If the human race keeps growing fatter at American rates, the Earth may face a rise in food demand equal to that of nearly a billion extra people, British researchers warned on Monday.

Expanding waistlines in the rich world pose a grave threat to our planet's finite resources, said the team, arguing that a population explosion in the Third World was often wrongly singled out as the chief menace.

"If all countries had the BMI distribution of the USA, human biomass would increase by 58 million tonnes, an increase in mass equivalent to an extra 935 million people of average body mass," said the researchers from the London School of Hygiene and Tropical Medicine.

A heavier body requires more energy, obtained from food, in both its active and restive states.

The world's over-15 population in 2005, when it was estimated at 4.6 billion, weighed a total 287 million tonnes, said the team -- an average of 62 kilogrammes (137 pounds) per individual.

Of the total, 15 million tonnes were ascribed to overweight -- a mass equivalent to 242 million people of average weight.

Biomass due to obesity was put at 3.5 million tonnes, the equivalent of about 56 million average-weight individuals.

A person is considered overweight if his body mass index (BMI), a measure of body fat based on height and weight, is over 25, while a BMI over 30 qualifies one as obese.

The food energy used to fuel the excess kilogrammes that overweight people carry around could nourish more than 100 million extra average-weight individuals, the team wrote in the journal BMC Public Health.

"The average (weight) is increasing everywhere. Everybody is getting fatter, even the thin people are getting fatter," co-author Ian Roberts told AFP.

"When people talk about the Earth and how many people it can maintain, they usually think of what is the population and how much is the population growing. Very quickly, people start pointing fingers at poor people in poor countries having too many babies."

But it was a mistake to count the number of mouths that need feeding and not the amount of flesh that needs sustenance, he said.

"Once you start thinking in terms of flesh, actually one has to start pointing the finger at fat populations in wealthy countries as well," said Roberts.

"It is not just about family planning in Africa".

The team said North America had six percent of the world's population, but 34 percent of mass attributed to obesity, compared to 61 percent of the population and 13 percent obesity mass for Asia.

"One tonne of human biomass corresponds to approximately 12 adults in North America and 17 adults in Asia," said the paper.

Seventy-four percent of the North American population was overweight, compared to 56 percent in Europe, 24 percent in Asia and 29 percent in Africa.

And in consuming more than they needed, rich consumers were also driving up prices, said Roberts.

"There is a certain amount of food on the Earth and the price is determined partly by demand," he explained.

"Over-consumption in wealthy fat countries contributes to under-consumption in poor, lean countries."

The world's nations are meeting in Rio de Janeiro, Brazil, for a UN Conference on Sustainable Development, discussing ways to end abuse of Earth's resources while also addressing poverty and hunger.