It's good to see people that get the full picture. To all those in this thread saying "Just count calories", I have a movie for you:
http://fedupmovie.com/#/page/home, it's on netflix.
Calories in for calories out is the way the food industry are trying to cover their faces for the horrible practices they are getting away with. I'm not even American, but I can see many things showed in the movie that I can relate to in my country,
You can check this page
A Calorie is not a Calorie and see how counting calories is just a small fraction of the problem.
Another issue treated in the film is that, even though you look skinny, your body might be as the body of a obese person. Maybe you don't have a belly, but eating all the crap that is handed by the industry is causing your body to develop the same issues and illnesses of an obese person.
I fully recommend the movie and to google some more about the issue, there are a lot of facts that get lost thanks to the great job of the food industry PR and lobbies.