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1. Chewing gum causes unnecessary wear and tear of the cartilage that acts as a shock absorber in your jaw joints. Once damaged, this area can create pain and discomfort for a lifetime.
2. You use eight different facial muscles to chew. Unnecessary chewing can create chronic tightness in two of these muscles, located close to your temples. This can put pressure on the nerves that supply this area of your head, contributing to chronic, intermittent headaches.
3. You have six salivary glands located throughout your mouth that are stimulated to produce and release saliva whenever you chew. Producing a steady stream of saliva for chewing gum is a waste of energy and resources that could otherwise be used for essential metabolic activities.
4. Most chewing gum is sweetened with aspartame. Short and long term use of aspartame has been closely linked with cancer, diabetes, neurological disorders, and birth defects.
5. If your gum isn't sweetened by aspartame, it is probably sweetened by sugar. Sugar is most likely the single greatest dietary cause of chronic health problems like cancer, heart disease, diabetes, and emotional disorders.
-- Says Dr. Ben Kim (Whoever the hell he is

).
http://drbenkim.com/articles-gum.htmlOh, and to whoever asked the swallowing gum question:
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Although chewing gum is designed to be chewed and not swallowed, it isn't harmful if swallowed. An old wives' tale suggests that swallowed gum sits in your stomach for seven years before it can be digested. But this isn't true. If you swallow gum, it's true that your body can't digest it. But the gum doesn't sit in your stomach. It progresses relatively intact through your digestive system and is excreted in your stool.
--
http://www.mayoclinic.com/health/digestive-system/AN01006I don't know how credible these sources are.