The world is facing a "population emergency" as soaring rates of obesity threaten a pandemic of cardiovascular disease, scientists have warned.

The spread of Western fast food was blamed as the tiny Pacific nation of Nauru was named as the fattest in the world. Its average Body Mass Index is between 34 and 35, 70 per cent higher than in some countries in South-east Asia and sub-Saharan Africa.

More than one in 10 of the world's population is obese – more than half a billion adults – and rates have doubled since 1980. The biggest increases are in the richer nations but almost every country has seen rates rise.

Only Bangladesh, the Democratic Republic of Congo and a few countries in sub-Saharan Africa and east and south Asia have escaped the rise. Yet even in these regions neighbouring countries have had widely differing experiences. The women of Southern Africa are among the fattest in the world.

The rise is being driven by increasing urbanisation, the growth of sedentary, office-based lifestyles and the substitution of Western-style fast foods for traditional diets. Researchers from Imperial College London and McMaster University in Canada, writing in The Lancet, describe it as a "tsunami of obesity that will eventually affect all regions of the world".

In its wake comes an epidemic of heart disease and stroke, linked with high blood pressure and raised cholesterol levels. Remarkably, high-income countries such as the US and UK have managed to avoid this, by reducing blood pressure and cholesterol with drugs and dietary changes, such as reducing salt and fats. Smoking too, one of the key causes of heart disease, has fallen. (Japan is an exception where historically low cholesterol levels, associated with the nation's high consumption of fish, have risen to levels seen in western Europe, as the Japanese adopt a Western diet.)

But in middle and low-income countries the outlook is "dismal". "Considering all risk-factor trends together, the forecast for cardiovascular disease burden... comprises a population emergency that will cost tens of millions of preventable deaths, unless rapid and widespread actions are taken by governments and health care systems woldwide," the researchers say.

Treating the consequences of the obesity explosion with drugs will create an "unsustainable financial burden" in these countries and there is an "urgent need" to understand why unhealthy behaviours are adopted by both individuals and communities.

With an increasing trend towards globalisation and urbanisation, the problem is likely to get worse rather than better. "Ironically the economic growth of low-income and middle- income countries is now threated by the projected cardiovascular disease epidemic," they say.

Citing the noted British epidemiologist Geoffrey Rose, the authors say: "Mass disease and mass exposures require mass remedies. Mass remedies require the masses to be part of the solution."

The world obesity map

Fastest growing: US

The US saw the biggest rise in BMI of all developed nations between 1980 and 2008, more than 1kg a decade. Increasingly sedentary occupations, less walking and cycling, more driving in cars and rising consumption of fast foods and sugary drinks are behind the rise which affects all high-income countries.

Slimming down: Italy

Italy is the only high-income country in Europe where BMI declined - for women, from 25.2 to 24.8. Even among men, Italy saw one of the smallest increases. The classic Mediterranean diet - pasta, vegetables and fruit - is one of the healthiest in the world.

Fattening up: UK

The UK has the sixth highest BMI in Europe for women and the ninth highest for men (both around 27). The rate of increase has been second only to the US for men. One in four men and one in three women is overweight and about 12 million are obese.

South America's biggest: Chile

Chile with an average BMI of 27.0 for men and 27.9 for women, was the heaviest country in southern Latin America. The scale of increase in obesity in southern Latin America is second only to the US among men and ranks fifth among women. Rates of obesity soared in Chile with the end of its dictatorship in 1990 and a surge in fast food restaurants and some critics are now calling for a junk food tax to be imposed.

World's thinnest: Bangladesh

Bangladesh is the world's thinnest nation, with an average BMI of 20.5 for women and 20.4 for men. Rice is the staple diet and millions go without enough to eat. More than half of children - more than 9 million - are underweight and have stunted growth.

Fattest on earth: Nauru