Written By: Brittani Kolasinski (BHSc Nut, Adv Dip Nut Med, ANTA Acc)

Soy has received a lot of press with many people avoiding it like the plague, and some embracing it as a health-promoting food. There are much confusion and confliction, even within the research, leaving many of us not knowing where we stand on the matter.


Soy is part of the legume family, coming from our humble soybean. It's rich in isoflavones, actually being one of the richest sources in the human diet. Isoflavones are polyphenolic compounds that are capable of exerting oestrogen-like effects in the body. Because of this, they are classed as phytoestrogens (plant-derived compounds with oestrogenic activity) along with other foods like flaxseeds and lentils.

Oestrogens are signalling molecules (hormones) that exert their effects by binding to oestrogen receptors in the body, similar to how a lock and key fit together so do oestrogen and their receptor sites. The difference with phytoestrogens is that although they mimic oestrogen in the body, once bound to the receptor they will exert a weaker effect to what the body’s true oestrogen would (80x weaker). Therefore, phytoestrogens are known to be amphoteric, meaning they can work both ways - to help build estrogen levels in a deficiency or to help reduce oestrogen levels when there’s dominance.

We have oestrogen receptors present in numerous sites within the body, not just within the reproductive organs. They are found in bone, liver, heart, and brain meaning that estrogen serves a far greater purpose for many other organ systems other than the female reproductive organs. Of course, like with anything, too much can also negatively impact as too little would. Balance is key.

In soybeans, the isoflavones are present as glycosides meaning they are bound to a sugar molecule. When fermented as with tempeh or digested, the sugar molecule is released leaving the isoflavone to remain as either genistein or daidzein (two different types of isoflavones). The effects of isoflavones within the body are greatly influenced by how they are metabolised. Their metabolism is dependent upon the activity of certain bacteria that colonise the human intestines. For example, the soy isoflavone daidzein may be metabolised in the intestine to equol, a metabolite that has greater estrogenic activity than daidzein but this would be dependent on the type of bacteria in the intestinal tract. Studies that measure urinary equol excretion after soy consumption indicate that only 33% of individuals from western populations metabolise daidzein to equol.


  1.  It could be due to the gut health of the individual, and since the standard Australian diet (SAD) does not support good gut health it would mean that soy metabolism would be impacted.

  2. We have unique genetic variations and vast differences to our microbiome. Our microbiome is like a fingerprint, each one different to another’s.

With these reasons in mind, we can understand how such differences occur with our isoflavone metabolism and utilisation of phytoestrogens.


The interest of soy consumption within the research lies mostly within the tissue-selective activities of phytoestrogens because anti-oestrogenic (reducing oestrogen) effects in reproductive tissue could help reduce the risk of hormone-associated cancers (such as breast, uterine, ovarian and prostate), while oestrogenic (oestrogen building) effects in bone could help maintain bone density.

We know from the research that the incidence of breast cancer in Asia where the average soy isoflavone intakes range between 25-50mg per day is lower than breast cancer rates in western countries where the average intake of isoflavones in non-Asian women are less than 2mg per day. However, many other hereditary and lifestyle factors could contribute to this difference.


The humble soybean in its natural form can be a nutritious food. Traditional Asian foods made from soybeans include tofu, tempeh, miso, and natto. Edamame refers to varieties of soybeans that are harvested and eaten in their green phase. Yet the soy products that are gaining popularity in Western countries include soy-based meat substitutes, soy milk, soy cheese, and soy yogurt. So likewise, with anything, the quality counts. Choosing less processed, organic and non-GMO sources of soy like that of tempeh, tamari, tofu, natto, miso, and edamame beans can be a part of a healthy, nutritious diet. On the contrary, there are certain circumstances when soy products should be avoided.


  • Thyroid conditions especially underactive thyroid or Hashimoto's disease. Soybeans contain plant compounds (glucosinolates) called goitrogens, which have been shown to block the thyroids utilisation of iodine. People with underactive or unstable thyroid conditions should avoid soy products. Individuals with overactive thyroid conditions such as Grave's disease may find some symptomatic
benefit from consuming goitrogenic food but their condition is often driven by immune system dysregulation.

  • Oestrogen dominant female reproductive conditions such as fibroids, endometriosis, heavy menstrual periods, early puberty and infertility.

  • Testosterone sensitive male reproductive conditions such as infertility and sperm irregularities, prostate problems

  • Weight gain when weight is contributed to a sluggish metabolism, soy consumption can interfere with thyroid function so can contribute to fluid retention and weight increases.

  • Nutritional deficiencies as soy is a legume, it contains phytic acid which reduces the absorption of nutrients such as calcium, magnesium, iron, and zinc.


Yes…better if it is fermented.

The fermentation process largely deactivates the phytates, enzyme inhibitors, and goitrogens. Moderate intake of fermented soy products have been shown to produce positive effects on these health conditions:

  • Osteoporosis prevention and treatment

  • Lowered risk of cardiovascular disease

  • Reduced incidence of prostate and breast cancer

  • Menopausal symptoms

Overall, my general recommendation is to always:

  • Choose whole soybean products (never isolates)

  • Ensure soy products are organic and free from contributing chemicals that aggravate health

  • Ensure soy products are free from genetic modification

  • Choose traditional preparations - fermented foods 

  • Keep consumption to a realistic level, around 50g per day as this is in line with the maximum average level Asian cultures consume

For most of us, we would assume that plant-based dieters would be consuming higher amounts of soy, but quite the contrary.  A typical western diet that’s high in processed and packaged foods can consume up to 250g per day coming from processed meats, confectionery, bars, and cereals.

If you decide to have yourself a coffee on soy each day, add some tamari as a dressing or the occasional tofu stir fry in line with a mostly unpackaged and unprocessed diet I wouldn’t lose sleep over it. If you are choosing more packaged foods always check the ingredients list and be mindful of your intake. Of course, as with anything, seek professional advice to tailor a diet to you if there are health concerns involved.

Yours in health,