If only it were that simple.
The government subsidizes corn and soybeans, which is why they're everywhere and in everything. Derivatives from corn and soybeans account for a staggering proportion of the ingredients, 'natural' and 'artificial,' that appear in the staple products offered by most of our biggest food companies. They're also the feed that raises all of our meat. In fact, it's not too much of a stretch to suggest that corn and soybeans are the nation's food supply. They are pretty much the only crops we can grow cheaply at massive mega-scale, and we derive almost all our nutritionally questionable foods from them.
Transforming businesses the scale of McDonald's and General Mills will require nothing short of a complete overhaul of our current agricultural economic policy. Best of luck with that...
For a more insightful article with a deeper look into the issue, I recommend the recent piece about McDonald's in the New Yorker.