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A 20% tax on sugary drinks would reduce the number of obese adults in the UK by 180,000, say researchers writing in the British Medical Journal.

The impact would be greatest in the under-30s, suggests.

But some groups say a tax is misguided and simplistic, and would not have an impact on older age groups who might benefit most from losing weight.

Earlier this year doctors called for a soft drinks tax to reduce sugar intake.

Sugar-sweetened drinks, when taken regularly, have been shown to increase the risk of obesity, diabetes and tooth decay.

A typical sugary drink can contain six to 15 teaspoons of sugar. One teaspoon of sugar is equivalent to 4g of sugar or 16 calories.

A 20% tax would add about 12p to the price of a 330ml can of fizzy drink bought in a supermarket or about 40p to the cost of two-litre bottle.

The researchers in this study estimated that such a tax could cut drinks purchases by 15% and lead to a reduced energy intake of 28 calories per person per week.

Overall, they estimated that such a tax would reduce the number of obese UK adults by 1.3% and the number of overweight adults by 0.9%.

Among younger adults, aged 16-29, who drink larger quantities of sugary drinks (300ml per day) than older adults (60ml per day), the researchers predicted a reduction of 128,000 in the number of obese adults - a bigger impact than for other age groups.

Complex causes

But Gavin Partington, director general of the British Soft Drinks Association, said soft drinks were not to blame for obesity.

"There's ample evidence to suggest that taxing soft drinks won't curb obesity, not least because its causes are far more complex than this simplistic approach implies.

"Indeed, the latest official guidance from the National Institute for Health and Care Excellence points to the need to look at overall diet and lifestyle.

"Trying to blame one set of products is misguided, particularly when they comprise a mere 2% of calories in the average diet."

Dr Adam Briggs, lead study author from the Nuffield Department of Population Health at Oxford University, said their research led them to conclude that taxing sugar-sweetened drinks "is a promising population measure".

He said: "Sugar-sweetened drinks are known to be bad for health and our research indicates that a 20% tax could result in a meaningful reduction in the number of obese adults in the UK.

"Such a tax is not going to solve obesity by itself, but we have shown it could be an effective public health measure and should be considered alongside other measures to tackle obesity in the UK."

The study modelled the health effects of a 20% tax using data from a number of different surveys, which provided information on drinks consumption, drinks-purchasing trends and the prevalence of obesity in the four countries of the UK.

Some experts, including Tom Sanders, professor of nutrition and dietetics at King's College London, felt the findings were "naive".

He said: "Most nutritionists agree it would be better to drink water than sugar-sweetened beverages. However, many consumers like sweet drinks and if they could not afford to buy sugary fizzy drinks they can always revert to drinking tea with added sugar as in the past.

"The cost of sugar-sweetened beverages is currently so low that any price increase would be so marginal that it would be unlikely to affect intake."

Better protection

In the past year, however, there have been calls for a tax on fizzy drinks from the Academy of Medical Royal Colleges and from food and farming charity Sustain.

Malcolm Clark, from Sustain, said the BMJ study provided more evidence that a duty on sugary drinks would improve the diet and health of children and young people.

He said the duty should cover all sugar-sweetened drinks, including those - like flavoured waters and juice drinks - that parents often don't realise have added sugar.

He added: "We challenge the government to show it has a public health backbone by introducing a sugary drinks duty, alongside better protection for children from junk food marketing and robust nutrition and sustainability standards for all the food served in schools, hospitals and other publicly funded places."

A spokesperson from the Department of Health said the Responsibility Deal, which a number of soft drinks companies have voluntarily signed up to pledging to reduce sugar in their drinks, was helping people to make healthier choices.