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The UK sugary drinks tax is working, or so says a study published this week. claim the tax prevents 5000 cases of obesity among 10 to 11-year-old girls in England every year.

Unfortunately, that may be optimistic. It is debatable whether any small drop in obesity rates in this age group is really due to the tax, and regardless, any such effects have been overwhelmed by the larger rise in children’s weight amid the covid-19 pandemic.

Rates of obesity have been increasing in nearly every country for decades. Many governments are considering or have brought in higher taxes on certain foods and drinks to nudge people into healthier diets. Sugary drinks are a common target, as they usually contain few other nutrients.

Mexico was one of the first countries to bring in such a tax, in 2014. The UK introduced one in 2018, causing some drinks to be reformulated with sweeteners to replace some of the sugar. Other places to have introduced similar drinks taxes include South Africa, Portugal and parts of the US, and there is fierce debate over whether to do so in Australia.

But do they reduce people’s weight? In the latest study, at the University of Cambridge and her colleagues say they have provided the first evidence that the UK rules have helped to stop some children from becoming obese.

Their analysis is based on a respected source of data: England’s National Child Measurement Programme, which records the height and weight of most school children in their first and last years of primary school, when they are aged 4-5 and 10-11, respectively. But their conclusions hinge on the uncertain science of modelling – in this case estimating what obesity levels would have been without the tax, based on the existing trend of a gradual rise in obesity rates, among 10 to 11-year-olds, since about 2014.

The team’s analysis suggests that, after about 2016 – when some drinks firms began reducing the sugar in their products in response to the tax announcement – the rise in obesity levels among girls of this age started to plateau. It finished at about 8 per cent lower in 2019 than it would have done if the previous trend had continued, the analysis claims.

But it seems unclear if this reduction can really be pinned on the sugar tax. Of the four cohorts studied – boys and girls, in the younger and older age groups – there was no apparent response to the tax in the other three groups. The researchers’ claim therefore has the appearance of cherry-picking the figures that support their conclusions.

There are possible explanations for why the older girls were more affected, says Rogers. Younger children have fewer soft drinks than older ones so would be less affected by the tax, she says. On the other hand, UK boys tend to consume more soft drinks than girls, so the sex difference doesn’t fit that explanation, says at King’s College London.

Older girls might have responded more than boys to negative publicity over sugary drinks in the media, says Rogers. She points out that a similar pattern of reduced obesity in teenage girls, but not boys, .

Rogers also says that when the statistics for older girls in England are broken down by income level, those from the lowest-income families had the biggest change in obesity levels from the predicted trend, and it is these families who typically consume more soft drinks.

Regardless of any trends before covid-19, childhood obesity levels in England jumped during the first year of the pandemic, . This is probably because school closures meant children spent more time being sedentary at home, with access to snacks.

Alarmingly, in the school year 2020 to 2021, obesity rose from about 18 to 22 per cent in 10 to 11-year-old girls – an increase of 22 per cent on the previous year, swallowing up any theorised effects of the tax. In boys the same age, there was a jump of about 26 per cent.

Rogers says the effects of the lockdowns make it all the more important to consider expanding food taxes to include fruit juices and milk drinks too. These are currently excluded from the UK sugar tax as they are considered to be more nutritious than soft drinks.

It’s certainly true that more must be done to tackle the obesity epidemic, as one of the world’s biggest public health problems. But so far, it doesn’t look like the evidence is there to support drinks taxes as an effective way to go.