Smed and Robertson bring some welcome economic realism to the idealistic proposals by public health specialists for taxes on "bad" foods. They consider the counter-strategies adopted by both consumers and companies. They are admirably candid in acknowledging the absence of evidence that such taxes improve health. But...

They never mention the politics of the matter. You would never know from their article that Denmark, where they both work, is in the middle of a debate to repeal the fat tax, introduced to such acclaim among nutritionists, only a year ago.

Nor do they mention similar negative political reactions elsewhere, like the controversy over the "pasty tax" in Britain, or the rejection of soft drinks taxes all over the US in 2010.

Like good academics, they propose more research, ever more detailed economic studies. But they never consider the obvious practical question: why are "health" taxes so unpopular with voters and politicians in the real world?

The morale of the story: you cannot write seriously about policy without writing about politics.

This was a commissioned article. The BMJ should commission a second piece from these authors next year, when the Danish debate over fat taxes has concluded. Living in Copenhagen, they are well placed to provide a detailed, objective report on how nutrition policies can fail, as well as succeed.

Competing interests: None declared