Does soy cause breast cancer?

We often hear a lot of things said on the internet about soy being "the evil” - but why would that be?

First of all, soy has phytoestrogens, which have a similar molecular structure to oestrogen, the female hormone.

Now, oestrogen does not 'cause' cancer.

However, women who already have breast cancer may be sensitive to excessive oestrogen levels in the body, which can cause hormonal imbalances and foment the proliferation of breast cancer cells.

In other words, excess oestrogen has a role in cancer growth (i.e. when you already have it) but it does NOT cause it on its own.

And what about cancer prevention? Or does plant oestrogen (phytoestrogen) increase your risk of cancer?

Overall, plant oestrogen is unlikely to increase your cancer risk. Epidemiological data suggests the opposite. Studies show that soy may protect you against breast cancer - for example, the women in Asia who eat the most soy have the lowest incidence of cancer.

Specifically, soy consumption in premenopausal women (i.e. those with regular female cycles) was shown to have decreased cancer risk.

This means that soy might be protective if you still have your female cycles. In particular tofu and soy milk (that is, the least processed types of soy products).

However, women with breast cancer, especially those that are hormone-receptor positive (oestrogen and progesterone) should speak to their doctors about consuming soy products. This is due to the cancer being activated by oestrogen through the receptors which soy may also bind to.

So if you already have breast cancer, you need to watch out for excessive phytoestrogens from your diet or anything that mimics oestrogen in your body, because you’d want to maintain hormonal balance. In this case, eating excess amounts of soy may not be a good idea. And by "excessive" we mean eating soy all day, as your main dish at every single meal. In other words, more than women in Asia do!

Which types of soy products?

Now, the one type of soy product that we strongly suggest that you avoid is “hydrogenated” soy oil, which uses an industrial process to harden the oil and increase its shelf life, so it can be used in junk foods.

Hydrogenated oils like this one are damaged 'trans fats', which provide no fuel to your body but harm it in
many ways.

Tofu and soy milk are okay. In fact, tofu seems to be more beneficial in terms of protecting against cancers like prostate cancer, and not just breast cancer.

However, soy sauce is a cause of concern: it turns out there are two different methods of brewing soy sauce. The traditional way is fermented (no harm done). But the other method creates the sauces by hydrolysing soy, which creates a number of unwanted by-products including MSG and potentially some carcinogenic chemicals. This Wikipedia article on soy sauce is very informative.

Fermented soy (like miso) has beneficial probiotics, like most fermented foods, such as yogurt or sauerkraut. But if you want the probiotic benefit of miso, the miso shouldn’t be cooked (it kills the beneficial bacteria).

Overall, soy’s isoflavones (mentioned in your course module) are good for us. Besides being cancer-protective, they help prevent osteoporosis in ageing women and heart disease in adults.

Here’s the links to the research:

Breast cancer

• Meta-analysis of soy intake and breast cancer risk
• Soy isoflavones consumption and risk of breast cancer incidence or recurrence: a meta-analysis of prospective studies
• Epidemiology of soy exposures and breast cancer risk
• Soyfood intake in the prevention of breast cancer risk in women: a meta-analysis of observational epidemiological studies

Prostate cancer

• Soy consumption and prostate cancer risk in men: a revisit of a meta-analysis

Heart disease

• Soy protein effects on serum lipoproteins: a quality assessment and meta-analysis of randomized, controlled studies
• Non-soy legume consumption lowers cholesterol levels: a meta-analysis of randomized controlled trials
• Relation between soy-associated isoflavones and LDL and HDL cholesterol concentrations in humans: a meta-analysis
• The effect of soy protein with or without isoflavones relative to milk protein on plasma lipids in hypercholesterolemic postmenopausal women.
• Association of dietary intake of soy, beans, and isoflavones with risk of cerebral and myocardial infarctions in Japanese populations: the Japan Public Health Center-based (JPHC) study cohort I.

What about soy consumption in males?

While soy contains a large amount of isoflavones (a type of phytochemical), when consumed in excess it can also mimic oestrogen in the body (which can be unwanted in some cases). Let’s look at a couple of studies in men…

A meta-analysis published in 2009 looked at the consumption of soy with prostate cancer risk based on data published worldwide. Soy foods significantly decreased prostate cancer risk. Tofu showed the largest
correlation, whereas there was no decreased risk with soymilk or miso.

In a small study of 29 men with prostate cancer, 50g of soy grits daily decreased PSA (marker of prostate cancer that is often elevated). This means that soy consumption might be not only beneficial for decreasing risk of prostate cancer, but also used as a potential adjuvant therapy.

What is considered an excess soy intake? An upper limit of about 25g soy protein per day (4 serving per day for adults) has been suggested by the United Soybean Board.

To conclude, whenever you read on the internet “soy promotes cancer growth” or “soy linked to cancer”, please take these statements with a pinch of salt, because every person is different, and things need to be considered on a case-by-case basis. Hopefully by now you would have an idea of where these claims come from and how to interpret them in a smart(er) way.

If you're interested in diving a lot deeper into the science of nutrition for cancer prevention and longevity, we have an entire advanced certification for that! You can download the full PDF curriculum here.

AUTHOR: Alex Ruani, Research Director

Updated on: 27/10/2026