WEDNESDAY, April 22 (HealthDay News) -- Watching your weight does more than protect your health. It also may help fight climate change.

Researchers at the London School of Hygiene & Tropical Medicine say that because food production is a major contributor to greenhouse gases, a lean population, such as in Vietnam, consumes about 20 percent less food and produces fewer greenhouse gases than a population in which 40 percent of people are obese, a rate close to that of the United States.

Also, less energy is required to transport slim people, say the researchers, Phil Edwards and Ian Roberts, of the school's Department of Epidemiology and Population Health.

They calculated that a lean population of a billion people would emit 1,000 million tons less transportation-related carbon dioxide equivalents a year than an obese population would emit.

Their research was published April 20 in the International Journal of Epidemiology.

"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 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 toward fatness and recognize it as a key factor in the battle to reduce emissions and slow climate change," they said.

However, they noted that the trend is in the opposite direction. The average body mass index (BMI) is increasing in nearly every country. The average male BMI in England, for instance, increased from 26 to 27.3 between 1994 and 2004, while the average female BMI increased from 25.8 to 26.9.

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