I'm 5 pages late on this, so forgive me if I chimed in on something already discussed. It takes an extremely large amount of calories to be fat. A lot of people eat shitty foods, but also eat it in very large quantities and don't exercise. Your body can really only process so many calories a day. Even if companies could make our foods 'healthier,' we would still have to combat the fact that some people literally eat a ton of food and will still be over weight. On top of that, we drink an astronomical amount of soda, beer, etc. Those are additions to the food we eat.
This really all boils down to personal responsibility. We can't regulate it. We can shift liability, but it's going to be very difficult without better habits from the people in general. However, I do think we need to direct more attention to biotechnology and other break throughs that could be both extremely helpful and potentially dangerous.