Chewing gum knocking you out of ketosis, unlilkely (never did for me or many other people i know who have been on this forum), but it does make you very hungry and so i wouldn't advise people to do it!
Just a bit of info for anyone who is interested..
Voluntary induced Ketosis,relies on something called the the Krebs cycle in the body completely collapsing. It has been known that people who consume too much citric acid may well re-activate the krebs cycle and thus have gluconeogenesis kick in, which will take you of of ketosis.
However, there isn't any hard evidence to show that just the presence of Citric Acid could interfere with ketosis, especially as many people in ketosis drink alot of diet soda, which contains citric acid, and these people haven't been kicked out of ketosis by it. The atkins diet for example, you can drink as much diet coke on that as you feel like and the diet still works by you being in a ketogenic state.
Lipotrim is a way of retraining your mind and body to food and so obviously they will always advise you to abstain from any food and drink in order to retrain yourself from a blank palette so to speak. They believe that a blank palette is experienced when nothing passes the lips other than what they tell you, which is obviously very true. The diet works!