Fermentation Does Not Neutrailize the Goitrogenic Effect of Soy — It Makes It Worse!

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Sarah Pope of The Healthy Home Economist alerted me to the fact that Dr. Mercola has been spreading the myth that fermentation destroys the goitrogens in soy.  Nothing could be further from the truth.

The isoflavones of soy, predominantly genistein and daidzein, are the estrogenic and goitrogenic toxins we are concerned about, and they exist in the unprocessed soybean attached to sugars.  We can only absorb the sugar-free form, and there is very little sugar-cutting activity in our intestines able to render these nasties absorbable.  Traditional fermentation, however, successfully frees many of these substances from their attached sugars and renders them more absorbable.

Fermentation does lots of good things too, like break down the phytate and produce vitamin K2.  Excess iodine appears to protect against the goitrogenic effect of soy, but exactly how protective it is and at what range of soy intakes is still left to the unknown.  Therefore, soy should be consumed in moderation as well as with plenty of iodine.

Here is Sarah's blog post on the matter:

Mercola Confusion: Fermented Soy IS Goitrogenic

Here's the link to my Special Report, the most pertinent information from which Sara nicely summarized:

Thyroid Toxins: The Double-Edged Swords of the Kingdom Plantae

Honey post is coming.  Sorry for the delay!

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  1. I realize that your article is over 12 years old at the time of my comment, but I’m curious your take on this 2019 meta-analysis: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6408586.

    The research article indicates that there’s no appreciable changes to thyroid hormone production and minimal TSH changes with avoidance or consumption of soy. The researchers didn’t review changes to thyroid antibodies (if any) which I think is an oversight on their part.

    Your thoughts?

  2. I’m curious …. If I took a bite of natto and swished a drink of water around in my mouth, with the beans and only swallowed the liquid mixture(including the stringy slimy substance) would I get the large amount of MK7s ? Would any of the inhibitors be present? I’m strongly avoiding the inhibitors…..

  3. I have just starting using Megumi Natto – fermented soybean with the strings. So this isn’t a good thing? I eat no other soy products and thought the bean itself was better than tofu, etc. Not able to take any kind of iodine and no longer trust seaweed products… Fukushima is still pouring out

  4. I don't trust Dr. Mercola. He sells a so called Liposomal C. Call or email them and they will tell you it has NO LIPOSOMES! Then they will tell you something lame like "Well, just add water."

    When I noted on a Mercola Youtube vid about Vitamin C that his liposomal C had no liposomes, well I got blocked so fast that I couldn't post under a different vid w/in 5 minutes. They could have explained and cleared things up (really not possible, the product is a scam) but no they just censored me in a heartbeat.

    Thanks for the info.

  5. Around here, cutting out soy sauce would reduce veggie consumption 30-50% or so. Hubby just doesn't like veggies much, but will eat a lot if stirfried with fresh ginger, fresh garlic and soy sauce.

    I figure the amount is too small to worry about and anything that increases veggie consumption is good. Also why I approve of lots of butter and deep-frying!

    We don't eat soy at ALL except for fermented tamari. No "extended" foods or vegan cheeses or even edamame. This is all our soy intake.

    And really, even if you pig out on the stuff, doing heavily sauced stirfries daily, you just don't get that much soy.

  6. Interesting post. It makes me wonder about a lot of things about soy because I thought it was good for the body. My rule of thumb is always to take it in moderation. Too much or too little of something is not going to do any good.

  7. Chris,

    I would love it if you could explore this issue further.

    I love traditional Asian foods and I would like to believe that it is not detrimental to use soy sauce and fermented bean pastes occasionally. Fortunately I have no interest in tofu.

    I don't really care for seafood much, but I could probably add shrimp to my diet if necessary. I do eat a lot of eggs, and occasionally dairy, which I know are decent sources of iodine. Do you think I should take a kelp supplement if I use soy products in my cooking?

    Thanks so much Chris. It would be great if you could investigate this issue further.

  8. Hey Sarah,

    You're welcome! Thanks for spreading good info!


    It's possible, but I think it's important to remember that traditional diets contained multiple interacting foods. The main protector against thyroid toxins from soy is most likely iodine, and that is primarily obtained from seaweed and other sea foods, not from fermentation of the soy.


    Ah, the expectations! Well, if I were to make it the best post ever, that would just suspend its release all the longer. So I will try to abandon my lofty hopes of explaining everything about honey and get the straight facts out in a post tonight. 🙂


    Abstinence is a legitimate choice, but if he really likes soy sauce, I don't think there is sufficient evidence to say that abstinence is superior to moderation in the context of an iodine-rich diet, and eating food you really like promotes happiness and happiness promotes health. Soy sauce is, by the way, very low in isoflavones compared to other soy foods, perhaps because it is diluted. Soy isoflavones can have various estrogenic or anti-estrogenic effects depending on species, tissue, amount, etc.

    Drs. Cynthia and David,

    Soybean sprouts have a lot of isoflavones, but much less than most other soy foods.


    Good points.


    Wary? Not so much. But realize that fermentation does more or less the same thing to these foods, but that iodine can completely overcome the goitrogens in sauerkraut. In fact, I feel much more confident that it is safe to eat substantial amounts of these as long as one has sufficient iodine. That said, they do compete with iodine for uptake not only into the thyroid gland but also into the mammary gland, so it's especially important for a nursing mother to get lots of iodine if she's going to eat lots of these foods. Used in small amounts as a condiment, I do not think it's even worth worrying about in most cases.


  9. My question pertains to fermented cruciferous vegetables, such as fermented sauerkraut and kimchee. Are you saying that one should be wary of consuming these types of fermented foods, also? Or do the benefits of lactic acid (for our gut bacteria) produced by these fermented veggies over ride any such hazards?

    Thanks, Klaatu

  10. excellent post, Chris. While Mercola is probably a good thing overall, for questioning the status quo and getting a lot of mainstream folk to question a lot of what they hear, time and time again his site is a source of misinformation. Overall he does preach a wise path to health i believe, but there are sloppy details all over the place.

  11. Moderation? Why not just avoid soy altogether and not take the risk.

    In a related vein I've heard that consuming soy either blocks testosterone production or increases estrogenic production in men. Any truth to that?

  12. The suspense for the upcoming honey post is building – at this point, I'm expecting the greatest post EVER!

  13. This is interesting. Fermented soy products have long been recognized as health-giving, invigorating foods. I wonder if there are other factors that are produced or increased via fermentation that help the body deal with the goitrogenic substances found in soy? While it is a bad idea to generalize, it seems that many properly prepared whole foods, especially those that are fermented and/or predigested, come in a neat "package" with constituent parts which our body utilizes to neutralize certain elements and enhance others.

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