Thinking that legislation is the solution to everything is part of so many problem in the US. You can't stop fat gain with laws any more than stopping people from driving over the posted speed limits on highways (which are government legislated and policed).
Education and free market effects are the only real and long-term solutions to these and other problems.
Educate people and they will vote with their actions. Food quality will improve overnight if Walmart shoppers stop buying everything with corn syrup and a myriad of unrecognizable chemical cocktails added for flavor, looks, preservation, whatever.
Finally, one needs to recognize that a certain percentage of the population will be fat no-matter what. I don't know what that number might be, but the key point is to face the reality that one can't fix every life. It's an individual choice.
If health insurance costs were paid directly by the policy holder and the market was free and open across state lines you might just see insurance policies that say things like "if your body fat content is over 20% your deductible increases by 100%, if it's over 33% your coverage is cut in half." or some such thing.
People react immediately to things that affect them directly. This sort of a free market approach is proven to motivate people to change behaviors. Your automobile insurance gets more expensive if you accumulate speeding tickets, therefore you slow down.
Keep government out of it. Almost everything they touch turns into an ugly mess.