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Welcome to my third article on soy bean based foods, where I help to debunk myths around whether soy foods are healthy or harmful.

In my first article found, I answered the top 10 questions around soy foods that people ask me on a daily basis. I then mentioned that I will be going into each one of these topics in depth, so you can be informed and empowered about the choices you make around food and your health.

If you haven’t already, you may want to learn about Soy Fact #1: Soy & Hormones – debunking the myths -:

So here is soy fact #2:

Soy foods can help protect against certain cancers

Does eating Soy foods cause Cancer?

The relationship between diet and cancer

Anyone who has been touched by cancer in some way, shape or form will know just how devastating the disease can be. This year alone, 138,321 people have been newly diagnosed with cancer in Australia, with rates in males a tad higher than those in females. This is over 3 times higher than rates in the 1980’s. It is estimated that 48,500 people have died from cancer related deaths in Australia in the past 5 months alone, making this a very serious issue.

Numbers are also on the rise. So much so that according to Cancer Australia (2018); “in 2017, it was estimated that the risk of an individual being diagnosed with cancer by their 85th birthday will be 1 in 2 (for both males and females) (1-2).

But this article is not designed to make you feel fearful or pessimistic. It’s about providing you with the latest research so you can make informed food choices that best benefit your health and well being.

So is there anything we can do to help prevent or even treat cancer? Well yes, the top three most common cancers in Australia (breast, bowel and prostate cancer) all have dietary related risk factors. Migration studies (studies that show that when a population moves from their native country e.g. Japan to a different country e.g. USA they develop diseases they didn’t have before) also suggest that there are many lifestyle related risk factors for cancer, including what we choose to eat and drink.

In this article I will debunk myths around whether soy foods (tofu, tempeh, miso, soy milk, edemame, natto, soy beans etc) increase your risk of cancer, or if in fact they can actually help to reduce your risk or reverse the disease.

Soy foods and Cancer risk

Anti-cancer properties in soy foods

Soy isoflavones

The key ingredient in whole soy foods believed to inhibit or block cancer cell growth is a plant compound in the soy beans themselves called isoflavones. Isoflavones are a nutraceutical (like a natural plant nutrient version of pharmaceuticals). Soy foods and soy beans are actually the only food to contain decent amounts of isoflavones. Isoflavones are also heat stable, so you can cook soy foods without effecting the health promoting effects.

There are 3 major isoflavones in soy:
• Genistein
• Diadzein
• Glycitein

1 serve of soy foods (approximately 1 cup) contains 25mg of isoflavones.

Traditional Asian diets contain roughly 25-50mg soy isoflavones or 1-2 cups of whole soy foods (43).

The key isoflavone in soy reported to be ‘anti-cancer’ is genistein, which looks very similar to your body’s natural hormone oestrogen. Read my previous article here for more about Soy and hormones or check out the Cancer Council NSW website for a useful description of soy isoflavones here.

Soy isoflavones and hormones. Does soy cause Cancer?

Soy Intake and Global Cancer  Rates

If you compare the global incidence of cancer, which is how many people out of 100,000 are newly diagnosed (adjusted for age) per country, China comes out with the lowest global cancer rates at 19.1%. This is followed by Japan (19.7%), India then Sweden and Finland sit in the middle and right at the top with the highest global rates of cancer is the USA at 87% (3). Could it be that traditional Asian plant-based, soy heavy diets are cancer protective while a highly processed meat and dairy-heavy Western diet could be cancer causing…? Well this thought is where the research all started . (Note: genetic variations were accounted for in these figures).

Brast Cancer rates in Japan and US. Does Soy cause Cancer?

Well, there is some suggested truth to this in migration studies. This graph is from one such study. It shows consistently high rates of breast cancer in Caucasian Americans eating a Western diet. It also compares this with Japanese citizens who have immigrated to America and slowly adopted a Western diet in place of a traditional diet over a 10 year span. You can see that the Japanese immigrants slowly develop higher rates of cancer similar to their Caucasian neighbors the longer they spend time in America, and possibly the longer they eat American style food as well (4-5).

But this is just an observational study. There could be other factors like air pollution, so it doesn’t prove cause and effect. It only highlights trends.

Making tofu. Soy foods can help protect against certain cancers

Do other longitudinal and control studies paint a clearer picture? Read on to find out.

Soy and Breast Cancer risk

Originally it was believed that eating soy foods increased your chances of developing breast cancer. This is because high levels of a human hormone called oestrogen are linked with the development of breast cancer and soy beans contain a compound that looks very similar to estrogen, called isoflavones. This theory has since been proven to be a HUGE misconception.


  • Firstly, many of these studies were based on rats and mice, which metabolise soy very differently to humans.
  • Secondly, vegetarian and vegan diets which include soy foods tend to be lower in dietary fat and higher in dietary fibre, which both decrease oestrogen levels in your body (6).
  • Thirdly, soy isoflavones actually change or block the way your body metabolises (processes) oestrogen in a way that actually reduces ones’ risk of breast cancer. This is where we get sciencey:

The science behind soy and cancer

Soy isoflavones are what’s called a ‘selective oestrogen receptor modulator’ or SERM’s. A fancy term which means they help to activate oestrogen hormones in some parts of your body e.g. your bones. But they also block the activation of oestrogen hormones in other parts of your body like breast tissue. It does this because it looks like oestrogen but isn’t, so it binds to the oestrogen receptor and thus blocks actual oestrogen from binding and doing its thang. So, it actually BLOCKS the cancer causing activity of oestrogen in breast tissue. This is how one of the leading breast cancer drugs tamoxifen works; by blocking oestrogen activity in breast tissue.

So by blocking oestrogen, the isoflavones in whole soy foods help to inhibit the growth of breast cancer cells, hence being anti-cancer. So soy is like the drug…but without the nasty side effects and with the added bonus of other beneficial nutrients.

Oestrogen molecule. Soy foods can help protect against certain cancers

If you want some extra information read on:

This is because there are 2 types of estrogen receptors in the body: alpha and beta receptors (think A and B).

Soy isoflavones mostly attach to the B (beta receptors). Beta receptors are mostly in your bones, while alpha receptors are in reproductive organs. You would need to eat around 20-30 cups of soy foods to activate the A (alpha receptors) in breast tissue. To date, I have never heard of someone eating 20-30 cups of tofu, tempeh, soy beans, natto, edemame beans or miso paste in a single day (7-8)

In fact, a study by Korde et al (2009) found that eating 1-3 cups of soy foods daily in childhood, teenage years and adulthood was associated with 40-80% less risk (roughly half the chance) of developing breast cancer (9)

Studies show the quantity of phytoestrogens (isoflavones) found in just a single cup of soymilk (bean based-non GMO) alone may reduce the risk of breast cancer returning by 25%!

Soy Milk. Soy and Cancer risk

But it’s not just prevention. In 2009, a longitudinal study called the LACE study (Life After Cancer Epidemiological) Study found that those who ate the most soy isoflavones (20mg vs 5mg or 1 cup soy foods vs trace amounts) had the greatest reduction in breast cancer growth (60% less) than those who only used cancer drugs like Tamoxifen but ate no soy foods (10). Another long-term study of 5,000 breast cancer survivors found that “among women with breast cancer, soy food consumption was significantly associated with decreased risk of death and re-occurrence”(11). Since then multiple studies of thousands of breast cancer patients (of all ages and types of breast cancer) have found time after time again that women with breast cancer who eat the most soy foods (and soy isoflavones) have the lowest risk of cancer related death, longer life expectancy and lowest chance of the cancer re-occurring (12-20).

But how much did they consume to cut their breast cancer cell growth by half? More than the odd cube of tofu, we’re talking amounts similar to that consumed in traditional Asian diets. So 1-2 serves or cups daily. Which is actually quiet achievable in a balanced plant-based diet but may make someone consuming a typical Western diet balk.

Also, as I will explain in my next blog article, benefits are only found with WHOLE soy foods (edamame, soy beans, tofu, tempeh, soy bean based milks, natto, miso paste etc) and NOT ISOLATED SOY PROTEIN found in supplements and processed foods.

As such, the American Cancer Association national guidelines report that for breast cancer survivors, consuming soy and soy-based foods has the potential to have “a positive synergetic effect with tamoxifen” a leading cancer drug, meaning better results with the drug AND soy foods than the drug alone. They also state that there’s no evidence of soy foods having any NEGATIVE affects in terms of breast cancer growth AND that soy foods are an excellent source of protein and thus an alternative to meat consumption. This is important as people with or who have survived cancer often experience muscle wasting and need higher amounts of protein in their diet, which soy can provide (21).

Soy can reduce risk of some types of cancer, including breast cancer

This is also the position of Cancer Council NSW who state; “Cancer Council supports the consumption of soy foods in the diet. This is consistent with Cancer Council recommendations and national dietary guidelines to eat a diet high in plant-based foods (22).

So the verdict: Soy foods don’t promote breast cancer cell growth and actually appear to help prevent breast cancer cell growth. And for people diagnosed with breast cancer, eating soy foods can be a beneficial adjunct therapy that has no negative consequences (unlike many cancer drugs) and is a nutritious source of protein and other vital nutrients including fibre.

Soy and Prostate Cancer risk

Prostate cancer is similar to breast cancer in that rates are highest in Western countries such as the USA and Australia and lowest in soy food-consuming Asian countries such as Japan (23-24). As such, The American Cancer Society states that eating soy foods is one way to reduce your risk of developing prostate cancer (21).

International studies suggest that a high meat and dairy-based diet may be linked to greater risk of developing cancers including: breast, colon, prostate, renal and endometrial cancers (25-26). Thus, considering that soy foods offer a nutrient rich replacement for meat and dairy, could this be the key?

But studies have found that there’s more to soy foods that just replacing potentially pro-cancer foods. Control studies using mice injected with human prostate cancer cells found that those who were fed soy had slower tumour cell growth or even blocked (inhibited) tumour cell growth (27). They also found that the isoflavones in the soy foods have various ways of de-regulating cancer cells, like turning the off switch.

This is mainly due to the isoflavone called ginistein, which I mentioned before. In these studies, ginistein blocked the growth of the two main types of prostate cancer and also block the current cancer cells from spreading (metastasizing) elsewhere in the body (28-30).

However, these are just animal studies. Humans are not rodents. Studies in actual humans have had some mixed results and quite frankly better and longer designed studies are needed. Nevertheless, there is some true promise for eating soy foods to prevent and treat prostate cancer. I’ll fill you in on what we know so far.

Soy reduces risk of Prostate Cancer

A 5-year observational study in America of 12,000 Seventh Day Adventist men showed that those who drank a cup of soy milk once to twice daily had 70% less chance of developing prostate cancer (31).

Another study of Chinese men from 12 different cities found those who ate roughly ½-1 cup of soy food daily had 50% less risk of developing prostate cancer than the men who ate less than 1/4 cup daily (32).

In terms of treatment, there’s some promise there too. A small study conducted in 2002 of 40 men with prostate cancer who weren’t responding to typical medical treatment (chemotherapy, pharmaceuticals etc) found that up to 70% who ate 120mg soy isoflavones daily had lower PSA scores (33). PSA is a marker for prostate cancer growth.

Since then, eight randomised control trials on the subject have been conducted. Two of these studies found that prostate cancer cell growth halved when the men ate soy foods daily. None of the studies showed any negative effects from eating soy foods (34-37).

Tofu burgers. Eating Soy foods can reduce risk of prostate Cancer

Two other meta-analysis in combination looked at 28 studies and also found that eating more soy foods was associated with 69-75% decreased prostate cancer risk (38).

The verdict:

Overall, it appears that soy foods (and soy isoflavones) seem to reduce men’s risk of getting prostate cancer and potentially could help treat prostate cancer.

However, more studies longer than 1 year are needed as it’s still unclear exactly how soy foods effect prostate cells. Nevertheless, no harm has been shown from consuming soy foods even at high doses for long periods of time. So they’re a healthy nutritious food to include in the diet for anyone, including people with prostate cancer too.

Soy and Ovarian and Endometrial Cancer

The positive anti-cancer effects of soy are also seen in other cancers of the reproductive organs. In a systematic review of 7 longitudinal studies, women who ate the most soy (again 1-3 cups or 20mg isoflavones vs the least <1/2 a cup or 5mg isoflavones) had roughly 50% less risk of developing ovarian cancer and 70% less risk of developing endometrial cancer (40).

Soy and Cancer. Eating soy foods may reduce your risk of Ovarian Cancer

Results are a little mixed though. Another study of 47,140 Swedish women found that eating isoflavone containing foods had no effect on whether the women developed ovarian cancer or not. However tofu, tempeh and similar soy containing foods are not a big part of Nordic diets. Thus it’s likely that the Swedish women never actually ate ENOUGH isoflavones to have an impact on cancer cells (41).

Soy and Cancer. Eating soy foods may reduce your risk of Endometrial Cancer

A more recent prospective study of 46, 027 American women of various ethnicities found that those who ate the most isoflavone-containing foods (legumes, soy beans and tofu) had the lowest risk of developing endometrial cancer. The women who consumed more than 7.82mg of isoflavones per day (less than half a cup of these foods) had 34% less risk of developing endometrial cancer than the women who ate less than 2mg of isoflavones containing foods (practically none) daily. The isoflavone called daidzein appears to be especially protective. The women who consumed the most daidzein per day (3.54mg vs <0.70 mg) had 36% or over 1/3 less chance of developing this type of cancer. When adjusted for age, those who ate the most isoflavones from soy foods and legumes have half the risk of developing endometrial cancer in their lifetime (42).

The verdict:

Ladies, the current evidence suggests that if a we eat enough isoflavones daily from foods such as tofu, tempeh, soy milk, soy beans, natto and edamame beans as well as other legumes (more than 1 cup daily) you can reduce your chance of getting ovarian or endometrial cancer by 50% to 70% which is pretty significant.

I recommend aiming to make these foods non-GMO or organic where possible.

Final thoughts on Soy and Cancer


Tofu. A summary of the research on Soy and Cancer risk
So there you have it, a quick summary of Soy and Cancer. Eating whole (non-GMO) soy foods like tofu, tempeh, natto, soy beans, edemame beans and certain soy milks are a good source of protein, fibre and other nutrients, both for people looking to prevent cancer as well as those who have been diagnosed with cancer. But more than this, these foods and their unique isoflavone compounds also appears to help prevent and treat reproductive (or hormone related) cancers such as breast, ovarian and endometrial cancer, and likely prostate cancer too. But a little more evidence is needed for prostate cancer. Either way there are only benefits in consuming whole soy foods for these cancers, and no known negative effects even if you eat tonnes of them for long periods of time.

However, as I have alluded too in this blog post, my professional recommendation is to eat whole soy foods that are either made from organic or non-GMO soy beans.

Find out why in my next article…

Related articles:

This article is brought to you by plant-based dietitian (APD) Amber Sewell-Green


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Amber Sewell-Green

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