Should I change my diet to improve focus and intelligence in school and on mcat?

I eat a lot of junk food and fast food, but my goals are more extreme, like completely eradicating artificial sugar and fast food, rather than eating it occasionally or in moderation. I'm wondering if anyone has tried something like this to retain information, because I watched an educational video all the way back in middle school which I still remember saying not to eat too much sugar or caffeine before an exam.
Source: Student Doctor Network - Category: Universities & Medical Training Authors: Tags: Medical Students (MD) Source Type: forums