Chef and television personality Jamie Oliver was a major proponent of a sugar tax in the UK.

You would think, then, that a sugar tax would be a real triumph for the chef turned lobbyist. Yet with barely enough time to celebrate, Jamie Oliver and health experts acknowledge this is only a small part of the policy package.

So if a sugar tax in and of itself is not a promising solution to obesity, what is it good for?

A likely answer is that a sugar tax is simply a chance to shift the Overton window. For those unfamiliar, the Overton window is the range of policies voters deem acceptable at any given time. As voters get accustomed to an idea, more radical policies can then be introduced.

An editorial in the Listener has suggested as much, arguing the tax will probably have little direct effect on obesity, but could have symbolic value.

In a sense, sugar taxes themselves are a result of shifting voter expectations. The public have already become accustomed to tobacco and alcohol excise, making food taxes seem like the next logical step.

The reality is, despite (varying forms of) sugary drinks taxes being implemented in seventeen countries, there is still little evidence that these taxes actually reduce obesity.

Much of the overseas evidence, where that evidence exists, indicates a reduction in spending on that taxed item, or a reduction in overall consumption. But that does not consider whether people switch to cheaper versions of the same product, or switch to untaxed but equally unhealthy foods.

Most importantly, it does not prove that the tax will have a significant effect on obesity. The Morgan Foundation has helpfully pointed out a study they deem reliable, which shows the effect on obesity and BMI is tiny. The largest effect was a 0.06 BMI reduction for a 20 percent tax.

Health Minister Jonathan Coleman has rejected a sugar tax for now, but that does not mean we can remain complacent. After all, this is the same government that places regulatory restrictions on e-cigarettes, despite convincing evidence they can reduce harm to smokers wanting to quit.

The Overton window has already shifted in New Zealand without many people making much fuss. That is the whole point.

While public health is the flavour of the month today, there is absolutely nothing stopping evidence-poor policies being introduced in other areas too.

Sugar taxes is just one subject in The New Zealand Initiative’s upcoming report on public health and lifestyle regulations. Join us for a panel discussion where we will be discussing this and other regulations that limit the freedom we have to make decisions about our own bodies.