Wish it weren't that way, but...
I don't know how any "agency" could prevent/outlaw unhealthy food. If you think gun ownership/violence prevention is difficult, outlawing unhealthy food, IMO, would be even harder to do.
Better education about how what one eats affects health and making healthy food available and affordable for all, especially in poor neighborhoods, IMO, is the only possible way. Large cost penalties on health insurance due to obesity, etc. would make the nation's health even worse, plus discrimination against pre-existing conditions is mostly illegal.