Why Avoid Soy
Although soy is definitely off the Paleo menu, there are some benefits to fermenting it (as is the case with soy sauce). The main reason we avoid legumes like soy is because they are very high in anti-nutrients, such as phytic acid and lectins. These tend to punch through the lining of our guts, letting toxins and undigested foods into the blood stream and causing inflammation.
The Benefits of Fermentation
Fermenting legumes eliminates some of these anti-nutrients, but not all of them. This makes it a better choice than unfermented soy, but it doesn’t do anything to help the fact that soy is still estrogenic and possibly even carcinogenic. Add to that the fact that most soy sauce contains gluten (although there are gluten-free varieties available), and there’s just no place for it on the Paleo menu.
Mark Sisson talked about the pros and cons of fermented soy, but still refused to give it his seal of approval.