14 February 2022

Is soy milk safe? Oestrogen, breast cancer and soy – the answers…

Do soy products affect breast cancer risks and does soy help or hinder recovery from breast cancer? And what of soy milk specifically? Is it safe?

Despite years of research and clinical evidence these questions still seem confusing to many. So this week, a research and experiential soy summary, then the soy milk specifics; plus a bonus - the role of oestrogen in COVID, but first

    Thought for the day

The person who asks a question

Is a fool for one minute.

The person who does not ask a question

Is a fool for life.



The activity of most breast cancers are influenced by hormonal levels, especially the main female sex hormone oestrogen – basically more oestrogen is bad, less is better. Oestrogen is produced within the body and also can be taken in via common foods we eat; the phyto-oestrogens. 

How oestrogen influences breast cancer is by attaching onto the breast cancer cells and stimulating their activity. 

Some compounds can block this interaction; the oestrogen blockers. 

Some phyto-oestrogens stimulate breast cancer, some act as oestrogen blockers.

So the effect oestrogen has on breast cancer is a combination of the amount produced within the body plus any oestrogen-like foods consumed, minus the mitigating effect of any oestrogen blockers.

But now, consider this - most breast cancers occur after menopause when the ovaries have stopped producing oestrogen, so where does the oestrogen come from? 

Is it only from the outside? Well, no, some still comes from the body - from fatty tissue, muscle and breast cells. But also, breast cancer cells produce their own oestrogen that then can go on to stimulate more cancer growth! 

So the question is, what can be done to block the adverse effects of oestrogen? 

There are 2 possibiities

1. Blocking the adverse effects of oestrogen

In mainstream medicine a number of drugs are used as oestrogen blockers – the oldest being Tamoxifen, while more recent ones include Arimidex, Aromasin and Femara that are only suitable for women who have completed menopause.

And what about phyto-oestrogens? 

Do they act as oestrogen stimulants or as oestrogen blockers? 

In previous posts the research evidence has been made clear. 

Soy products are effectively oestrogen blockers. 

But then the question of what type of soy is OK for you to eat?

Minimally processed soy foods like cooked soybeans, tofu, tempeh are all OK. 

Highly processed soy, not so good; in fact better to minimise or avoid foods based upon Textured Soy Proteins like veggie sausages, mock meats and so on.

2. Stopping breast cancer cells from producing oestrogen

It turns out oestrogen is produced in breast cancer cells using 2 enzyme pathways. In fact, inhibiting this oestrogen production has been shown to be more effective than just trying to block the effects of the estrogen, “suggesting that the inhibition of estrogen synthesis is clinically very important for the treatment of estrogen-dependent breast cancer.”

There are drugs that do this – the common aromatase inhibitors are letrozole, anastrozole and exemestane. But how about this? The plant world is so amazing! Turns out soy phytoestrogens not only is a oestrogen blocker, it also reduces aromatase production. 

In a 2015 paper, the authors concluded “In general, phytoestrogens act as aromatase inhibitors by (a) decreasing aromatase gene expression, (b) inhibiting the aromatase enzyme itself, or (c) in some cases acting at both levels of regulation.”

Lephart ED. Modulation of Aromatase by Phytoestrogens. Enzyme Res. 2015;2015:594656. doi:10.1155/2015/594656



What does the research offer? In a randomised study, Japanese women were asked to add 400ml per day of soy milk to their diet or not for just 2 months. 400ml is quite a lot! Oestrogen levels dropped about a quarter in the soy milk supplemented group. 

Nagata C et al. Effect of soymilk consumption on serum estrogen concentrations in premenopausal Japanese women. J Natl Cancer Inst. 1998 Dec 2;90(23):1830-5. 

Japanese women live in a culture where soy is a common food, so what about in America where it is not? 

In a small study, women aged 22 – 29 consumed 350 ml soy milk and within a month halved their circulating oestrogen levels. 

Importantly, the oestrogen levels stayed down for a month or two even after the subjects stopped drinking soy milk, which suggests you may not need to consume soy daily to gain the cancer protective benefit.

The authors concluded “These results suggest that consumption of soya diets containing phytoestrogens may reduce circulating ovarian steroids and adrenal androgens and increase menstrual cycle length. 

Such effects may account at least in part for the decreased risk of breast cancer that has been associated with legume consumption.”

Lu LJ,et al. Effects of soya consumption for one month on steroid hormones in premenopausal women: implications for breast cancer risk reduction. Cancer Epidemiol Biomarkers Prev. 1996 Jan;5(1):63-70.

Isoflavins and breast cancer risk

Still not fully confident? Isoflavones are only one of the major phyto-oetrogens, so this research is not the same as testing soy as a whole, however, in this recent meta-analysis, the authors did conclude “Taking into account the risk of bias and methodological limitations, there is little evidence that isoflavone treatment modulates risk factors of breast cancer in pre- and postmenopausal women.” 

This is a conservative finding with no positive benefit claimed, but it does state clearly the research finds no discernible risk. 

Finkeldey L, Schmitz E, Ellinger S. Effect of the Intake of Isoflavones on Risk Factors of Breast Cancer-A Systematic Review of Randomized Controlled Intervention Studies. Nutrients. 2021 Jul 5;13(7):2309. 

Lifetime effects of phyto-oestrogenss on hormones

Need more? Another recent major study concluded “In premenopausal and postmenopausal women, the reported impacts on hormones are inconsistent, although beneficial goitrogenic effects and improved glycemic control and cardiovascular risk markers have been described in postmenopausal individuals.” 

Good for diabetes and heart disease, but again, no mention of adverse effects from soy. 

Domínguez-López et al. Effects of Dietary Phytoestrogens on Hormones throughout a Human Lifespan: A Review. Nutrients. 2020 Aug 15;12(8):2456.



In my opinion, eat minimally processed soy foods regularly. 

Soy milk is OK. Bonsoy remains the best choice. 

Rest easy. 

Stay healthy.

Heal well when needed...


AND FINALLY - Oestrogen may protect from COVID-19

It has been widely observed that adult men of all ages are at higher risk of developing serious complications from COVID-19 when compared with women. This large study investigated the association of COVID-19 positivity and severity with estrogen exposure in women. 

The findings support a protective effect of estrogen exposure on COVID-19, based on positive association between predicted COVID-19 with menopausal status, and negative association with use of the combined oral contraceptive pill. 

HRT use was positively associated with COVID-19, but the results should be considered with caution due to lack of data on HRT type, route of administration, duration of treatment, and potential unaccounted for confounders and comorbidities.

Costeira R et al. Estrogen and COVID-19 symptoms: Associations in women from the COVID Symptom Study. PLoS One. 2021 Sep 10;16(9) 


The latest on soy and breast cancer 

Is soy safe – Part 1

Is soy safe – Part 2


1 comment:

  1. Thanks Ian! I’ve been following you on this issue for 16 years (after bilateral oestrogen sensitive breast cancers) so really appreciate this update.