Chewing sugar free gum increases the flow of saliva in your mouth. If you chew it after eating, the increased production of saliva can help wash away and neutralise the acids that are produced when food is broken down by the bacteria in your mouth. Acid can break down tooth enamel creating conditions for decay to occur. Increased saliva flow also carries with it more calcium and phosphate to help strengthen tooth enamel.
Clinical studies have found that chewing sugar free gum for 20 minutes after eating can help prevent tooth decay. It is still important to brush twice a day and floss once a day but chewing sugar free gum can help, particularly if you have a high acid diet.