Staying slim helps reduce greenhouse gas emissions because heavier people tend to use vehicles more, say researchers who compared lean and obese populations.

In Sunday's edition of the International Journal of Epidemiology, British researchers estimate that a lean population of one billion people would emit one billion tonnes less carbon dioxide equivalents per year, compared with a fat population.

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

The heavier that people's bodies become, the harder and more unpleasant it is for them to move about. So they become more dependent on vehicles.

In addition, higher demand for food puts pressure on the transport system.

Staying slim "is good for health and for the environment," the researchers concluded. "We need to be doing a lot more to reverse the global trend towards fatness, and recognize it as a key factor in the battle to reduce emissions and slow climate change."

In Vietnam, people consume almost 20 per cent less food than in the U.S., where 40 per cent of people are considered obese, said Dr. Phil Edwards and Prof. Ian Roberts of the London School of Hygiene and Tropical Medicine.

Based on their model, the researchers figured a lean population of one billion people would emit up to one billion tonnes less carbon dioxide from food production and car travel each year than an obese population.

People are generally becoming heavier worldwide — a worrisome trend because of associated health risks such as Type 2 diabetes and cardiovascular disease.

Between 1994 and 2004, the average male body mass index in England increased from 26 to 27.3, with the average female BMI rising from 25.8 to 26.9.

BMI is used to determine whether someone is overweight or obese, based on height and weight. A score of 18.5 to 24.9 is considered a healthy weight for adults.

Almost two-thirds of Canadian adults are deemed overweight and almost one-quarter obese, according to Obesity Canada, a non-profit organization. Among children, one in four is considered overweight and one in 10 obese.