A sluggish metabolism does in fact affect your ability to burn calories. If you have a faster burning metabolism you might tend to burn calories at a higher rate and therefore not gain weight. The bottom line is, obesity in children and adults is climbing at an alarming rate and fast food does not help.
Fast food chains usually serve a larger portion than you would serve yourself at home. Over time, your body adjusts to the larger amount and you start wanting to eat more than is necessary on a regular basis. That of course can contribute to weight gain. You start eating a lot more than you need, and not even realize it.
Whether fast food causes obesity is only part of the problem. No matter what your weight, fast food is just not good for you. It contains high quantities of sodium, bad fats, sugar in some form, and chemicals (including MSG which is bad for you). One expert I heard said that the chemicals in the food are designed to make you crave that food and it can actually be like an addiction. You keep coming back for more. When you eat that nice juicy burger which tastes so delicious, it is usually the chemical flavoring in the food more than the meat itself that you are really enjoying.
Often prepackaged foods are no different. Many contain so many artificial flavorings (chemicals which are what make the meal taste so good), and fats and sugar. The eBook Healthy Urban Kitchen is a terrific way to learn to cook more healthy and lose weight doing it.
Avoid fast foods, and prepackaged foods. Eat and cook from scratch. When cooking a meal, prepare at least two meals at once. Freeze one and use it another day. That will help take away the urge to buy something already prepared when time is a factor.