Almost daily I hear someone feeling confused about soy, should they, shouldn’t they eat or drink soy products?

I mean, a few weeks ago I was standing in line to order a coffee and the barista gave the customer ahead of me a hard line of questioning as to why he was choosing soy milk over almond milk in his coffee! The interaction ended with the customer saying he will risk the “possibility of developing man boobs for the taste”

First of all, I was completely outraged at the fact that someone would question another person about their milk choice, like REALLY? Second of all, no food should ever be THAT controversial, it’s just milk for goodness sakes! And finally, are these even scientifically founded?!

Anyway, I can see my clients are confused and so is the general public and I hear you! So here we are, a blog just talking about soy, I’ve talked about it a bit in my last blog on menopause and in a recent podcast about nutrition for PCOS.

Especially, working in women’s health and talking about the complexities of hormones and their role in health, I see lots of soy avoidance! But what happens if you’re vegan or vegetarian? What about men? What happens in PCOS or endometriosis?

So, for the situations below I will be answering – to soy or not to soy?

Of course, everyone is different and individual differences will come into play, so get in touch with your local Accredited Practising Dietitian to get tailored nutrition advice.

Before we jump into it, let’s talk about soy foods and the component that makes them such a hot button topic.

What is in Soy?

Soy foods are made from the soybean, whether eaten as edamame, tofu or tempeh or drizzled on dumplings as soy sauce or added to your cappuccino as soy milk, it has moved away from just being associated with traditional Asian cuisine and has integrated into the Western food scene, especially with the rise of veganism and plant-based protein.

Soy contains a type of chemical compound called phytoestrogens, which have oestrogen-like effects on the body, it’s like the plant version of oestrogen! The type found in soy foods are called isoflavones (and there are different types of isoflavones and so on), which are essentially a kind of antioxidant. Remember, oestrogen is a sex hormone found in both women AND men, just in differing amounts!

Soy foods richest in isoflavones are:

  • Soy protein
  • Miso
  • Edamame
  • Soy milk
  • Tofu & tempeh

Isoflavones are also found in other foods, such as:

  • Legumes & beans like chickpeas & fava beans
  • Peanuts & pistachios
  • Some fruits
  • Other nuts

Essentially, what we know about soy’s phytoestrogen effects is that:

  • It exerts an oestrogen effect when circulating levels of oestrogen are low
  • It exerts an anti-oestrogen effect when circulating levels of oestrogen are high (i.e. competition occurs)

Soy also contains phytic acid, which is known as an anti-nutrient, that is it opposes the action of other important nutrients, notably calcium and iron. This can be easily overcome by cooking or fermenting soy.


PCOS (Polycystic Ovary Syndrome) is a hormone condition that affects 18% of Australian women, it is characterised by menstrual irregularity, signs of excessive androgen hormones (i.e. “male” hormones, like testosterone) which can result in acne, excessive hair growth or hair loss and sometimes the presence of cysts on the ovaries.

There has been some high-quality research on the role of supplementing a particular isoflavone, called genistein found in soy products, for women with PCOS which has shown promise in improving the hormone profile, menstrual regularity & fertility as well as managing some of the risk factors for future heart disease like high cholesterol, after 3 months (Sources: Khani et al., 2011; Romualdi et al., 2008).Genistein, a type of isoflavone, has shown evidence to improve both the hormones & metabolic outcomes of women living with isoflavone

The dosages used in the study were 18 mg twice daily of genistein, however, the author notes that these dosages may not be optimal, but at this time is the best evidence that we have.

Looking at your oestrogen levels from blood tests with your dietitian and discussing whether this may be appropriate to include as part of your management plan, or simply eating more soy products and legumes, like chickpeas, could be something worth discussing to see if it can help!

So, it looks like soy foods could potentially be helpful for PCOS (remember, to account for other factors like low oestrogen which can subsequently affect bone mineral density and therefore consuming lots of uncooked soy rich in phytic acid may block calcium absorption and subsequently worsen bone health).

Read more about PCOS on my blog.

Breast Cancer

1 in 8 Australian women will be diagnosed with breast cancer in their lifetime (Source: National Breast Cancer Foundation). Breast cancer as a disease is strongly associated with oestrogens, therefore, soy consumption is called into question from both a prevention of onset & recurrence of breast cancer.

The incidence (i.e. how many diagnoses occur in a year) rate of breast cancer in countries like China & Vietnam is much lower, compared to Australia & New Zealand (Source: Youlden et al., 2014).

Obviously there are a lot of different factors to consider beyond just soy consumption between Australian women and women from countries like China, however it has been proposed that soy consumption may be a potential key factor, based on animal studies, soy has been shown to be protective and in studies of populations of people, non-fermented soy like soy milk and tofu were more likely to be protective or not associated with cancer, whereas fermented soy, like miso, showed no consistent pattern with cancer risk (Source: Messina et al., 1994).

The Cancer Council states there is not enough conclusive evidence or it is limited and suggestive towards a reduction of risk for a number of cancers. They do note their support of soy foods in the diet as per our national dietary guidelines which favours the consumption of plant foods, and do not advocate for soy protein or isoflavone supplements for health men & women for the purpose of cancer prevention.

You can read the position statement from the Cancer Council here.


For women who have already been diagnosed with breast cancer, a large cross-sectional study of over 5,000 female breast cancer survivors in Shanghai were interviewed over 60 months, showed that soy food consumption was significantly associated with a reduced risk of death and cancer recurrence (Source: Shu et al., 2010).

Looks like, it could be positive for soy when it comes to breast cancer, but without enough research it is hard to know whether we should be gunning for soy for breast cancer prevention. It is best to stick to soy foods rather than supplements if you’re at risk of breast cancer or have been diagnosed, based on current evidence.

Note: the Cancer Council cautions soy for women receiving breast cancer treatment, specifically Tamoxifen.


I wrote about the role of phytoestrogens in my most recent blog on Eating for Menopause, and we have pretty good evidence to show that including phytoestrogens are effective at reducing hot flush frequency in menopausal symptoms, however, are not necessarily useful for other symptoms of menopause and (bonus!) no serious side effects. This review was done using supplemental isoflavones, but there is no reason why you cannot apply this to including more soy foods in your diet – remember, food first! (Source: Chen, Lin & Liu, 2014)


Heart Health

Heart disease is still the number one killer in Australia, claiming nearly 50,000 lives in Australia every year, that’s one every 12 minutes (Source: Heart Foundation Australia)

Whilst some risk factors for heart disease cannot be changed including family history, genetics and age, there are lots of modifiable factors including maintaining a healthy body weight, eating a balanced diet, regular exercise, reducing alcohol intake, and quitting smoking to name a few.

Soy food consumption has been positive associated with heart health as it can lower LDL cholesterol (the type of cholesterol we would ideally like to lower), when replacing animal proteins in the diet, however it may not just be the isoflavones that are having a positive effect on heart health, soy foods are also a source of fibre and other compounds which can improve blood pressure, blood glucose control, inflammation and obesity, how exactly we are not 100% sure, once again – more research is needed! (Source: Ramdath et al., 2017)

Looks like another positive for soy foods when it comes to keeping your ticker happy and healthy.

Vegan Diets

When it comes to vegan diets, soy becomes a critical source of high quality protein, energy and an array of other nutrients (calcium and iron – when products are fortified), excluding soy can really limit your options to really just legumes and their products as a source of satiating protein in your meals.

For example, per 100 g firm tofu provides 12 grams of protein, compared to 100 g of lentils which provides 7 grams. Whilst both valuable sources, it can often be more efficient to include soy as part of a vegan diet to meet protein needs.

Soy milk in particular is an important dairy alternative for those wanting to follow a more plant-based diet as it has a more similar nutrient profile in terms of protein to dairy protein, and are more often calcium-fortified compared to nut milks like almond milk. You can check if your milk is fortified, by looking for at least 120 mg per 100 mL in the far right column of your soy milk.

Man Boobs?!

So, if you’re a man and just want to eat some more soy or maybe you follow a vegan or plant-based diet, does that mean you are at risk of developing man boobs (medical term: gynecomastia)?

A 2010 review of the evidence shows that there is no feminizing effects on men when consumed in level at or above a typical Asian male’s intake (Source: Messina, 2010). There has been one case of a 60 year old man drinking nearly 3 L of soy milk a day developing breast tenderness and also had elevated oestrogen levels. But let’s put this in perspective, 3 litres A DAY! That is a huge volume and the author even notes how unusual this case is! (Source: Martinez & Lewi, 2008).


Based on observation, many Asian men from Korea & China and other South-East Asian nations consume soy foods regularly and there does not seem to be a man boob issue being reported. There definitely needs to be more research to confirm this observation and a randomised controlled trial comparing oestrogen levels in men after a soy-rich diet would be best, whether this manifests as anything clinical (i.e. feminine traits like breast tissue growth) would also be interesting.

Safe to say, if you stick to a sensible amount of soy products in a day, 1-2 servings (i.e. a cup of soy milk and serve of tofu) you should be right and avoid any risk of man boobs.


It appears that soy does not seem to impact the thyroid, however regular consumption and/or use of isoflavone supplements may not be the best idea for those with hypothyroidism, and can interfere with levothyroxine – a medication used for hypothyroidism (or low thyroid hormones, Source: Fruzza et al., 2012).

Read more about eating for hypothyroidism on my blog.

Are there any risks?

  • From a safety perspective, the long-term use of isoflavone supplements may increase the risk of endometrial hyperplasia (a thickening of the endometrial lining, which may lead to cancer), however soy foods appear not to have this effect
  • If you have a soy allergy, all soy should be excluded in the diet and if you have an intolerance, talk to your dietitian about your tolerance level.

Some more great resources on soy:

Get personalised advice, and apply to work with us one-on-one today – let’s develop a tailored plan to help you reach your goals!

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