Alea's right. I eat nothing but trash but because of my job I am constantly running around the office with breaking news stories, etc., and playing in my band is an intense workout in itself most of the time. That is the only reason why I'm not a fat slob; I have many friends with similar eating habits to mine who are three times my size because of it. Now bear in mind I'm not exactly thin, but I'm also not a fat mass either. I don't have a gut, I don't have to wear stuff the wrong size so it reaches around my waste...I have one friend who now has like a hunchback because for ten years he was so self conscious about his man tits that he hunched up his shoulders to hide them and has now damaged his posture permanently because of it. Another friend I have actually tucks his stomach's overhang into his pants to look thinner despite it being incredibly uncomfortable. But both of these dudes do little besides go to work then come home and lay around all day. So therein lies the key difference.
I once lost 20 lbs. because I worked in a mall and the only thing to eat there was Burger King, but I was walking and lifting books all day long at work. So I don't know if we can lay the blame on fast food entirely.
A friend of mine made an interesting point once, about how McDonald's type food has so much of everything in it...but theoretically if that was pretty much all you ate that day, you would still burn it off by default.
Now here's an interesting observations: one of my friends, he sleeps A LOT, and he eats really bad food (like, he's one of those people that goes to McD's and gets a big huge meal and an extra sandwich and nuggets on the side). My other buddy, he eats fairly normally though (better quality food and in less portions than I do), and yet he stays big. So friend A is definitely in large part responsible for his own obesity but I don't think friend B is so much.
The sad thing is it doesn't take much to shake off this stuff. If I had an exercise bike or something I could do for 20 minutes a day, three times a week, I would look absolutely great despite my fast-paced newsroom lifestyle all but requiring me to eat fast food 3-4 days a week. I think I look pretty good now, but like I always say, much of that comes from discipline. My friends take up lazy hobbies because they are lazy (well, Friend A, at least).
You take video games for example. I am HIGHLY passionate about video games. As a result I have learned tons of stuff about computers, hardware, technology, industry trends, etc...all sought out by me, out of curiosity and interest. I work hard to afford the multitaps, the newest games, the best hardware, the good net connection. My friends don't do that. Yeah they work hard but only because if they didn't they'd be out on the streets because they couldn't afford rent. Since my landlords are my mother and father they allow me to live here for a lot less than if the downstairs was being rented out, but yet I still work long hours doing very stressful work. Why? I'm actually about to move into new place that is literally 4x as expensive as here. Why would I do that, if I don't have to? Why would I work this tough job's tough hours if I don't HAVE to?
I think at the end of the day it is that fundamental difference of mind. I set my alarm every day whether I have work or not; my buddies sometimes dedicate entire weekends to extended unadulterated sleeping. Today is a saturday and I was up at 9am, because I want to maximize my time and get a bunch of stuff done. My buddies wouldn't do that. If there isn't a necessity involved, or at least a strong irrefutable need for a means to an end, they would just as soon sleep all day and never do anything. I mean, seriously...I can't tell you how many times I call these dudes at any given time of day only for their family/roommates to tell me they're asleep. Been happening for 20 years.
So really I think there's more to it than meets the waistline.