If you are concerned about your weight, you can't eat fast food. Period. Here are the reasons. This is not a diet, this advice falls into the same category as my dad in North Dakota advised me: "never eat yellow snow." That's not a dietary restriction; it is part of growing up nutritionally.
Here's why. Fast food is not food. Not food in the traditional sense of things that grow and our ancestors flourished on well enough to become our ancestors. Fast food, or faux cuisine if you're French, is not grown. It is assembled by technicians not as smart as God and with intentions not as pure.
Fast food is a huge chemical experiment. Normally, people won't participate in experiments with their lives, so the industry spends 32 billion dollars a year advertising it. If you like math, that's about a hundred dollars a person a year. They bribe small children with toys, they provide mothers with playpen sitters, and they sponsor virtuous looking things like walks for cancer and houses for sick children. This is cover for the fact that their products suck, nutritionally. Most of you, if you're reading about weight control, have read some of the horrible things they insert into their food. The trans fats will remain with you forever. A McDonald hamburger bun has so much sugar in it that it qualifies as cake by confectioner's standards. The oils in the salad dressing are more suitable for automotive lubrication. As a matter of fact, they do double as diesel fuel.
Why 'Fast' is also not a good thing
But besides the awful food-like substances purveyed, the "fast" part is a threat, too. According to medical lore, the human stomach requires about 18 - 20 minutes to register the arrival of "enough" food and send this important message back to your brain. I am not clear why this information takes so long, but apparently it does. In twenty minutes you will have eaten way more than you should if you eat fast - the advertised reason for coming to the emporium. You are filling your tank without any gustatory shut-off valve. This short circuits a million year old system of stopping eating when you're full. If your Enneagram style loves speed (7's and 3's, pay special attention), this is particularly relevant.