Soy contains chemical compounds that inhibit the body's utilization of certain nutrients, such as iron, zinc, calcium and iodine. Soy also contains tripsyn inhibitors, which prevent the proper digestion of protein.
Goitrogens are chemical compounds found in soy that interfere with thyroid activity by inhibiting the body's ability to metabolize iodine. This can lead to slower metabolism and a goiter.
Phytates found in soy bind to calcium, niacin, iron and zinc, reducing their absorption during digestion. While this can lead to a host of health issues, a diet high in these nutrients is generally enough to counter these effects.
Fermenting soy reduces or removes the phytates and tripsyn inhibitors found in natural soy. Goitrogens are broken down during fermentation, but fermented soy may still have a goitrogenic effect. So a diet high in fermented soy should also contain an adequate amount of iodine.