The problem is that this sets the stage that allows them to ban whatever they deem harmful. Salt, sugar, white flour, you name it. All of those will be put up on the chopping block, now. And the .gov will be telling us what we can and cannot eat. Care to show me where that power lies in the Constitution? I'll give you a hint, it doesn't appear anywhere int he document. This is an overstepping of boundaries.
Constitutionality? Pfff, our government don't need no stinkin' constitution to do what it wants! As far as what will be banned have no fear, whatever Monsanto creates in a laboratory will be approved by the FDA in record time for human consumption. They'll concoct some sort of petroleum-based carcinogenic pile of **** that our government will deem to be safe to replace all those other items you just listed.
The sad part is, no purple is needed for this.