In the most recent issue of the Toulouse School of Economics (TSE) Magazine (pg 8) features some work by Vincent Requillart and Celine Bonnet on ability of nutrient taxes (like soda taxes) to fight obesity.
Soda and sugar taxes don't always have the anticipated effect:
“The fact that we take into account the way the industry and retailers react via their pricing decisions. Most research assumes that the tax is passed on to the consumer. There’s no reason that should be the case! Firms are not passive, they develop strategies. They can raise prices more than is strictly necessary to cover the tax or, on the contrary, reduce their profit margins so as to maintain their sales.”
The point out that the effects of a sugared-soda tax are small, and that the actual policy passed in France (taxing all sweetened drinks - even those with artificial sweeteners) would not be expected to reduce weight.
“Taxing all drinks, be they sugar-sweetened or light, is counter to health recommendations. In practical terms, the tax implemented does not reach its goal of reducing sugar consumption. It acts primarily as an instrument to increase the State’s budget revenue.”
They seem to favor voluntary arrangements between food companies and the government to reduce sugar and salt content. Even still, in places like the UK, where such an approach has been taken, the effect appears to be virtually nil.
“Having said that, despite all the measures implemented, obesity has not been eliminated. ”
One of the challenges is the complexity of it all
“ In the case of food, defining what is good and what is bad when dealing with a large number of nutrients, is complex.What’s more, eating habits change very slowly.”
For a more in depth and academic treatment of the topic, you might check out some of the published work by these authors.