptors: a technical review of the observational and clinical data, Critical Reviews in Food Science and Nutrition, DOI: 10.1080/10408398.2021.1895054.
