If you do any form of excercise, try this:
Go without ANY fast food for 30 days, do your normal excercise routine (for extra credit, eat lots of veggies during this time)
Then, after 30 days, go eat a BIG meal at your favorite McDonalds type fast food place and do your routine about 2 hours later....
The results are astounding and had it not been for my wife challenging me to this diet test, I'd probably still be eating fast food 3 times a week.