If all of the world's population was as overweight as Americans, the resulting increase in global human weight would be equivalent to adding nearly an extra billion average-sized people, says a new study.
The striking numbers come from British researchers who argue that discussions about overpopulation shouldn't be limited to head counts but should also factor the impact of some countries' heavier population.
The paper by six researchers, mostly from the London School of Hygiene & Tropical Medicine, was published Monday in BMC Public Health, a peer-reviewed journal.
Mining World Health Organization statistics, they used national data for body mass index (BMI) and height distribution to calculate the world's global biomass.
Based on 2005 data, the world's overweight people account for an extra biomass equivalent to 242 million average-sized people, the study says.
Ever since 1798, when Thomas Malthus first warned that humans would eventually outstrip their ability to feed themselves, concerns have focused on unchecked population growth in less-developed countries in Asia and sub-Saharan Africa, where fertility rates are high.
The study's authors say, however, that even small population increases in wealthier nations will exact a toll on global resources because their heftier citizens require more food and energy.
The population of the United States for example is expected to increase from 310 million in 2010 to 403 million by 2050. Most of that increase will come from immigration but newcomers tend to adopt the diet and lifestyle of their new countries, with the resulting increase in body weight.
"The increased global demand for food arising from the increase in body mass is likely to contribute to higher food prices. Because of the greater purchasing power of more affluent nations (who also have higher average body mass), the worst effects of increasing food prices will be experienced by the world's poor," the study noted.
The energy requirement to sustain overweight people (those with a BMI over 25) is actually smaller than the energy requirement to sustain those with a BMI under 25, said one of the study's authors, Sarah Walpole.
"It is not a weight for weight equivalent energy requirement," she said.
Still, if all countries had the same BMI distribution as the U.S., this would increase the total global human biomass by the equivalent of 935 million people of average body mass.
The amount of energy that would be required to sustain that increased mass would be the equivalent of the energy requirement of 473 million people, the study found.
"Increasing population fatness could have the same implications for world food energy demands as an extra half a billion people living on the earth," the authors wrote.
The researchers compared the U.S., where the population is saddled with an average BMI of 28.7, with Japan, a similarly developed country where a leaner diet and a more physically active population has resulted in a national average BMI of 22.9.
If all world's countries had the same BMI as Japan, the paper says, the global energy needs would drop by the equivalent of having 107 million fewer adults to feed.
According to the study, the world's 10 heaviest nations are the U.S., Kuwait, Croatia, Qatar, Egypt, the United Arab Emirates, Trinidad and Tobago, Argentina, Greece and Bahrain.
Canada is in 20th position, sandwiched between Chile and Germany.
The lightest 10 countries are North Korea, Cambodia, Burundi, Nepal, the Democratic Republic of Congo, Bangladesh, Sri Lanka, Ethiopia, Vietnam and Eritrea.
On average, it takes 12.2 Americans to make up a tonne of biomass, whereas one would almost need 20 Nepalese or Sri Lankans to make up a similar mass.
While the top 10 heaviest countries represent a broad geographical range, the U.S. demographic weight means that North America has the highest average body mass of any continent.
North America has 6 per cent of the world's population but 34 per cent of world biomass due to obesity. Asia has 61 per cent of the world population but 13 per cent of the biomass due to obesity.
"Although the concept of biomass is rarely applied to the human species, the ecological implications of increasing body mass are significant and ought to be taken into account when evaluating future trends and planning for future resource challenges," the study concludes.
"Tackling population fatness may be critical to world food security and ecological sustainability."