Following a sugar challenge, measures of interdental plaque pH, using pH indicator strips (Spezialindikator, pH range 4.0-7.0; Merck, Darmstadt, Germany), will be performed at 5 different points: at baseline, after 6 months of chewing gum use, after 12 months of chewing gum use, 6 months after the end of chewing gum use (18 months from baseline) and finally 12 months after the end of chewing gum use (24 months from baseline).