I've also read that study about how chewing gum improves concentration. After I told that to all my physio students...they suddenly all showed up next class chewing gum! haha it was good times.
and btw, I chewed gum at one of my MCAT administrations. I had it in my mouth before I went in to sit down; and I think I ended up swallowing it.
The key thing here is to respect your fellow test takers (as some have already alluded).
For instance, during my last MCAT we all sat in swivel chairs, and the kid next to me had a slight squeak in his. So he found the sweet spot for the squeaky noise and rotated back n forth on it making all kinds of distracting sounds. Myself and the guy next to me took off our "sound-proof" headphones to find out where the noise was coming from and then we saw the culprit.
We just kinda shook our heads and returned to our test, all the while this kid swiveled and squeaked in his chair. I felt I was going to report him to the testing supervisor but he didn't do it that much throughout the rest of the test. I was close though!