Aiki alerted me to possible issues with soy protein a long time ago on a totally different internet forum. At first I thought he was a loon (and I sort of still do) but the more I read the more I agreed. There is an overriding assumption that soy is great for all people in all quantities. Healthy lean protein right. Not so much.
Aiki can provide more links that I - at least I think so.
Just do a google search of 'soy protein estrogen' and you might think twice about it.