My brain isn't working properly today. You'll have to give me an example of some sort, because I can't think of one.
Think of all the vitamins and minerals you consume. No burnable energy in the copper, iron or calcium. There are many compounds and foods that likewise pass completely undigested through the system. Fiber, for instance.
Well well well.. Where to start?
Coke (or any soft drink, diet or not) is so acidic from the phosphoric acid (!) in it. But diet drinks don't feed the plaque and bacteria like HFCS or sugar in nondiet does. So instead of bubble and dissolve your teeth out and feed the bacteria, you just dissolve your teeth out.
Another urban legend/ old wives tale. Acidic drinks will not dissolve your teeth unless you're fond of taking a huge sip and holding it in your mouth for hours without swallowing.