4. Soy contains goiterogenic compounds, which undermine the healthy functioning of the thyroid gland. As Dr. Harold Kristal points out, "Sub-clinical hypothroidism is already such a common health problem that caution is certainly warranted."
In addition, three further points need to be made.
One: Most of the soybeans produced in the U.S. are genetically engineered, the implications of which have not yet been fully understood.
Two: The healthiest soy foods are the fermented ones such as tempeh and miso, which don't have any of the problems mentioned above but aren't necessarily the ones we're eating the most of.
Three: Probably most importantly, the phytoestrogens in soy -- those very compounds so touted for their health benefits -- are actually a mixed blessing. Yes, they are weaker than "real" estrogen, and yes they bind to the estrogen receptors in your body, which partially prevent the body's own estrogen from binding to those sites and possibly causing mischief. But they can theoretically help reduce the downside of estrogen (breast cancer, for example). Doesn't it make sense to consider whether that benefit might be washed away by consuming so much of the phytoestrogens that you might as well be taking the "real" thing?
I posed these questions to Dr. Barry Sears, who just wrote a very good book about incorporating soy into his Zone-type diet (The Soy Zone) when he was on my radio show last week. Going down the list of arguments made by Fallon and Enig, I basically said to him "Barry, you got 'lot of 'splaining to do!"
While I can't say he put my mind at ease 100 percent, his answers definitely shed some light on the situation.
For one thing, in the best of all possible worlds, Dr. Sears doesn't recommend that you get all of your protein from soy. A mix of soy and animal products would be ideal, and with that, I agree completely.
For another, Dr. Sears agrees that while the health benefits of some of these soy compounds are unquestionable, it's the old American propensity for thinking "if some is good, a hundred times that is better" that is causing a lot of the problem. It's one thing to get a reasonable dose of isoflavones -- such as genistein -- from soy. It's quite another to take isolated supplements of these compounds in amounts several hundred times what is found in the food. In high amounts, isoflavones may have negative effects on other hormones (such as thyroid), and taking hundreds of times the dosage of a weaker estrogen might well be erasing the benefits of phytoestrogens in the first place.
Finally, who knows what the cumulative effect of having this much phytoestrogen exposure in our food supply has had on sexual maturity and development in young people?
I agree completely with Dr. Sears that soy can be part of a healthy diet, but for right now, I don't think we should eat unlimited amounts of it, and I definitely don't think we should be taking supplements of concentrated isoflavones.
And I am pretty sure that the best soy products continue to be those that are fermented, rather than those that are highly processed.