When a person hears the word cancer, many things change. Especially the view on food! Questions quickly arise on forums, in books, and during conversations with others: can I eat soy? Is it safe? Could it even help or harm in breast cancer treatment? It seems everyone has their own story and advice, but what does science say and what about common sense?
In recent years, soy, this modest legume from Asia, has become the subject of numerous debates. Some praise it as a remedy, while others warn against it, especially in women with a diagnosis of breast cancer. But let's see what the scientific assessment says about this and whether soy really deserves so much distrust.
Firstly, what is the problem with soy?
Soy contains isoflavones, natural compounds structurally similar to the female hormone estrogen. Because of this, they are often referred to as phytoestrogens or plant estrogens. Phytoestrogen means that the substance has a similar shape to estrogen but works much weaker. It can bind to estrogen receptors in the body, but does not trigger the same effect as real estrogen.
It is precisely because of this similarity that isoflavones have raised concerns in the past that they could mimic the effects of a hormone often associated with the growth of certain types of breast cancer. However, things are not so simple. An increasing number of studies show that isoflavones from food do not act like estrogen, but often even the opposite. They block the action of real estrogens and can thereby slow down the growth of certain types of cancer cells.
Large study: more soy, fewer deaths
One of the most important studies on this topic was published after a nine-year study involving 6,235 women from the USA and Canada who already had a diagnosis of breast cancer. Scientists monitored their diet, especially the amount of isoflavones consumed through soy products such as tofu, soy milk, tempeh (fermented soy), and miso soup.
The results were surprising: women who consumed more soy isoflavones had a 21% lower risk of death compared to those who consumed them the least. This is a significant number. And it wasn't just one study! Similar findings were also recorded in Asian countries where consuming soy is part of daily life.
Interestingly, those who benefited the most were:

• those not taking so-called antiestrogen therapy (therapy that suppresses estrogen activity),
• those with tumors not sensitive to hormones (referred to as negative hormone receptors).

In simple terms, soy acted as a protective factor in these women. And more importantly, even in those undergoing hormone therapy, consuming soy did not increase the risk, as scientists once feared.
What do other sources say?
A very similar effect was also observed in an analysis of 14 studies published in the journal PLOS ONE (2014), where they found that higher soy intake was associated with a lower rate of cancer recurrence and higher survival rates. On average, the risk of cancer recurrence decreased by 26% if women regularly consumed soy after diagnosis.
In Asian countries, where women consume an average of up to 50 mg of isoflavones per day (compared to around 1 to 5 mg in Western countries), the breast cancer rate is one-third lower than in the Western world.
Of course, this does not mean that soy treats cancer. It does mean that it does not harm, as some once claimed, and in certain cases, can even be beneficial.
Why is there so much confusion then?
Confusion arose because some older studies, often conducted on animals or in test tubes, showed that isoflavones could accelerate the growth of certain cancer cells. However, scientists later found that the doses in these experiments were unreasonably high and that animal organisms are not a good approximation to the human body when it comes to soy metabolism.
Additionally, dietary supplements with isolated isoflavones (e.g., in capsules) were much stronger than those consumed in food. And this is a crucial difference. Isoflavones from food act differently than those in tablets.
Therefore, it is recommended today: if you do consume soy, it is better to do so in its natural form rather than in concentrated supplements.
What does this mean for us?
If you or someone close to you has been diagnosed with breast cancer, there is no reason to remove soy products from your diet. Of course, this does not mean that you should only eat soy. Like with all things in nutrition, the key to success is balance.
Fermented products like tempeh, miso, and natto not only contain isoflavones but also beneficial bacteria that have a positive impact on digestion and the immune system. This is why they have been recommended in traditional Chinese and Japanese medicine for centuries as foods that strengthen the body after illness.
What about other types of cancer?
Soy has not only been studied in breast cancer. Data is also emerging for prostate, colon, and lung cancer, where the effects can be positive. In a study from 2020 (American Journal of Clinical Nutrition), it was found that men who regularly consume tofu and soy milk have a 19% lower risk of prostate cancer. This is especially important for those with a family history of the disease.
It is worth emphasizing here: soy itself is not a cure, but it is not an enemy, as some wanted to portray it.
Common sense and science hand in hand
In traditional medicine, soy is known as a food that cools the body, meaning it alleviates inflammation and has a calming effect, especially in menopausal women. This aligns nicely with scientific findings since inflammation plays a significant role in the development of many diseases, including cancer. If food can be used as an aide in treatment, why not use it?
As an old saying goes: what is natural should be a support, not a replacement for discernment.
Let soy not worry you, but rather empower you
If until now you thought soy was dangerous in cancer, you are right. Soy products are not only safe but can even help in certain cases, reduce the risk of disease recurrence, and contribute to a better quality of life.
It is important to opt for unprocessed, fermented, or minimally processed products, and to see food as part of a holistic approach to healing. Soy should be part of a balanced diet, rich in vegetables, legumes, nuts, and healthy fats... This way, you will do the most for your body that you can.
Therefore, do not be afraid of tofu or soy milk, if they come from reliable sources and are part of a diverse diet, they can support you on the path to health.