Popular Fact

Soy’s phytoestrogens cause a variety of negative health effects due to interfering with hormones.

See our article: Soy contains estrogen for more information on phytoestrogens.

Reality

TalkVeganToMe’s layman’s understanding from surveying the material available online is that:

Soy contains phytoestrogens which are structurally similar to human estrogen (specifically isoflavones) which can have hormonal effects on the body.
The maximum safe dosages are not well understood.
It is unlikely that you will ingest a high enough dosage to affect your health as part of a balanced diet provided you do not have any special circumstances (e.g. hypothyroidism, infancy, etc.). But you should ensure that you vary your protein sources and consult your dietitian for special circumstances (e.g. hypothyroidism, infancy, etc.) see Soy, Soy Foods and Their Role in Vegetarian Diets - 2018 below.
All articles on the topic caveat their conclusions with: ‘more research is needed’.

This is a highly complex and fast moving topic in terms of research. We’ve discounted a number of the most popular pro-soy sources (see Conflicts of Interest below). We’ve also intentionally put the position of the Vegan Society to one side until we understand the original sources properly (see Vegan Society below).

What remains is the truest representation of the research we’ve surveyed from our layman’s ability to digest them. Despite assessing nearly 80 articles and meta-analyses we have relied particularly heavily on Soy, Soy Foods and Their Role in Vegetarian Diets - Nutrients - 2018 as it is a highly relevant, highly recent, and seemingly unbiased meta-analysis of the research.

You may come to a different conclusion based on the evidence provided, which is why we have cited, quoted, and linked to our sources so you can make an informed decision. If you wish to reach us regarding anything we’ve said in this article, please contact support@talkveganto.me

1. Phytoestrogens can have hormonal (and anti-hormonal) effects.

This was our biggest surprise in researching this topic, as we had heard over and over again that soy contained plant estrogen, you don’t need to worry about plant estrogen as they can’t interact with the human body hormonally!

Unfortunately phytoestrogens are not so easily dismissed. See Soy contains estrogen for more.

2. Dosage is hard to understand

This assertion is based on:

How much phytoestrogen is in food varies wildly
How much is safe to eat is not well understood

2.1 Hard to tell how much phytoestrogen is in food

The amount of phytoestrogens (e.g. isoflavones) in soy based products varies wildly depending on the product, the origin of the soybeans, and the cooking method.

Even if there were a Recommended Daily Allowance, it would be quite hard to tell whether you were above or below it based on the nutritional information available.

In Canada, isoflavones [a type of phytoestrogen] in soy beans vary from 360 to 2241 mg per kg. Isoflavones content in soy beans from Romania ranged between 210 and 1340 g per kg, 1176-3309 mg per kg in USA and 525-986 mg per kg in India. - Soy, Soy Foods and Their Role in Vegetarian Diets - Nutrients - 2018 - p7

So even at the source the amount of isoflavones per kg can vary massively depending on the local conditions of the crop.

In soy flour, total isoflavone concentrations range from 60 to 265 mg per 100 g, tofu may contain between 5.1 and 64 mg per 100 g of total isoflavones, soy milk 1.3-21 mg per 100 g, tempeh 6.9-63 mg per 100 g, soy sauce 0.1-2.3 mg per 100 g, miso 23-126 mg per 100 g and natto 20-124 mg per 100 g.
[…]
Temperature is critical for retention of isoflavones: in fact, soymilk film formed during boiling, called Foo joke or Yuba, has been reported to have 196.05 mg per 100 g of isoflavone compared to 44.67 mg per 100 g after cooking
[…]
The most refined soy foods can lose up to 80-90% of isoflavone content during processing. Soy burgers have 0.1-26 mg per 100 g of total aglycone equivalents, soy yogurts 1.6-11.8 mg per 100 g, soy drinks 1.0-21 mg per 100 g and soy cheeses 2.3-33 mg per 100 g. Usually, bakery soy-based products retain low concentration of isoflavones. Soy milk formulas may contain up to 31 mg per 100 g.
- Soy, Soy Foods and Their Role in Vegetarian Diets - Nutrients - 2018 - p18

In other words, foods containing soy can have very unpredictable amounts of phytoestrogens, processed foods can lose a lot of their isoflavone content, and typically cooking soy based foods reduces the amount of isoflavones dramatically.

2.2 How much is safe to eat is not well understood.

While there is no RDA currently published by any body we’ve been able to find, there are some indications of what might constitute a dosage which would have an effect on the body.

At the same time, to have the beneficial effects of soy isoflavones, intake should be at least of 60-100 mg per day, at present not easily reached in Western countries.
- Soy, Soy Foods and Their Role in Vegetarian Diets - Nutrients - 2018 - p21

This talks specifically about beneficial effects (which has been the focus of a lot of research due to health claims for tofu in the 1990s), the full scope of large dosages of phytoestrogens (whatever may constitute large dosages!) can have negative effects.

There are also real concerns that excessive amounts of isoflavones in serum may promote other hormone-related problems.
- An updated review of dietary isoflavones: Nutrition, processing, bioavailability and impacts on human health - Critical Reviews in Food Scient and Nutrition - 2017

and

[P]hytoestrogens are also considered endocrine disruptors, indicating that they have the potential to cause adverse health effects such as infertility and increased risks of cancer in oestrogen-sensitive organs.
- The potential health effects of dietary phytoestrogens - British Journal of Pharmacology - 2017

That said, the few studies that show negative effects have major drawbacks such as:

Being performed on animals, which apart from the ethical concerns do not even necessarily yield valid results for humans!
Being performed in-vitro (i.e. in a petri-dish on small clusters of cells)
Drawing conclusions from statistical (e.g. epidiemological) data which risks confusing mere correlation with cause.

These adverse effects have been mainly suggested based on data from in vitro, animal or epidemiological studies. Clinical studies often report the absence of adverse effects. - The potential health effects of dietary phytoestrogens - British Journal of Pharmacology - 2017

and

Concerns about the adverse effect of soy consumption were originally based on animal data.
- Soy, Soy Foods and Their Role in Vegetarian Diets - Nutrients - 2018

3. It’s unlikely you’ll ingest enough to have adverse effects, but vary your protein intake anyway!

Having a varied plant-based diet is rarely bad advice. And while it seems like you are unlikely to eat enough to adversely affect your health, it’s also pretty much impossible to measure how much phytoestrogen you’re ingesting on a daily basis without specialised equipment. And even if you could, there are no official recommended daily allowances to stay below.

Overall, the low content of bioactive compounds in second generation soy foods [(e.g. soy based mock meats)] and moderate amounts in traditional soy preparations offer modest health benefits with very limited risk for potential adverse health effects.
- Soy, Soy Foods and Their Role in Vegetarian Diets - Nutrients - 2018 - p21

An interesting point in the same meta-analysis was:

[T]o have the beneficial effects of soy isoflavones, intake should be at least of 60-100 mg per day, at present not easily reached in Western countries.
- Soy, Soy Foods and Their Role in Vegetarian Diets - Nutrients - 2018 - p21

4. More research is needed

This was repeated by every single study we read. Dietary conclusions are extremely difficult to come to as they can be affected by everything from genetics of the person ingesting it, to the conditions in which the crops were grown, to even the way the food is cooked.

This article will likely be updated in the future as new studies come out. But for now the answer is a firm: nobody quite knows for sure.

Despite the high intakes in Asian countries, there is still a paucity of long term data on soy consumption in Western countries. Currently, the use of soy foods in infancy, including soy-based infant formulas, was not linked to adverse events. However, absence of evidence is not an evidence of absence. With the growing adoption of vegetarian diets and with the choice of soy formulas by Western vegetarian parents, a broader characterization of the role of soy foods in Western nutrition could help the acceptance of plant-based new generation foods.
- Soy, Soy Foods and Their Role in Vegetarian Diets - Nutrients - 2018

and

Having said that more rigorous studies are required to assess dose-response relationships while consuming soy food and supplementation.
- An updated review of dietary isoflavones: Nutrition, processing, bioavailability and impacts on human health - Critical Reviews in Food Scient and Nutrition - 2017

and

Given the rapid increase in global consumption of phytoestrogens and the fact that phytoestrogens are present in a wide range of dietary food supplements and widely marketed as a natural alternative to oestrogen replacement therapy, further insight into the risks and benefits of these phytoestrogens seems essential.
- The potential health effects of dietary phytoestrogens - British Journal of Pharmacology - 2017

Vegan Society & British Association of Dietitians

Evidence suggests that soya is not a threat to our health and is in fact a reliable and healthy protein source.
- Vegan Society - General FAQs

TalkVeganToMe reached out to The Vegan Society to enquire after the sources for the evidence. The Vegan Society replied incredibly promptly and directed us to The British Association of Dietitian’s as their source.

There is now scientific agreement that the controversy around isoflavones’ potential to have bad side effects in humans is fuelled only by findings from laboratory or animal studies using pure isoflavones or high doses. It is well established that animals metabolise isoflavones in a different and much more efficient way to humans and results from such studies cannot be compared to any human outcomes. Additionally, using high doses of pure isoflavones cannot be compared to consuming isoflavones from whole soya foods as they provide lower quantities and are a combination of many biologically active molecules. Comprehensive reviews by the European Food Safety Authority, World Cancer Research Fund and the World Health Organisation all conclude that soya foods as part of a healthy balanced diet are safe.
- British Dietetic Association - Soya and health - the basics - 2017

The supplementary information sheet includes citations for:

One article from the WHO called “Protein and Amino Acid Requirements in Human Nutrition” which doesn’t seem to make a healthfulness claim for soy except as a source of protein.
One article from the World Cancer Research Fund titled “Food, Nutrition, Physical Activity, and the Prevention of Cancer: a Global Perspective” that talks a lot about the potential cancer-curing effects of isoflavones but does not appear to talk about the potential for adverse effects.
No articles from the EFSA. The EFSA has published two articles pertaining to the safety of soy, one studying peri and post menopausal women, and another pertaining to soy’s potential positive effect on cardiovascular health by way of cholesterol.

We have reached out to the BDA to seek further details.

Conflicts of Interest

Mark Messina

A number of the pro-soy articles we’ve found have Mark Messina listed as an author. As declared on one of zirs articles, Mark Messina is “The executive director of the Soy Nutrition Institute, an organization funded by the United Soybean Board and its soy industry members”.

We don’t mean to suggest any impropriety on Dr Messina’s part but when citing articles we want to avoid the merest suggestion of bias as it may undermine your ability as a reader to confidently cite your sources.

As a result, when researching this article, we excluded the following articles from our investigation:

Sources

Organisations

Articles

You can see a full list of articles we have considered for this document in our Soy Sources Sheet. If you’ve got any you think we absolutely should have a look at, email support@talkveganto.me.