Difficulty does not change damage dealt. It pretty much just changes how often you get into combat, whether food lasts, whether you can starve to death and whether or not you succumb to addictions. In particular when getting captured the starving is probably what is actually killing you as even if you take an absolute beating by your captors you usually have enough recovery by the next day to survive... If you have decent constitution anyway. Some captors are so viciously brutal that survival is hilariously unlikely though. However I've tested playing in just about every captured scenario and even with just 10 constitution I've been able to survive and escape. Most of the time you have options to play along(such as David Born's camp), in other scenarios they'll actually keep you alive(ork-kinds/abominations), and in the human captor scenarios which are the absolute most brutal possible... You pretty much need to either use stealth to stun/steal/hide, whoring to collect semen to eat at the end of the day before you go to sleep along with your bread, omnivore to eat rats and insects, or my personal favorite you aggro the enemies moving up and down causing them to hit eachother where they will kill each other and if you have cannibalism that's even more food to survive.
The ultimate requirement is just having enough food to recover for the next day. As long as you can keep it up you'll keep gaining huge amounts of XP which gives you levels that eventually can lead to your escape. That's not to say there isn't a clock though, if you get pregnant you got like 14 days left tops to get out of there or you will die from the ravages of a pregnancy/abortion.