While they are relatively new to western menus, tofu and the fermented soybean paste miso have been on the menu throughout Asia for centuries. Today, soyfoods are finding their way onto more American tables than ever. Packed with high quality protein, these foods also provide healthy fats and beneficial phytochemicals like isoflavones. Research suggests that isoflavones lower risk for heart disease and certain cancers.
Recently, their relationship to thyroid function has become an active area of research. The thyroid gland, found at the front of your neck, regulates metabolism and energy use through production of two thyroid hormones, T3 and T4. Synthesis of these hormones requires the mineral iodine, which attaches to tyrosine, an amino acid.
In animal studies, iodine can also attach to the soy isoflavone genistein, which has a similar chemical structure to tyrosine. That’s raised concerns that eating too much soy could lead to a reduction in active thyroid hormone. But until now, it wasn’t known whether this happens in humans. However, a new study from the Czeck Republic has allayed concerns about the effects of genistein on thyroid hormone production.
The researchers provided subjects with about 80 milligrams of isoflavones—the amount in about three servings of soyfoods—to men and women for 3 months. All of the subjects were consuming adequate amounts of iodine. At the end of the trial, thyroid hormone levels were essentially unchanged. The research did find that some iodine attached to genistein, but the amount was insignificant, representing less than 1% of the total iodine consumed. Based on these findings, the authors concluded that soyfoods have no impact on thyroid function as long as iodine intake is adequate.
Sosvorova L, Miksatkova P, Bicikova M, Kanova N, Lapcik O. The presence of monoiodinated derivates of daidzein and genistein in human urine and its effect on thyroid gland function. Food Chem Toxicol 2012.