I don’t think there’s any food as controversial as soy these days. I’m asked about its use (and safety) almost daily. Is it safe? It is healthy? Will it hurt my thyroid? Does it cause breast cancer? Will it harm male fertility? Does it enhance female fertility? While I hope to answer most of these questions, it’s a very broad topic, so I think I have to break this into pieces.
Part 1 – Understading phytoestrogens.
Phytoestrogens are simply plant constituents that are able to bind to the estrogen receptor. And, most of the time they act like estrogen modulators. This means that in a high estrogen environment they help to reduce estrogen dominance, but in a low estrogen environement they can help to normalize low levels of estrogen. Many foods contain phytoestrogens: beans, flax, nuts, etc. The phytoestrogens of interest in soy foods are daidzein and genistein. These are called isoflavones.
So, are these isoflavones dangerous to human health? Well, maybe, but not likely. And, here’s why. In order for a hormone to have an effect in the body, it must be bound to a receptor. No receptor = no action. A common binding protein is SHBG (steroid hormone binding globulin). Interestingly, soy foods actually increase the production of SHBG, making fewer isoflavones available for binding to receptors. And, those that do bind, bind very weakly. It is estimated that phytoestrogens from soy only bind at 1/1000 the strength of even our bodies own estrogens. So far, we have a weak estrogen that’s not very available.
Let’s put a real life example in here. Is a pre-menopausal woman in her 30’s, who is still having regular menstrual cycles, likely to have any negative effects from consuming soy foods on a regular basis? Ie. soy milk, tofu, etc. Given that most of her receptors are taken up by her own estrogens (or stronger estrogens from the environment – pesticides, plastics, etc), any further stimulation by soy (remember, it’s activity is 1/1000 of human estrogen) will be minimal. But, what will the effect be 15-20 years later when she is menopausal? Given that most of her receptors will be sitting empty (hence the hot flashes), the isoflavones may exert a stronger estrogenic effect, which isn’t a bad thing in this case, as we know that estrogen is protective for our bones, brain and heart.
Another key point is that soy’s ability to bind to estrogen receptors (and take binding spots away from stronger estrogens like pestidices) is one of the postulated theories for soy’s possible protective effect. So, if an isoflavone is bound to the receptor, the stronger (more harmful) estrogen is out of luck.
So in summary, when a woman has little natural estrogen production (post-hysterectomy or postmenopause), isoflavones can attach to open estrogen receptor sites and produce a weak estrogenic effect. When there is too much estrogen (during PMS, and endometriosis for example), isoflavones can compete with estrogen for receptor sites, and because their effect is weak in comparison, they blunt the estrogenic effect.
I want to emphasize that we are talking about soy foods, not supplements. Soy foods include tofu, tempeh, soy beans, soy nuts and soymilk. For the time being, I don’t recommend soy supplements or powders on a regular basis. And, always choose organic soy, as almost all non-organic soy is made from genetically modified soy.
So, that’s a good place to start. Next up – Soy and Thyroid function. I hope to have part 2 up in the next week or so, assuming my kids don’t bring home another set of germs.