In my experience I have been down to 56kg weight (from 59-60kg) but that takes an enormous amount of riding to do - that's doing consistent 800-1000km weeks because I have no interest in radically changing my diet. At that level I basically eat heaps. You need it to keep going when you ride that much.
I was riding a 7.2kg bike at those times. The really light frames weren't as common back then. I would have liked to have had my Canyon back then - I was very fast at that time. It was still 4 years away from being released.