Because food production is a major contributor to global warming, a lean population will consume almost 20% less food and produce fewer greenhouse gases than a population in which 40% of people are obese (close to that seen in the USA today). This is according to Phil Edwards and Ian Roberts of the London School of Hygiene & Tropical Medicine's Department of Epidemiology and Population Health.

“When it comes to food consumption, moving about in a heavy body is like driving around in a gas guzzler”, say the authors. “The heavier our bodies become the harder and more unpleasant it is to move about in them and the more dependent we become on our cars. Staying slim is good for health and for the environment. We need to be doing a lot more to reverse the global trend towards fatness, and recognise it as a key factor in the battle to reduce emissions and slow climate change.”

Transport-related emissions will also be lower because it takes less energy to transport slim people. The researchers estimate that a lean population of 1 billion people would emit 1.0GT (1,000 million tonnes) less carbon dioxide equivalents per year compared with a fat one.

In nearly every country in the world, average body mass index (BMI) is rising. Between 1994 and 2004 the average male BMI in England increased from 26 to 27.3, with the average female BMI rising from 25.8 to 26.9 (about 3kg - or half a stone - heavier). Humankind - be it Australian, Argentinean, Belgian or Canadian - is getting steadily fatter.