There is so much misinformation about this online so I just wanted to clear it up with a short and concise post today to help you know what the truth actually is.
Soy contains compounds which are called Isoflavones and these are a type of phytoestrogen. They’re a plant-derived compound that has a similar chemical structure to human oestrogen, so this did lead to some concerns because some breast cancers are oestrogen receptor positive, which means they are stimulated by oestrogen.
Some early animal studies, which were generally done using rodents, did sometimes show that high doses of isolated soy compounds stimulated cancer growth.
But, it’s really important to remember that we are not rats! Humans metabolize soy differently to how rodents do, so the findings from these early studies were often misinterpreted or oversimplified.
We now have an overwhelming body of evidence that consistently shows that soy consumption is not only safe but may actually be protective, particularly against breast cancer.
There have been population studies, particularly conducted in Asian countries where soy is of course a dietary staple, that have shown a significant correlation between higher soy intake and a lower risk of breast cancer.
Some of these studies actually showed a reduction of around 30% in breast cancer risk among women with higher soy consumption in their diet.
A systematic review and meta-analysis by van Die et al. (2023) found that soy isoflavones were associated with a 26% reduced risk of recurrence, particularly among postmenopausal and oestrogen receptor-positive survivors.
Another meta-analysis by Boutas et al. (2022) concluded that soy isoflavones can reduce the risk of breast cancer in both pre-menopausal and post-menopausal women.
What changed?
More research & a better understanding of the mechanisms.
We now fully understand how phytoestrogens interact within our bodies. While they are structurally similar to oestrogen, they don’t act in the same way as the oestrogen produced by our bodies
Human oestrogen primarily binds to oestrogen receptor alpha (ER-alpha), which can stimulate cell growth.
Phytoestrogens, like those found in soy (genistein and daidzein), preferentially bind to oestrogen receptor beta (ER-beta).
This binding can actually have anti-proliferative effects, meaning they can help slow down cell growth, or act as weak oestrogen antagonists - blocking stronger human oestrogen from binding to the receptors.
The American Institute for Cancer Research (AICR) now states beyond doubt that soy foods present no increased risk for breast cancer survivors.
In fact, they highlight that some evidence suggests a protective effect.
How to include Soy in your diet:
If you’re considering adding more soy to your diet, focus on whole, unprocessed soy foods. These include:
Tofu: A versatile protein source that absorbs flavours well.
Edamame: Delicious steamed or roasted, perfect as a snack or in salads.
Tempeh: A fermented soy product with a firm texture, great in stir-fries or sandwiches.
Soy Milk: A plant-based milk alternative; but be mindful of added sugars.
I highly recommended you choose these whole food forms and avoid highly processed soy isolates or supplements; which have unnaturally high levels of the isoflavones and can cause contraindications with your treatments.
A few considerations:
As with everything, more isn’t always best, and we don’t need to go to extremes when adding soy to our diet.
Soy contains goitrogens, which can interfere with thyroid activity. A moderate soy intake doesn’t harm healthy thyroids, but if you’re on Thyroid medication, you should consult your Doctor as soy can interfere with medication absorption.
Phytates in soy may slightly reduce the absorption of minerals like iron and zinc, so if you know you are already low in these then please check with your Doctor before increasing your intake.
If you have IBS or gut issues, soy can be a trigger, so excessive intake should be avoided.
I hope that has helped to clear this myth up for you and to help you understand how you could introduce a little bit more soy into your diet if you choose to.
As always, my purpose is to empower you with the information to make informed choices for your health. I am never here to judge. I just want you to know all of the info so you can make the best decision for you.
If you’re looking for support with your strength or nutrition during or after cancer treatment, please get in touch using the links below. I would love to have a chat and see how I can help you.
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