Sugar taxes appear to work, but the tax rate sweet spot is not yet known, public health researchers say.

In recent years the concept of imposing a tax on the sale of sugary beverages has gained popularity as a public health measure to tackle obesity. Sugar taxes are recommended by the WHO and now legislated in parts of the US and more than 20 other countries, where they are proving an effective strategy to help drive down obesity rates.

But unanswered questions remain around what is the most appropriate type of regulation and whether taxing should be supplemented with providing consumers with nutritional information, according to the authors of a new systematic review of studies investigating the impact of taxes on sales of  sweetened beverages.

The review looked at 17 studies – five examining real-world tax scenarios and 12 using virtual or experimental models – carried out in the US, Mexico and Canada between 2011-2017.

Results from real-world studies showed a tax of more than 8% drove down the purchase and sales of sugary drinks, and triggered consumers to buy non-taxed drinks in the US and Mexico, with reductions higher in areas of lower socio-economic advantage. However, a sugar tax of 5.5% was not effective, one of the studies found.

Results from virtual or experimental studies also showed a sugar tax drove down the purchase and sales of soft drinks, or the desire to buy them.

And a survey of 354 shoppers in an Australian convenience store found a 20% price hike for non-diet soft drinks and energy drinks would reduce beverage volume sales.

Meanwhile, randomised controlled studies evaluating behavioural intent carried out in restaurants, convenience stores and schools showed a tax of more than 19% was effective on reducing intent to purchase, and the inclusion of warning labels with a 20% tax in particular had a “significant effect”.

“This systematic review showed that taxes on sugar sweetened beverages [SSBs] have a significant impact on sales and purchasing or intended purchases. Thus, they are a useful tool for reducing total calories and sugar purchasing behaviour or sales as well as increasing the behavioural intent to reduce consumption in the future,” the authors concluded.

But the type and the quantity of tax and whether it should be adopted as a single or joint measure with other information or nutritional measures still need to be established, they wrote in the American Journal of Clinical Nutrition.

“An indirect tax on SSBs, which would result in a sales price increase to the consumer of 20% to 25%, seems to be the most appropriate measure,” they suggested. “However, the type and quantity of tax and whether it should be adopted as a single or joint measure (with other interventions) still need to be established.”