159 students were given a battery of demanding cognitive tasks, such as repeating random numbers backward and solving difficult logic puzzles. Half of the subjects chewed gum (sugar-free and sugar-added) while the other half were given nothing.
Those randomly assigned to the gum-chewing condition significantly outperformed those in the control condition on five out of six tests… The sugar content of the gum had no effect on test performance.
While previous studies achieved similar results — chewing gum is often a better test aid than caffeine — this latest research investigated the time course of the gum advantage. It turns out to be rather short lived, as gum chewers only showed an increase in performance during the first 20 minutes of testing. After that, they performed identically to non-chewers.
Article here. Mechanism apparently unknown.