Sophie, this behavior is an indicator of disordered eating. Here are a few resources:
http://www.nationaleatingdisorders.o...ning-signs.php
http://www.nedic.ca/knowthefacts/definitions.shtml
http://www.mayoclinic.com/health/eat...CTION=symptoms
There is a difference between chewing and then spitting out regularly for satisfaction versus what Tejas mentioned she has done on occasion- started taking a bite and realizing she shouldn't be eating this. Chewing to enjoy the taste and mimic the process of eating without the intention of swallowing is different from taking a bite and realizing it's not healthy for you to do so and stopping.
Chewing and spitting will not "make you fat" but neither will eating. Eating too much will. Chewing and spitting to avoid eating for fear of getting fat is a sign of a disordered relationship with food.
Perhaps you want to talk to someone about this behavior. It can lead to more dangerous disordered eating down the road.
Wish you all the best.