Q. But even diet soda contains acid from the carbonation (carbonic acid) as well as citric acid and even other forms of acid added to enhance the flavor. Since these acids occur in diet soda as well as in sugared soda, why is it that diet soda doesn’t cause decay??

A. All the non sugar related acids in soda (including carbonic acid) are so soluble in water that they are washed off the teeth almost immediately before they can cause much decalcification of the tooth structure. On the other hand, the sugar in regular soda is very sticky and remains on the teeth for a long time. In addition, the bacteria in plaque use sugar as a raw material to create dextrans which are the molecular units composing the viscous sticky stuff that makes plaque adhere to the teeth. The dextrans have the property of absorbing more sugar which is turned into acid by the plaque bacteria causing the plaque to remain acidic for twenty minutes or more after each exposure to sugar.

I have never tried to dissolve a nail in a bottle of Coke, but if it did dissolve, it would be from the prolonged exposure of the iron to the carbonic acid from the carbonation in the soda, and not from the sugar. If a diet soda is drunk in the ordinary way, the carbonic acid caused by carbonation does NOT spend enough time in the mouth to damage the teeth. However if a patient habitually engages in soda-swishing, which is the habit of swishing the soda around in the mouth for prolonged periods before swallowing in order to remove the carbonation (because the carbonation hurts the throat), then over many years, the acids in even diet soda can cause serious erosion of the enamel and exposed dentin. Note that this form of erosion is NOT the same as decay. Decay is caused by bacterial action while erosion is caused by direct acid attack. Of course, soda swishing with regular sugared soda causes rampant decay as well as serious tooth erosion.