I don't believe that the government has any place in regulating what people eat. But they can regulate what can be advertised and what information must be disclosed.
For example, how about a regulation that bans television advertising on all foods that fit within a certain criteria. How about a further requirement that all restaurants must list certain information on the menu, such as how many calories, fat content, and cholestoral is in each menu item.
Years ago they banned tabacco advertising on television, and required the surgeon general's warning on all tabacco products, and the end result is a society today where much fewer people smoke.
Prohibition, and more recently the "war on drugs", have proved that you cannot tell people what they can and cannot have. But you can surely educate them about what they are eating (or drinking), and make sure they are not lied to by false advertising.
http://www.youtube.com/watch?v=gCMzjJjuxQI
http://www.youtube.com/watch?v=2r4Book3JQQ