Obesity, along with rising population, is draining earth's resources. The 'weight of a nation' is as important as the number of people living in a country, says a new study.

The extra food required to feed the overweight and obese people, many of whom reside in rich countries is almost the same as feeding a billion extra people, BBC reports.

According to the study, the total weight of all adult humans in the world is about 316 million tons, or 632 billion pounds. Of this total weight, about 16 million tons is due to people who are overweight. Obese people around the world add up to about 3.8 million tons to the total human weight.

Average weight of a person in the world is 136 pounds, while average weight for an adult in the U.S. is 178 pounds.

America with about 6 percent of the world's population contributes to about 34 percent of weight of the world human biomass. Some 13 percent of the world's adult biomass is due to the 61 percent of humans who live in Asia.

"Our results emphasize the importance of looking at biomass rather than just population numbers when considering the ecological impact of a species, especially humans," said Sarah Walpole, one of the authors of the study from London School of Hygiene and Tropical Medicine.

Obesity is seen as a problem faced by rich countries, which to certain extent is true, but being rich doesn't always translate into being obese. One of the leading examples of how even a developed nation can keep its weight down is Japan.

"The Japanese example is quite strong. Average BMI (Body Mass Index) in USA in 2005 was 28.7. In Japan, it was 22. You can be lean without being really poor, and Japan seems to have pulled that off," Professor Ian Roberts one of the authors of the paper told BBC.

It isn't just about how many people reside in a country, researchers say, but also about how much they collectively weigh. The world needs better control over both population and fatness to preserve the natural resources.

"Everyone accepts that population growth threatens global environmental sustainability – our study shows that population fatness is also a major threat. Unless we tackle both population and fatness - our chances are slim," Prof Ian Roberts said.

The study is published in the journal BMC Public Health.