It depends on the type of gum, and specifically what type of sweetener is used to flavour the gum. Chewing gum that contains sugar is bad for your teeth, whereas sugar-free gum can actually help prevent decay.
The sticky white substance that builds up on your teeth is called plaque, and consists of millions of bacteria. These bacteria need fuel to grow, and their favourite food is sugar (particularly sucrose, the refined sugar in sweets and some chewing gum). When bacteria and sugar combine in the mouth, tooth decay follows!
Chewing sugar-free gum does not provide fuel for the bacteria, and can stimulate saliva flow, which has a protective effect and helps prevent decay. Certain types of sweetener are better than others for prevention of decay – chewing gum that contains xylitol is best.
Xylitol is a naturally occurring carbohydrate that looks and tastes just like regular table sugar. It is a natural sweetener that is extracted from renewable resources such as corn cobs, and as hardwood. Xylitol also occurs naturally in our bodies – in fact, an average size adult manufactures up to 15 grams of xylitol daily during normal metabolism. Pure xylitol is a white crystalline substance that looks and tastes like sugar. Xylitol is more expensive and less widely used that other sweeteners, and so is rarely used in most sugar-free gums. Chewing gum, mints and sweets containing xylitol can be ordered online, xylitol.org offers a resource for more information and links to purchase xylitol products.