The tax on sugary drinks may have prevented more than 5,000 cases of obesity every year among girls in their final year of primary school, research suggests.
Experts led by the University of Cambridge analysed the impact of the sugar tax, which came into force in 2018 as part of plans to tackle childhood obesity.
The tax has led to drinks companies reformulating their drinks to contain less sugar. Failure to do so leads to firms paying a levy per litre of drink.
Dr Nina Rogers from the Medical Research Council epidemiology unit at Cambridge University, and first author on the study, said: "We urgently need to find ways to tackle the increasing numbers of children living with obesity, otherwise we risk our children growing up to face significant health problems.
"That was one reason why the UK's soft drinks industry levy was introduced and the evidence so far is promising.
"We've shown for the first time that it is likely to have helped prevent thousands of children each year becoming obese.
"It isn't a straightforward picture, though, as it was mainly older girls who benefitted.
"But the fact that we saw the biggest difference among girls from areas of high deprivation is important and is a step towards reducing the health inequalities they face."
The new study, published in the journal PLOS Medicine, looked at the impact of the levy on Reception-age children (aged four and five) and those in year 6 (aged 10 and 11) 19 months after the tax came into force.
Researchers also tracked changes in levels of obesity in children between 2014 and 2020, comparing what would have happened if the levy had not been introduced.
The data found that the sugar tax seems to have had a measurable effect on older girls' obesity levels, leading to an 8% drop in obesity in Year 6 - preventing 5,234 cases of obesity per year in this group alone.
Reductions were greatest (9%) among girls whose schools were in deprived areas (40% of schools studied), where children are known to consume the largest amount of sugary drinks.
However, the experts found no measurable effect of the sugar tax on obesity levels among year 6 boys.
The researchers suggested this may be because boys are more susceptible to advertising by food firms, are more likely to believe energy-dense foods will boost athletic performance, and that girls are more likely than boys to make healthier food and drink choices.
The study also did not find a correlation between the sugar tax and reducing obesity in Reception-age children, with the experts suggesting this may be because younger children are less likely to get their calories from sugar-laden drinks that fall under the tax.
Fruit juices are also not included in the levy, but contribute similar amounts of sugar in young children's diets as sugar-sweetened beverages, they said.
Figures published in November by NHS Digital found that almost one in three children in their final year of primary school in deprived areas of England are obese.
Overall, 31.3% of year 6 children in deprived areas were obese - more than double the 13.5% in the least deprived areas.
Across the whole of England, 23.4% of children in year 6 were obese, of whom 5.8% were severely obese.
A further 14.3% were overweight, meaning almost four in 10 children aged 10 and 11 are overweight or obese in England.
Senior author Professor Jean Adams, also from Cambridge, said: "We know that consuming too many sugary drinks contributes to obesity and that the UK soft drinks levy led to a drop in the amount of sugar in soft drinks available in the UK, so it makes sense that we also see a drop in cases of obesity, although we only found this in girls.
"Children from more deprived backgrounds tend to consume the largest amount of sugary drinks, and it was among girls in this group that we saw the biggest change."
Dr Simon Steenson, nutrition scientist at the British Nutrition Foundation, said the study offered important insight.
"However, the underlying causes of obesity are complex and so it is difficult to directly attribute a change in obesity levels to a single factor," he added.
"For example, the announcement and implementation of the sugar tax occurred alongside a programme of voluntary sugar reduction in other products commonly consumed by children, including biscuits, breakfast cereals, yogurts, and confectionery.
"The authors have importantly highlighted that there are limitations to the interpretation of thei r study, which only shows an association rather than a causal link."
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