"The whole world's diet is being Westernized," says Neville Rigby, director of policy at the London-based International Obesity Task Force, part of an umbrella research group with 10,000 members in 50 countries. "Poverty is part of the problem - fat and sugar are cheap products, and people are eating the wrong stuff."
At the same time, city-dwellers in the developing countries are leading increasingly sedentary lifestyles, taking buses or cars to work rather than walking or cycling, and generally getting less exercise.
In some countries - such as the Pacific island nation of Nauru, where 70 percent of the inhabitants are obese, according to WHO studies - lifestyle and dietary shifts have happened too quickly for people's bodies to adapt to the changes. Recent research suggests that habitual food scarcity breeds a "thrifty gene" which boosts the efficiency of people's metabolism. When food becomes more plentiful, and "people are exposed to a carbohydrate-rich diet they were not built for, they run into problems," says Josef Schmidhuber, an analyst with the UN Food and Agriculture Organization (FAO).
The growing incidence of overweight people in developing countries threatens to overwhelm fragile healthcare systems. Problems related to obesity cost the US $117 billion last year, according to the US Surgeon General: Few developing countries could pay that sort of bill.
The WHO is responding with what spokesman David Porter calls "a toolbox of options" to help governments set policies on diet, physical activity, and health.
The strategy offers the sort of dietary advice mother may have given you - limit sugar, fats and salt, eat more fruit, vegetables, and legumes - and encourages daily exercise. It calls on the food industry to cut back on the saturated fats, trans fatty acids, salt, and sugar in its products, and to label them more clearly. And it suggests that governments might want to impose "Twinkie taxes" that discourage unhealthy eating habits, or to regulate the way junk food is marketed to children.
Controversy has focused on a report by a group of independent experts that provides the scientific foundations for the strategy, and particularly on the scientists' recommendation that consumers should limit "free sugars" to 10 percent of their calorie intake.