The fact of leatus is if you give to much it causes liver damage muck like alcohol to humans (kills cells andcleans out old ones) but to a resposble owner a small slice of a leaf per month is ok as it cleans out dead cells that are sent there anyway. This leads to better digestion and bodily working funtions ( like pooping )
However Too much lettus will harm your pig no matter what kind! - This is becauise of all the empty calories and Vitamin C and A. So it is ok to feed small amounts of lettus, its just better to feed too little of the lettus than too much (SMALL PORTIONS! 