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Blog Post


Written by Chloe Carroll, July 18th 2021, Student Dietitian, Bond University


Popular Science

The good, the bad & the ugly truth about soy

For centuries, soy foods, such as tofu, soy milk, miso, and soy sauce have been a long-term staple in the diets of many Asian countries. In more recent decades, with a growing body of literature supporting the purported health benefits of soy, this has contributed to an increase in the popularity and consumption of soy-based foods in the West.  


While there is evidence to support the nutritional quality and health benefits of soy, including a lower risk of heart disease, type 2 diabetes, and a decreased risk of hormone specific cancers (breast and prostate) - the soybean remains to be one of the most controversial foods in the world! So, what is soy bad about it?


Soy contains a class of compounds called isoflavones, many of which are often referred to as phytoestrogens (Phyto meaning plant) due to their ability to weakly bind to estrogen receptors in the body. Because of this estrogen-like effect, soy-based foods are often demonised, particularly from men whose greatest concern is whether soy is feminising them! But is there any merit to this?

Image by kimi lee

Kimi Lee, Unsplash

The truth is there is a small amount of evidence to support this theory. From the past 30+ years of soy-related health research published, there are two rather notorious medical reports that have linked a reduction in testosterone levels to the consumption of soy.


In the first case, the report described a 60-year-old male who had developed gynecomastia –  swelling of breast tissue in men, erectile dysfunction, and a decrease in his libido in response to the consumption of soy milk. They found his levels of the hormone estrogen to be around 4 times the upper limit of the reference range for men.


The other case was a 19-year-old vegan who had developed hypogonadism (reduced testosterone and sperm production) and erectile dysfunction from the ingestion of a variety of soy-based products. The reason both studies were able to link soy as the culprit, was that after both of the men had discontinued their consumption of soy foods, over time their symptoms resolved, and their hormone levels returned to normal. But is this enough evidence to support the notion that soy is feminising men, and should all men abstain from consuming soy foods?


Have your say?


Is soy really feminising men?

If you are going off of those two cases alone, then you may be disinclined or perhaps a little apprehensive to consume soy foods. But, what the highest level of evidence tells us, is that you have got nothing to worry about! A recently published, 2021, meta-analysis of 41 studies looking at soy or isoflavone intake and its effect on reproductive hormones in men, found that regardless of dose and duration of the study, neither soy nor isoflavone exposure affected testosterone levels in men.


Not surprisingly, the meta-analysis also referred to the two case studies mentioned above. They stated that both the 19 and 60-year-old males consumed an “estimated 360mg per day of isoflavones, which is approximately 9 times the typical intake of older native Japanese men” whose intake of isoflavones from soy is said to be the highest in the world at around 30-50mg per day.


To put this into perspective you would need to consume around 100g tofu, 1 cup of soy milk and around 1/2 a cup of edamame beans, 9 times over every day for months, possibly even years before the isoflavone content would likely have an effect on hormone levels. Not to mention, the consumption of high quantities of soy foods is likely to displace other foods and nutrients in the diet that are likely to have an effect on normal bodily functions e.g., hormones. It’s a case of you really can have too much of a good thing! For an overview of the isoflavone content of popular soy foods, see the table below.

Isoflavone content of popular soy foods

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Adapted from Harvard’s – The Nutrition Source, Straight Talk About Soy

The take home message

Regardless of gender, soy food can be included as part of any diet, and you can feel rest assured knowing that soy is more likely to benefit your health than cause you any harm. The key, like everything in life is to enjoy soy foods in moderation!

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Does a plant-based diet really help beat COVID-19?

Ref. Meta-analysis

Katharine E. Reed, J. C., Jill Hamilton-Reeves, Mindy Kurzer, Mark Messina. (2021). Neither soy nor isoflavone intake affects male reproductive hormones: An expanded and updated meta-analysis of clinical studies. Elsevier, 100, 60-67. doi:doi.org/10.1016/j.reprotox.2020.12.019

Looking to add more soy to your diet?
Check out these recipes!
Kung Pao Tofu