Critique
I find it hard to believe that dentist would say that, although not an MD most people know that when carbohydrates are eaten and digested they turn into BLOOD sugar (blood glucose).
Tooth decay has A LOT to do with genetics and with acid. I think you may have misunderstood what the Dentist was trying to say. When foods containing sugars and starches get left on teeth and then it mixes with the bacteria that is in the mouth the mouth starts digesting the food turning it in to acid.