I'm very fat, and despite making some effort to correct it I still have a long way to go. Meanwhile, my best friend is working to overcome his anorexia. He's 6'0, and just got his weight up to 125 (which might be the highest it's ever been). We make quite a pair in public.
As I see it, there are 3 main factors: exercise, quality of food, and quantity of food. To hell with special diets and pills, those are really the only 3 things that matter. Overall as a society we have been ignoring exercise while shoveling tons of crap down our mouth. We just have to learn to do better.