British scientists say the global obesity epidemic needs to be reversed not only to save lives, but to save the environment.

The researchers from the London School of Hygiene and Tropical Medicine say overweight people cause excess greenhouse gas emissions because they eat more than thin people and are more likely to travel by car.

Phil Edwards and Ian Roberts compared lean and obese populations in a study published in the International Journal of Epidemiology.

They calculate that a fatter population needs 19 per cent more food energy for its energy requirements. The production of that extra food requires machinery that emits greenhouse gases, as well as transport systems that emit pollution.

What's more, the researchers say, a heavier population is more dependent on greenhouse gas-emitting cars to help move around its people who have grown too obese to walk.

"When it comes to food consumption, moving about in a heavy body is like driving around in a gas guzzler," the researchers wrote in their study.

They estimate that each fat person is responsible for about one tonne of carbon dioxide emissions a year more, on average, than each thin person. That adds up to an extra one billion tonnes of CO2 a year in a population of one billion overweight people.

The scientists say the global obesity epidemic needs to be reversed to save the environment.

"We need to be doing a lot more to reverse the global trend toward fatness, and recognize it as a key factor in the battle to reduce (carbon) emissions and slow climate change," they write.