Image caption Extra weight could be the equivalent of adding an extra billion people to the planet.

Researchers say if increasing levels of fatness are replicated globally it could mean the equivalent of an extra billion people on the planet.

The team estimated the total weight of people on the planet and found that North America had the highest average.

Although only 6% of the global population live there, it is responsible for more than a third of the obesity.

The research is published in the journal BMC Public Health .

In their report, the researchers from the London School of Hygiene and Tropical Medicine calculate the weight of the global population at 287 million tonnes. They estimate that 15 million tonnes of this mass is due to people being overweight, and 3.5 million tonnes due to obesity.

Using World Health Organization data from 2005, the scientists worked out that the average global body weight was 62kg (137lb). But there were huge regional differences. In North America, the average was 80.7kg (178lb), while in Asia it was 57.7kg (127lb) .

One of the problems with definitions of obesity - it fosters a them and us ideal - when actually we're all getting fatter Prof Ian Roberts, London School of Hygiene and Tropical Medicine

While Asia accounts for 61% of the global population, it only accounts for 13% of the weight of the world due to obesity.

One of the authors of the paper, Prof Ian Roberts, explained the thinking behind the calculation.

"When people think about environmental sustainability, they immediately focus on population. Actually, when it comes down to it - it's not how many mouths there are to feed, it's how much flesh there is on the planet."

Weight of the world

The researchers say that just focusing on obesity in individuals or in groups is divisive and unhelpful.

"One of the problems with definitions of obesity is that it fosters a 'them and us' ideal. Actually, we're all getting fatter." Prof Roberts told BBC News.

The scientists also compiled tables of the heaviest and lightest countries according to their estimates.

The US, with its well documented problems with weight, is top of the list. If the rest of the world were to emulate the Americans, Prof Roberts says, it would have dramatic implications for the planet.

Image caption Japanese people have a low average BMI but high standards of living.

"If every country in the world had the same level of fatness that we see in the USA, in weight terms that would be like an extra billion people of world average body mass," he explained.

While countries like Eritrea, Vietnam and Ethiopia are at the other end of the scale from the US, the researchers argue it is not sufficient to say that being skinny is just a factor of poverty. The researchers point to a country like Japan which, according to Professor Roberts, could be a model for others.

"The Japanese example is quite strong. Average BMI (Body Mass Index) in USA in 2005 was 28.7. In Japan, it was 22. You can be lean without being really poor, and Japan seems to have pulled that off."

Car culture

But other countries in the top 10 most weighty are more of a surprise, and include Kuwait, Croatia, Qatar and Egypt.

Prof Roberts says that the high number of Arab countries is due to the impact of the automobile.

"One of the most important determinants of average body mass index is motor vehicle gas consumption per capita. So, it is no surprise to see many of the Arab countries in the list - people eat but they move very little because they drive everywhere."

The research team hopes its work will prompt new thinking about how the world weighs up issues of consumption, weight and population growth.

"We often point the finger at poor women in Africa having too many babies," says Prof Roberts. "But we've also got to think of this fatness thing; it's part of the same issue of exceeding our planetary limits."