Soybeans contain healthy plant compounds called phytoestrogens. They are a good, non-cholesterol protein source, a natural source of lecithin, and high in essential fatty acids including omega-3.
Soy is also difficult for humans to digest. It contains more phytic acid than most beans, which can affect mineral absorption. Some feel its natural enzyme inhibitors block protein absorption, and claim eating too much soy can lead to allergies, sexual dysfunction, adverse affects on hormone development during puberty, thyroid deficiencies and slow growth in children.
Processed soy derivatives such as soy flour, textured soy protein, partially hydrogenated soybean oil and soy protein isolate all raise concerns. These highly processed soybean products have become common ingredients in pre-packaged or fast foods. Products made from soy derivatives such as vegetarian cheeses, margarine, burgers, hot dogs and bacon are staples in many vegetarian diets. These products are often less healthy than their animal counterparts.
Traditional Japanese diets found healthy ways to use soybeans. They did not invent or eat soy protein isolate. They ate carefully crafted, fermented soy products in small amounts. The fermentation process deactivates phytic acid and renders the bean more digestible. Fermented tamari, shoyu, miso and tempeh add flavor and digestibility. Tamari and shoyu are soy sauces derived from a long aging process. Good brews are fermented at least three to six months. Less healthy mass-market soy sauces are not fermented, and use hydrolyzed vegetable protein, corn syrup, caramel color and MSG to mimic natural fermentation.
Tofu, though not fermented, is made from cooked and strained soybeans that have coagulant added. Luckily the beans phytic acid is mostly found in the fiber which is discarded when tofu is made.
Cultures traditionally eating soy also included sea vegetables. If theres concern about mineral absorption or thyroid deficiencies from eating soy, the plethora of minerals (including iodine) in sea vegetables counters it.