The link to the Eades book excerpt that Ray posted points out that 
flat bread was fed to the workers. Apparently the leavened bread was 
reserved for the upper classes.

From: http://forward.com/articles/10411/leavened-or-unleavened-a-history/

"food historians are of the opinion that leavened bread originated in 
Egypt, probably less than a millennium before the pyramids were 
built. Egyptian culture was the first to produce leavened bread, and 
leavened bread was a symbol of Egyptian culture."

While the upper classes also ate meat and fish, that workers didn't 
get to eat, they won't nullify the negatives of their bread eating.

Don.