"Goitrogens" is a term that is seldom used in peer-reviewed research studies, but in past years it was used to refer to substances that could interfere with thyroid metabolism, production of thyroid hormones, and could potentially cause the thyroid to increase in size (a condition called goiter). No large-scale human research studies have been conducted that examine the thyroid-related effects of long-term consumption of whole, natural soy foods consumed in ordinary amounts. Studies in this area have mostly focused on rats fed dietary supplements containing soy components like isolate soy protein or soy isoflavones.

In the human studies that we have reviewed, only one repeated finding has given us cause for concern when it comes to adult consumption of whole soy foods and thyroid-related effects. That concern involves individuals who regularly consume soy foods while at the same time following a diet that is deficient in iodine. That combination of iodine deficiency and regular consumption of soy foods may increase risk of thyroid problems above and beyond the risk posed by iodine deficiency alone. (Iodine is a mineral that is essential for production of thyroid hormones by the thyroid gland.) If you are an individual who is at risk of iodine deficiency, I definitely recommend that you consult with your licensed healthcare provider before making the decision to include soy foods in your Healthiest Way of Eating.

Soybean agglutinin (SBA) is a protein- and carbohydrate-containing molecule (called a glycoprotein) that also falls into the category of substances called lectins. In legumes (including soybeans), lectins might play a key role in allowing certain soil bacteria to work together with the roots of the soybean plant. There are hundreds of lectins found in legumes, and even though researchers aren't yet certain about their function, it is likely that most of these molecules play important roles in cell-to-cell communications occurring within the plants.

How SBA affects human health is a more complicated question that has yet to be clearly answered in research studies. Adverse reactions to food lectins are well documented in scientific research and sometimes referred to under the heading of "food intolerance." For this reason, we'd place soybeans higher up on the list of foods potentially able to cause adverse reactions. But it's also important to note that the research on SBA is clearly mixed in terms of benefits and risks, and that large-scale human research on whole soy foods is still non-existent in this area. SBA and other soy lectins appear to have inflammatory effects under certain circumstances and anti-inflammatory effects under others. They also appear to have different effects on different types of cancer cells when studied in extract form in laboratory settings. From my perspective, no strong conclusions can be reached at this point with respect to SBA and soy lectins, except to reinforce awareness of soy food in general as potentially more likely to cause adverse reactions in susceptible individuals.

Soy also contains phytate (also called phytic acid) that can sometimes decrease mineral absorption, including absorption of the minerals calcium, magnesium, iron, and zinc. Each of these minerals has a role to play in our health. Traditional methods of soybean fermentation appear to lower the activity of phytates found in soy. With cooking alone, there is more debate about the changes in phytate level. I have not seen any research that would support avoidance of whole soy foods for the sake of optimizing calcium, magnesium, iron, or zinc status. But I have seen research suggesting that highly processed soy foods—like commercially produced soy milks—may best be fortified with minerals like calcium in order to assure healthy mineral absorption. All of the precautions listed above are important considerations when deciding the role of soy foods in your Healthiest Way of Eating.