Over 12 per cent of adults are now obese, a ratio that has more than doubled since 1975 and will swell to 20 per cent by 2025, a major survey reported Friday.

Of about five billion adults alive in 2014, 641 million were obese, the data showed, and projected the number would balloon past 1.1 billion in just nine years.

In 2014, the world’s fattest people lived in the island nations of Polynesia and Micronesia, where 38 per cent of men and more than half of women were obese, said the study.

“There will be health consequences of magnitudes that we do not know,” study author Professor Majid Ezzati of Imperial College London said.

The survey, published in The Lancet medical journal, claims to be the most comprehensive of its kind conducted to date.

People are divided into healthy or unhealthy weight categories based on a universally adopted measure of Body Mass Index (BMI) which is calculated by the ratio of weight-to-height, squared.

A healthy BMI ranges from 18.5 to 24.9 and a person is considered obese above 30, when the risk of diabetes, stroke, heart disease and some cancers escalates massively.

With a BMI of 35 a person is categorised as severely obese, and from 40 upward as morbidly so.

Among men globally, obesity tripled from 3.2 per cent of the population in 1975 to 10.8 per cent in 2014 and from 6.4 per cent to 14.9 per cent for women – an average increase of 1.5 kilograms every decade.

“If the rate of obesity continues at this pace, by 2025 roughly a fifth of men (18 per cent) and women (21 per cent) will be obese,” according to a statement by The Lancet.

More than six per cent of men and nine per cent of women will be severely obese.

Current government recommendations for 30 minutes of exercise five times a week could be met in less than half that time.

“Over the past 40 years, we have changed from a world in which underweight prevalence was more than double that of obesity, to one in which more people are obese than underweight,” Professor Ezzati said.

At current rates, more women will be severely obese (a BMI of 35 or more) than underweight by 2025, and the world will miss its stated target of halting obesity at 2010 levels.

Nearly a fifth of the world’s obese adults (118 million) lived in six high-income countries – the United States, Britain, Ireland, Australia, Canada and New Zealand – with the US accounting for one in four severely obese men and one in five women.

At the other extreme, the paper said, Timor-Leste, Ethiopia and Eritrea had the lowest BMI numbers in the world, with averages as low as 20.1.

“The global focus on the obesity epidemic has largely overshadowed the persistence of underweight in some countries,” the research paper said.

The paper says stomach-shrinking bariatric surgery may become the “most effective intervention for weight loss and disease prevention” as waistlines continue expanding.

The data was compiled from 1698 studies involving 19.2 million adults from 186 countries which are home to 99 per cent of the world’s population.

- AFP