SCOTLAND is set to shed its reputation as the "fat man of Britain" over the next 20 years as it is overtaken by both England and Wales for rates of morbid obesity, researchers have claimed.

Despite Scotland's persistent reputation as the fattest nation of the UK, the first study to predict future trends in morbid obesity suggests that a country notorious for deep frying everything from Mars bars to pizzas is going to turn the table on its neighbours.

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It is thought that tougher action to fight obesity in Scotland compared to south of the border may be behind the shift.

A paper due to be presented today [Saturday] at the European Congress on Obesity in Vienna forecasts that the percentage of adults in Scotland who are morbidly obese - classified as body mass index exceeding 40 - will barely change between 2015 and 2035.

The researchers estimated that it will grow by just one per cent, rising from four to five per cent of the population.

In comparison, the proportion of massively overweight adults in Wales is expected to almost quadruple - from three to 11 per cent - and nearly triple in England, from 2.9 to eight per cent.

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One of the authors, Dr Laura Webber of the UK Health Forum, said the results were "surprising" but suggested it was down to "stronger policy drives" in Scotland.

She said: "The Scottish government did a big push to reduce obesity in 2010 and they developed a route map and also started to collate and share interventions that seemed to be working. That may have had an impact.

"What’s important to note though is no single intervention offers a ‘silver bullet’ and many interventions are needed.

"That morbid obesity is predicted to increase across all three countries is worrying, and it's time to consider upping bariatric surgery coupled with effective interventions to help obese and morbidly obese individuals to lose weight."

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Dr Webber added that interventions such as the UK-wide tax on sugar-sweetened beverages was welcome, but "not enough".

She said: "It's a very good policy, but compared with tobacco tax it's actually quite a small amount of tax so we would need a duty escalator on that."

The projections have been extrapolated using data from the Health Survey for England, Welsh Health Survey, and the Scottish Health Survey between 2004 and 2014.

However, the authors stress that "past trends do not always predict future trends" and they cannot account for factors such as potential new drugs or changes in policy.

The Scottish Government wants to roll out a raft of measures including bans on junk food multi-buy deals, restrictions on portion sizes in commercial premises such as restaurants, takeaways and cinemas, and mandatory calorie labelling on all menus in a bid to curb obesity.

The proposals have the backing of health campaigners, including celebrity chef Jamie Oliver, but are likely to face legal challenges from the food and drinks industries and retailers.

Morbidly obese individuals are at significantly higher risk of type 2 diabetes, coronary heart disease, stroke, mental illness and some cancers.

Despite the comparatively smaller increase in morbid obesity predicted for Scotland, the authors warned that the outlook is "worrying" for the NHS.

They said: "Our study reveals a worrying picture of rising morbid obesity across England, Wales, and Scotland that is likely to weigh heavily on healthcare systems and economies.

"Strong measures to reverse this future trend must be an important public health priority."

The research will be presented by co-author Dr Laura Keaver, from Ireland's Institute of Technology.