Some people like to believe that fermentation (or more specifically bad bacteria) are IBS culprits. There is some data, but not a lot. People have experimented with replacing ALL of oneï¿½s gut bacteria with new bacteria. Thatï¿½s not a procedure for the faint of heart, though.Anyway, fermentation should only be an issue if bacteria and substrate (sugar) meet. That generally canï¿½t happen in the upper gut because it washes away most bacteria, so the only way things could be problematic if the sugar gets through. Lactose gets through in a fair number of people, but for some reason, doesnï¿½t seem to cause a problem despite the popular belief. People have proposed the reason is that the good bacteria eat the lactose! In fact, a popular idea is to feed the good bacteria special sugars they like. Known as fructoligosaccaharides, or FOS. The Japanese have experimented with this with apparently mixed results. Another sugar which can get through is fructose or fruit sugar. That could make fruits problematic, but which ones, I am not sure. It also depends on the load of the sugar..small amounts shouldn't be a problem, but one study found bad bacteria and fructose as an apparent cause for IBS symptoms. Incidentally, when fructose is combined with glucose (it is when you eat sucrose, or table sugar), the fructose absorbs better. Another thing of note is that prunes have a known laxative effect, but it doesn't seem to be related to its sugar content. I don't think any of this is related to the outside of the gut, so that throws out the blood sugar concept. You don't get alter that too much without get very ill (diabetes).The upper gut symptoms could be related to intrinsic motility disorder of the stomach.