Think about what you just said... "namely excessive irritation from acidity in our undigested foods." Main word being ACIDITY...which is more likely to cause your saliva to become acid since lone behold, your stomach is closer to your lips than your bowel is. I believe that if "The acidic food particles then leaks through the bowel wall into the blood stream and this may be how the lips get affected by the acidity" was true than we would have a hell of alot more problems than just our lips. Why aren't other organs that would be exposed to this 'acidic' blood be affected? And if this was the case than taking pepto bismal and tums should get rid of our problem of bowel irritation. Our blood has natural buffers in it that work very effectivly "If the pH drops below 6.8 or rises above 7.8, death may occur. Fortunately, we have buffers in the blood to protect against large changes in pH." (http://www.chemistry.wustl.edu/~edudev/LabTutorials/Buffer/Buffer.html).