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Soy & Estrogen

"Don't Eat That! You'll Grow Boobs!"

If you’ve heard of soy, you may have heard a man somewhere say; “I can’t eat soy, I’ll grow boobs!” Well.. does this conversational anecdote have any factual backing? Let's discuss:

Soy is a legume that contains isoflavones. Isoflavones are a type of polyphenol found in legumes, including soybeans, chickpeas, fava beans, pistachios, peanuts, and other fruits and nuts. Of the above foods, soybeans are the richest source of isoflavones. Isoflavones protect plants from outside dangers, such as pests and drought. In the human body, isoflavones have powerful antioxidant properties. Antioxidants are necessary during normal metabolic processes to clear oxidative stress. Reducing oxidative stress enhances recovery from physical exercise and can prevent disease and slow aging.

The isoflavone in soy is referred to as a plant estrogen. When this plant estrogen is consumed by humans, we raise the question of what does consuming more “estrogen” do to our bodies? Before answering this, it is important to note that isoflavones are not the same as human estrogen. Isoflavone is considered a “weak” estrogen as it can bind at estrogen receptor sites in the body and only has a weak estrogen effect. In fact, isoflavone will compete with the body’s natural estrogen for the available binding sites and can actually reduce the total estrogen effect on the tissues.

All males and females make estrogen from testosterone and typically females have a lot more estrogen in circulation.


There is no clear evidence that eating soy increases estrogen levels in women or men. Below you will find three prominent studies on the matter;

In 2010, a meta-analysis published in Fertility and Sterility shared the results of more than 30 studies involving hundreds of men. These studies showed that soy does not increase estrogen or decrease testosterone in men or women.

A 2019 study published in the journal "Nutrition Reviews" looked at the results of 112 studies on the effects of soy on estrogen levels. The study found that soy had no significant effect on estrogen levels in women, and may actually lower testosterone levels in men. But this lowering of testosterone levels is miniature and definitely not enough to cause a development of breasts.

Another study, published in the journal "Cancer Epidemiology, Biomarkers & Prevention" in 2018, found that women who ate the most soy had a slightly lower risk of breast cancer than women who ate the least soy. However, the study did not find a link between soy intake and the risk of other types of cancer.


As with all foods, the less processed the food is the more balanced and more healthful. When it comes to soy foods for health, as with all foods, avoid highly processed foods.

  • Unprocessed: Soya beans, edamame.

  • Minimally processed: Soy milk, tofu, tempeh, soy nuts.

  • Moderately processed: Miso, natto, soy yogurts.

  • Highly processed: Extruded soy proteins (like soy protein isolate), soy oils and faux meat

If you are concerned about the effects of soy on your estrogen levels, you can always consult with your doctor or opt for plant based sources that do not include soy, as there are an abundances of plant-based proteins.

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