I say No... the government should not be regulating the diets of individuals. I don't think the government should regulate the health related decisions of individuals period.
My main reason would be that the Government is not capable of forcing people to be healthy. Endless laws and regulations don't guarantee that the public will adopt healthier lifestyles. There are numerous examples that support my stated reason.
Drugs, Prostitution, and Bar Fights all carry health risks and they are all subject to legal penalties. Does that stop people from engaging in such activities. Well maybe some, but for the most part people do it anyway and many incur legal and health consequences.
Drinking alcohol in large quantities is not the best course of action for one's health. Of course we all know that Prohibition was a giant failure and people still pay higher sales taxes to consume adult beverages, but the consumption still persists in heavy doses.
As for the recent happy meal ban in San Francisco, there is no guarantee that irresponsible parents won't substitute giving their kids a happy meal with something else that's even less healthy. Who knows maybe some will just buy their kids a large bag of candy every week or introduce them to smoking cigarettes so they don't have to listen to their kids whine about not getting toys with their greasy fast food meal.
What will the powers that be in SF do then? Will they pass new laws that will create new unintended consequences? Then what? Will they create a new government agency and hire new police to monitor everyone to make sure everyone in the city is taking their vitamins and eating their wheaties?!
And what will all the health consequences of that be? Why doesn't anyone ever think about the health risks of an oppressive government that meddles and interferes in people's lives? After all taxes, traffic tickets, acquiring the required licensing standards, etc., etc. are all sources of unwanted stress that wear us down mentally and aid the destruction of our health.
Healthy choices have to come from individual desire not the "manipulation" and "control" of the government.