Phytoestrogens… as bad as they say?!

You may have heard people say that guys should avoid soy, that it ‘feminises’ men or, more specifically, reduces their fertility. This ‘advice’ stems from the fact that soy contains high levels of phytoestrogens – oestrogen mimics that can have oestrogen like effects in the body. The question is – are phytoestrogens really that bad? And should guys avoid them? And what about us gals?


To start, a quick recap on hormones (which is what oestrogen is). Hormones are ‘instructions’ that the body produces to tell other parts of the body to do things. Hormones travel around the body in the blood and other bodily fluids until they find a cell that will respond to them; by this we mean cells that have a specific receptor that the hormone can bind to … a bit like a key (hormone) in a lock (receptor). By binding the hormone causes the cell to do something … like a key opening a lock. So in the case of oestrogen – oestrogen binds to cells that have oestrogen receptors and causes those cell to do something(s).


So what is the deal with phytoestrogens? Phytoestrogens are structurally like oestrogen, which means they can bind to oestrogen receptors. They are found at high levels in:

  • Soy (particularly when fermented)
  • Tempeh
  • Flaxseeds

They are also relatively high (although many orders of magnitude less than the above) in:

  • Oats
  • Barley and hops
  • Lentils
  • Apples

The question is, when we eat phytoestrogens:

  1. Do they enter the blood?
  2. Do they bind to oestrogen receptors and cause those cells to respond as they would to oestrogen?

What the research suggests  

  1. Do they enter our circulation when ingested?

Yes. They are small single molecules so they are not broken down in the gut and can be absorbed into the body as they are during digestion. A significant amount of the phytoestrogen ingested from food is in the biologically inactive form, however certain microbes in the gut can convert them to the biologically active form, and there is also a certain amount of natural conversion (non-enzymatic).

  1. Do they elicit a biological response?

As with everything in biology, nothing is black and white!!

The research is clear that phytoestrogens can cause a response in the body. And the evidence suggests that this is at least in part through their action on oestrogen receptors, because they do cause a response when they bind to them. They are therefore what are known as ‘endocrine disruptors’.

It has been clearly shown in animal (mice) models that they can disrupt lactation, development of reproductive organs, puberty, fertility, and sex specific behaviour. It should be noted that in these studies the phytoestrogens are typically administered at very high doses in the biologically active form. The thing about animal models is that they are performed on colonies of mice that are genetically and epigenetically very similar – this is necessary to reduce the variables in the experiment (i.e. so that you can distinguish the results of whatever perturbation you are doing in the experiment … e.g. pumping them full of phytoestrogens!!), but it also means that you may not necessarily even get identical results in another colony. More and more now scientists are validating results across cells and multiple colonies to prove their results, but in a lot of these studies it was single colonies that they were performed in. It is hard to replicate such studies in humans (shades of eugenics!!), but studies have shown increased rates of infertility that correlate with increased consumption of phytoestrogens. It is hard to prove cause here, as other factors will also have changed across this time (not least rates of obesity, consumption of corn syrup, sedentary lifestyle …!). However, it has been shown in certain cases where intervention to reduce intake of phytoestrogens has been possible, some of the effects on fertility have been reversed.

On the flipside, certain studies have also indicated that phytoestrogens have benefits such as reducing risk of osteoporosis, and certain cancers.

A couple of interesting and relevant points to consider:

  • Phytoestrogens are not as potent as oestrogen. Some people hypothesise that this means phytoestrogens could potentially ‘dilute’ the oestrogen response in women, therefore also negatively impacting their fertility and other consequences of oestrogen.
  • At least in some cell types, the signal you get from phytoestrogen is slightly different to the response you get from oestrogen. This means that phytoestrogens do not exactly mimic oestrogen – they cause their own slightly unique response.

Should I be worried?

I think it is likely that genetics, age, gut microbiome, health status, concentration of exposure, and natural oestrogen levels (and likely other factors) play a role in how any one person responds to phytoestrogen … in the same way you can have two 20 a day smokers and only one develops lung cancer. I think more studies need to be done on mammals, and also looking at the cellular level.

I think people who eat a lot of processed foods that have high levels of processed soy derivatives should look to reduce consumption – as they will not only be getting high doses, but also in a lot of these highly processed soy products it seems the phytoestrogens are in the biologically active form. However, people whose diet is fresh whole foods, but also eat edamame and tofu, I would not be so worried about (providing these are not the only things they eat!!).

I don’t have high levels of processed foods and soy derivatives in my diet and so, personally, of much more concern are the man made and even more potent xenoestrogens, such as BPA. Xenoestrogens are man made compounds that are structurally similar to oestrogen. They are present in many food additives, cosmetics and plastics. The plastics point is interesting because BPA is frequently found in Tupperware and food packaging, and the xenoestrogens can transfer into the food item contained within. These are what I am focussed on trying to avoid.


Read More

Bennetau-Pelissero, C. Risks and benefits of phytoestrogens: Where are we now? Current Opinion in Clinical Nutrition and Metabolic Care. 19(6):477-483. 2016.

Patisaul, HB, Jefferson, W. The pros and cons of phytoestrogens. Frontiers in Neuroendocrinology. 31(4):400-419. 2010.