Is Running Rotting my Teeth!!!

When I went dentist recently she was surprised how rotten one of my teeth had become in such a short time and questioned me about diet. I have no suger in hot drinks, and don't eat mints. However, due to my marathon training which started last summer, I now eat alot more fruit, drink alot more isotonic drinks and eat jelly babies on long runs.<br><br>
When I went back this morning, she told me that unless I address my diet quickly, I will be having alot more dental treatment and it will be costly.<br><br>
Please help.......<br><br>
Are some fruits more acidic than others?<br>
Would I be better taking jells with water and cutting out the isotonic drinks and jelly babies?<br>
What else can I do?<br><br>
I don't want false teeth inside 3 years!!!