We tried a tax on fatty foods in Denmark and it was a huge mess. People would just drive to Germany to get the stuff, and managing it was an administrative nightmare: http://www.nytimes.com/2012/11/13/business/global/fat-tax-in...

Interesting anecdote, but

1. It's quite possible (though it's a huge rabbit hole we shouldn't plumb here) that they were taxing the wrong macronutrient entirely, that is to say, that fat on the body is more a factor of sugar going in than fat going in. So in addition to imposing costs it may have failed to give benefits (whereas had it worked to improve health outcomes maybe it would have been worth the costs of enforcement etc.)

2. The Danes taxing butter?!? That seems extremely odd given the dairy industry and culture. But I suppose major kudos for being able to overcome such prior biases to even run the experiment.

That's like taxing it only in Idaho. Of course it must be done in an economic block, like the US or the EU or China.