Whats wrong with McDonald's?
1. Sodium- (as in most fast food joints) is usually to the power of 10 in their prepared foods. The high sodium effect is usually felt more accutely as one ages.
2. Preservatives and natural (read artificial) flavorings- Hepatotoxic to say the least (co-morbidity but alas, a contributor)
3. Sugar-High Fructose Corn Syrup (knowing the physiokinetics of this ratio of fructose in the liver should be enough for any GetBigger to moderate it, which is impossible to do if one eats at Micky D's.
4. Lack of substantively nutrient dense veggies
5. Impossibility to recognize a food as fresh
6. Taste, I believe someone said goat's ass, that about sums it up.
Some of these are just how a public health professional views the fast food scene and apply to most if not all of those places.
As far as studies go on sodium and sugar, all studies have an external validity that is applicable in the case of fast food, so pick yer study and remember: experience is as valid as experiment- never discount how you react personally to things found in the laboratory we call life.