“Over-proteining”

This is something I see people talk about a lot, but have never seen shown to be really true: the idea that if you put a lot of protein on your hair, it will become brittle and breakage-prone because there’s “too much” protein.  Similarly, that certain hair types shouldn’t have brazilian keratin treatments because it will “over-protein” their hair (though it seems to me that breakage would likely come from the repeated flat-ironing that’s a part of the straightening).  A lot of no-pooers wash their hair with egg occasionally, but are warned away from doing it too often with the same line, “too much protein”. 

I haven’t been able to find any scholarly evidence to show that this actually happens though, and the posts I’ve seen here and The Natural Haven seem to indicate that all protein does is create a slightly-conditioning film.  Is it possible to “over-protein” your hair?  As an addendum, how high would hydrolyzed protein generally need to be on an ingredient list to have an actual effect?