A report by the New Zealand Institute of Economic Research (NZIER) concluded that a sugary drinks tax by itself would not significantly reduce obesity. This has fuelled the merchants of doubt who are opposed to including it in a multi-strategy approach to reduce our appallingly high rates of childhood obesity and dental caries.

The report drew the wrong conclusions because it dismissed the estimated changes of body weight as trivial when in fact, applied across the whole population, they are substantial. For example, an 800g reduction in weight from a 20 per cent tax would have an impact equivalent to halting the rise in population weight gain for three years.

Science historians Naomi Oreskes and Eric Conway coined the term Merchants of Doubt in their expose of how Big Tobacco and Big Oil paid scientists and conservative think-tanks to dispute the scientific evidence on tobacco harm and climate change. Doubt in the evidence was the "product" they were marketing to avoid government actions which might threaten big business profits.

A health levy on sugary drinks, with the money going to prevention, is the new target. As expected, Big Food and Beverage and business interest groups are the leading merchants of doubt but conservative politicians and economists steeped in last century's economic theories are joining their ranks.

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Obesity, a complex problem, is on the rise across the globe and New Zealand has the third highest rates in the economically developed world. There are no magic-bullet solutions and the World Health Organisation (WHO) and every other authoritative health group is calling for a multi-strategy approach. But always at the top of those lists of cost-effective strategies is a health levy on sugary drinks. More than 30 jurisdictions have implemented sugary-drinks levies but New Zealand is dragging the chain.

The previous Minister of Health, Jonathan Coleman, was a leading chain-dragger and a vocal merchant of doubt saying that there is no concrete proof that a levy by itself could reduce obesity rates. On the one hand he was demanding magic bullet evidence for this single strategy, while on the other hand he was promoting his meek childhood obesity plan, filled with 22 largely evidence-free strategies, as his answer to the problem. The NZIER report was commissioned under his watch.

The report acknowledges the enormous burden of unhealthy diet plus obesity - more than twice the impact of the next biggest burden, tobacco. They also note the huge inequities, with these burdens falling much harder on people in disadvantaged situations. Good - we agree on the magnitude of the problem and urgent need for action. Then it gives a sermon from the gospel of the Rational Economic Man, the non-existent human who makes decisions based on 20th century economic theories. These are the same flawed theories that underpin the hyper-consumptive economies which produced the global obesity epidemic and climate change. They are just not fit for a 21st century clean-up of that mess.

The report acknowledges that studies in Mexico and elsewhere show that a sugary-drinks levy reduces consumption. Good - this is its purpose. Sugary drinks have a direct effect on oral health, diabetes, and risks of heart disease. This positive outcome of the review has been lost next to the conclusion that the effects on obesity are likely to be small and insignificant. This is where NZIER misses the boat.

The review was not systematic with fewer than half the relevant studies in the literature included. It also focused purely on the price pathway of impact, ignoring reformulation, signalling, and the health-promotion package that the tax might be introduced with.

The report quoted the work of Dr John Gibson from the University of Waikato, whose studies, based on Mexican data, suggested one of the steps in the model was overstating the impact. That may be fair enough. All the steps in these mathematical models warrant ongoing scrutiny to get the best possible model.

His revised estimates suggest a 20 per cent levy on sugary drinks would reduce weight by 800g which he dismissed as too trivial to be of health importance. While this may be trivial for an individual, across the population, it is important. The New Zealand population is gaining weight at the rate of 260g per year. Thus, 800g is the equivalent of stopping the rise in obesity for three years. Far from trivial. If we only had had other such powerful interventions we could make progress on reducing obesity.

When the merchants of doubt are asked what strategies they would propose to reduce obesity which could jump over the high evidence bar they have set for levies on sugary drinks, they meekly suggest things such as health education and bike tracks. These are good things to do anyway, but the evidence backing them up as cost-effective health measures barely leaves the ground, let alone jumps over the high bar of proven health impacts.

WHO, health experts, and a majority of the New Zealand public support a substantial health levy on sugary drinks with the funding contributing to prevention programmes. The vested interests of the food industry will never support it. It remains to be seen what level of leadership this Government will show on this central strategy to reduce obesity, especially in children and adolescents.

• Boyd Swinburn is Professor of Population Nutrition and Global Health at the University of Auckland and has conducted studies on the impacts of levies on sugary drinks to reduce obesity in New Zealand, Australia, the US, and Thailand.