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It is not uncommon in spaces where trans women congregate online to find some women talking about phytoestrogens as a possible alternative to or additive to standard hormone replacement therapy, especially for those who are attempting self-medication or those who favor alternative medicines. Anecdotally, some women claim to have had success with phytoestrogens in terms of triggering or maintaining a hormone therapy, while others declare them ineffective. What does the science say?

Phytoestrogens, (also called xenoestrogens and isoflavones), are plant-derived hormones that imitate endogenous estrogen (such as estradiol) though are not actually endogenous estrogens. The most common source of phytoestrogens in the modern diet is soy and soy-derived products. Most often they are marketed as herbal supplement treatments for menopausal hot flashes. They must be marketed as supplements because they have not met standards of evidence to be considered an actual pharmacological by the USFDA — that is, they’ve never been conclusively shown to have any real efficacy.

There has been both general interest in the possible health benefits of, and general concern over the dangers of, increased phytoestrogens consumption in the Western (and especially American) diet. For example, one of the most common accusations leveled against phytoestrogens is that they may increase breast cancer risks, though there is inconclusive evidence concerning this. There is also general fear floating around that consuming too many phytoestrogens can have a negative body impact on cis men, possibly even triggering breast growth. While occasional case studies have appeared suggesting that extreme circumstances can trigger gynecomastia (usually when coupled with other concurrent conditions that reduce testosterone and/or otherwise disrupt normal metabolic function), phytoestrogens aren’t generally a potential “feminizer” of cis men. In fact, a 2010 meta-analysis of research on soy isoflavones found no reliable evidence of adverse effects on the adult male anatomy at all.

All in all, there is little evidence that phytoestrogens are impactful at standard doses in a healthy adult body. But what does that mean for its use in alternative HRT? Can phytoestrogens in a higher dose serve as a replacement for, or supplement to take with, standard estrogen regimens?

The short answer is no. There are several reasons for this. 

Lack of Evidence. While the effects of phytoestrogens on transgender HRT have not been studied extensively, a lot can be gleaned by the aforementioned studies done on phytoestrogens generally. They have been found to have a very weak biological impact at best in terms of estrogen effect,. The most charitable thing one might say is that we’re not sure yet and that whatever the effect is, it’s probably subtle.  Trans women’s bodies are still biological human bodies and there’s nothing at all to suggest that phytoestrogens will somehow magically have a more potent impact on trans bodies they don’t have on general human biology.

Dangerous Dosage. Like any chemical compound, the biological impact of phytoestrogens is related to dose — the more taken, the stronger the impact. Phytoestrogens have been found to have an impact up to 1000x weaker than biological estrogen. Given what we know about the lack of biological impact at average levels, a trans woman would have to take a lot of phytoestrogen supplements to reach the free estrogen levels of a natal woman — possibly up to 1000x the dosage. It’s almost certain that the supplement would reach dangerous blood toxicity levels long before the proper estrogen levels were reached, causing damage to the body and possibly even becoming a fatal dose. Remember, too, that phytoestrogens are not estrogen, only compounds that mimic estrogen. They have their own properties, including their own dangerous potential side effects, and those effects also get worse at higher doses.

Questionable Supplement Purity. Another concern with taking phytoestrogen supplements is that the FDA doesn’t stringently monitor herbal supplements for quality or purity. This means that any given phytoestrogen supplement could contain a lot, or a little, or none at all, depending on the ingredients and the manufacturing process. Even if we ignore the dosage danger mentioned above there’s still the problem of even assuring that we’re getting the dose we seek (or inadvertently giving ourselves a toxic dose).

There is no sound science that says phytoestrogen supplements will work as a natural alternative for HRT, and plenty to suggest that they won’t work, and could even be dangerous to your health. Tempting as phytoestrogens may seem to some women eager to start HRT or frustrated with the progress of their transition, they are not a wise choice. 

References & Further Reading

Clinical studies show no effects of soy protein or isoflavones on reproductive hormones in men: results of a meta-analysis. (2010, August). Fertility and Sterility94(3).

Herbal Hormones. (n.d.). 

Herbal Supplements: Helpful or Harmful? (n.d.). The Cleveland Clinic [website]. 

Kabat, Geoffrey. (2012, November 26). Natural Does Not Mean SafeThe Slate. 

Natural Institutes of Health. (n.d.). Herbal Supplements: Consider Safety, Too.

Phytoestrogens. (n.d.) NYU Langone Medical Center [website].

U.S. Government Accountability Office. (2009, January 29). Dietary Supplements:
FDA Should Take Further Actions to Improve Oversight and Consumer Understanding.

U.S. Office of the Inspector General. (2012, October 2). Dietary Supplements: Structure/Function Claims Fail to Meet Federal Requirements.




Sophia says:

Every estrogenic Isoflavone,
every Phytoestrogen, and
every Xenoestrogen,
is an Estrogen.
By definition.
They’re merely not endogenous Estrogens.

Phytoestrogens do not usually imitate Estradiol.
For instance, 8-Prenylnaringenin imitates Estron more closely then Estradiol.

It is not possible to “imitate Estrogen”, because “Estrogen” is a substance class, not a substance; a substance can merely be an Estrogen by itself.

This was all just one sentence, I’m not going to dissect the rest as well
Please get your content right, if you’re using “science” as a headline.

Regarding classing as estrogens: I have consulted several science-based texts and I have seen phytoestrogens described as “estrogen-like”, “estrogen imitating compounds” and similar verbiage. Could you cite me a source for declaring them all “estrogen by definition”? If so I’d gladly consider making a correction. In fact, to be safe, I’ve gone ahead and added the word endogeneous to that sentence, just to make sure the intent of the statement is clear.

Regarding the equivocating of phytoestrogen, xenoestrogen, and isoflavones (from your prior comment): Again, your characterization seems to disagree somewhat with the literature I consulted. For example, the Linus Pauling Institute classes soy isoflavones as a subclass of phytoestrogens, and phytoestrogens are by definition a subclass of xenoestrogens. Moreover, in casual conversation, especially related to the topic as it remains relevant to herbal suppliments often considered by transwomen. I feel confident in depicting them as sysnonymous for purposes of this piece as there’s nothing to suggest that any one compound stands out from the others in terms of use in HRT.

Regarding phytoestrogens and estradiol: This article would seem to disagree with your characterization of the phytoestrogen you cited. It seems to think that 8-Prenylnaringenin “[competes] strongly with 17ss-estradiol.” Further, while this article references 8-PN in terms of its potency compared to estrone, its conclusions seem to be more concerned with comparing it with estradiol (e.g. “Compared with estradiol, ethinyl estradiol and conjugated equine oestrogens, which all undergo extensive phase 1 metabolism and for which only negligible amounts are excreted as free or conjugated compounds”). But, again, if you can provide me with some contrary sources I would gladly consider making a correction.

Moreover, I think that my word choice is probably a bit more concrete than I intended; my intent was to indicate that phytoestrogens impacting estradiol were the ones of interest to transwomen, estradiol being the common component in HRT, and I apparently didn’t state that clearly enough. I have altered the phrasing for now, pending your forthcoming citations.

Thank you for your critique (though next time please get your science right if you’re going to correct another’s science).

Hormone says:

I have read countless blogs and stumbled upon many sites about estrogens and phytoestrogens but never did those mention that phytoestrogen are also called xenoestrogen because they derived from totally different compounds.

Phytoestrogens are a subcatagory of xenoestrogens — that is, all phytoestrogens are xenoestrogens, but not all xenoestrogens are phytoestrogens. For example, where many people encounter the word ‘xenoestrogen’ is in reference to bisphenol-A, which is most definitely NOT plant-derived and therefore the phyto- label would be incorrect. But phytoestrogens all fall under the xenoestrogen definition.

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