Debunking soy misconceptions

By Easha Vigneswaran

The world of nutrition is saturated with finding food alternatives to staple foods in our diet. One food product that has been popularized is soy. It is a good source of protein and fibre, contains high levels of vitamins and minerals and has very little cholesterol and saturated fat. Soy-foods have been increasingly incorporated into the western diet but have also been met with many misconceptions about the health risk associated with their consumption.

One myth surrounding soy products is that they cause breast cancer. Soy is a source for isoflavones which are chemicals found in plants and can also have similar effects on the human body as oestrogen does. High levels of oestrogen are linked to the increased risk of breast cancer especially with regard to oestrogen positive breast cancer.1 However, many studies have now shown that soy food consumption does not increase the risk of developing breast cancer. In fact, some studies have shown soy consumption, especially during childhood, has been linked to a reduced risk of developing breast cancer. Isoflavones act like oestrogen (although they are not as strong) and so they can also bind to the same receptors. Isoflavones can block the oestrogen receptors therefore reducing the oestrogen formation in breast tissue and so reducing the chance of cancer developing.2

Another misconception that arose because soy is isoflavone rich is that it causes feminisation in men. Isoflavones are classified as phytoestrogens and studies have shown that male rats with high levels of this also exhibit infertility. However, the same was not detected in male humans. 3 There were two reported cases of “male feminisation” and, though the link was made between soy food consumption, in both cases more than 360mg/day of isoflavones were present in the diet which is almost nine times higher than that of most men’s intake. However other studies conducted on men given almost 150mg/day of isoflavone in their diet showed no effect on oestrogen levels.4 Furthermore, various other clinical trials were conducted which concluded that neither isoflavones nor soy protein had any effect on the level testosterone and oestradiol in male humans.5

In relation to the male infertility, there are some concerns associated with soy consumption and fertility issues. Some small studies found that there was a link between soy consumption and low sperm concentration. It should be emphasised that this does not mean a low sperm count was observed. The concentration appeared to decrease because of the volume of semen produced increasing in men consuming higher proportions of soy. The small scale of the study and the limited availably of data do not conclusively prove that soy can affect male fertility. Numerous other factors can influence findings like these and so the medical significance of the data is

Another concern that has arisen in relation to soy consumption is how it affects the thyroid function. In 2006, following 14 clinical trials, it was concluded that soy foods containing isoflavones had no effect on thyroid function in neither men nor women.6 A risk assessment conducted by the European Food Safety Authority (EFSA) also determined that isoflavones taken as supplements by women after menopause also do not affect their thyroid function.7 Though soy food does not affect people with a normal thyroid, those already suffering from hypothyroidism may need to increase their medication intake. The soy protein does not affect the thyroid itself but can affect the absorption of levothyroxine which is a medication used to treat an underactive thyroid.6 

Children fed soy-based formula during their infancy has also been met with the concerns that their cognitive function and growth might be affected. There have been issues raised that the soy-based formulas, due to their higher isoflavone levels could, affect infants’ development and adversely affect immune function. Studies conducted in 2012 compared the cognitive function of breast fed, cow-milk formula fed and soy-based formula fed babies. The study found that breast-fed babies had greater cognitive development however no difference was observed between all formula fed babies and they all grew normally. Recent research appears to concur with these findings that soy-based formula doesn’t seem to adversely affect child development.2

The nutritional benefits of soy foods have become more and more recognised and so have been incorporated into more individual’s diets globally. Their increasing popularity has also resulted in increasing scrutiny which led to the myths discussed above. Any foods in excess will always have adverse effects on the body potentially leading to chronic illness. When compared to other widely accepted western diet components, soy is ultimately healthier than many of them and can improve overall diet quality. 


  1. Hilakivi‐Clarke L, Andrade JE, Helferich W. Is Soy Consumption Good or Bad for the Breast?123. American Society for Nutrition; 2010. .
  2. Thalheimer, JC – Top 5 Soy Myths. Today’s Dietitian. Website. Available from: [Accessed 10th October 2021]
  3. Glover A, Assinder SJ. Acute exposure of adult male rats to dietary phytoestrogens reduces fecundity and alters epididymal steroid hormone receptor expression. The Journal of Endocrinology. 2006; 189 (3): 565-573. 10.1677/joe.1.06709.
  4. Messina M. Soybean isoflavone exposure does not have feminizing effects on men: a critical examination of the clinical evidence. Fertility and Sterility. 2010; 93 (7): 2095-2104. 10.1016/j.fertnstert.2010.03.002.
  5. Reed KE, Camargo J, Hamilton-Reeves J, Kurzer M, Messina M. Neither soy nor isoflavone intake affects male reproductive hormones: An expanded and updated meta-analysis of clinical studies. Reproductive Toxicology. 2021; 100 60-67. 10.1016/j.reprotox.2020.12.019.
  6. Messina M. Soy and Health Update: Evaluation of the Clinical and Epidemiologic Literature. Nutrients. 2016; 8 (12): 10.3390/nu8120754.
  7. EFSA. Risk assessment for peri- and post-menopausal women taking food supplements containing isolated isoflavones | European Food Safety Authority. 

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