User blog comment:Roranoa zoro/Latest Chapter Discussion/@comment-1940096-20141002195901/@comment-24638955-20141002201228

I don't remeber quite right but I think I heard somehing like that along the lines ln real live that rather than the age of death was fertility was reduced by generation.

Edit: I remember, it was called heavy metals which are toxic metals that  a part of them can be passed down to the generations if the mother gets intoxicated.