Obesity is threatening the world’s future food security, according to a study published Monday that calculated the weight of the global population at 316 million tons.

Researchers from the London School of Hygiene and Tropical Medicine said increasing levels of fatness around the world could have the same impact on global resources as an extra half a billion people.

In a report published in the journal BMC Public Health [PDF file here], the researchers estimated that 17 million tons of the global body mass was due to people being overweight.

Despite only making up five per cent of the world's population, the United States accounts for almost a third of the world's weight due to obesity, the researchers found.

In contrast, Asia has 61 per cent of the world's population but only 13 per cent of the world's weight due to obesity.

When working out is too much of a good thing 

The study is published to coincide with the largest-ever United Nations conference, Rio+20, which will discuss sustainable development.

Using World Health Organization data from 2005, the scientists calculated the average global body weight at 137 pounds, but in North America the average was 178 pounds.

Get off your butt and exercise, orders your doc 

One of the authors of the paper, Professor Ian Roberts, told the BBC: "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 is how much flesh there is on the planet."

"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 added.

Roberts said health campaigns and urban design that promotes walking or cycling were among the best ways to tackle the problem, which was primarily caused by sedentary modern lifestyles.

“We do not move our bodies so much but we are biologically programmed to eat,” he told the Daily Telegraph. "We often point the finger at poor women in Africa having too many babies. But we've also got to think of this fatness thing; it's part of the same issue of exceeding our planetary limits."