Obesity is not just a serious health hazard, it's an environmental problem as well, as a new study has found that our fast growing waistlines are putting an extra weight of 242 million people on the Earth.

Researchers from the London School of Hygiene & Tropical Medicine (LSHTM) estimated that the adult human population now weighs in at 287 million tonnes, 15 million of which is due to the overweight and 3.5 million due to obesity.

That's the equivalent of the extra weight of about 170 military aircraft carriers or the weight of an additional 242 million people having an average body mass on the planet, the researchers said.

This is, they said, just an attempt to make humans feel uncomfortable about their expanding waistline; looking at the collective mass of humanity can improve understanding of the effects of population growth, LiveScience reported.

Writing in the journal BMC Public Health, the researchers said: "United Nations world population projections suggest that by 2050 there could be an additional 2.3 billion people. The ecological implications of rising population numbers will be exacerbated by increases in average body mass."

The argument is simple. More body mass takes more energy to maintain; therefore as someone's weight goes up, so do the calories they need to exist. It means increases in population counts don't tell the whole story when it comes to demand for resources, said the authors.

"Although the largest increase in population numbers is expected in Asia and sub-Saharan Africa, our results suggest that population increases in the USA will carry more weight than would be implied by numbers alone," they wrote.

Using data from around the world for 2005, the scientists used body mass indexes (BMI) and height distributions to estimate average adult body mass. They then multiplied these results by population size to get a total mass, referred to as biomass. The collective mass of the adult population in 2005 due to obesity was 3.5 million tonnes, they calculated.

Globally, average body mass for a person was calculated at 137 pounds (62 kilogrammes).

According to the researchers, the US ranked at the top of the "Heaviest 10" category, while the "Lightest 10" list is composed entirely of African and Asian nations.

For example, North America has six per cent of the world population but 34 per cent of biomass due to obesity, while Asia has 61 per cent of the world population but just 13 per cent of biomass due to obesity.

"Our scenarios suggest that global trends of increasing body mass will have important resource implications and that unchecked, increasing BMI could have the same implications for world energy requirements as an extra 473 million people," the researchers wrote.

"Tackling population fatness may be critical to world food security and ecological sustainability," they added.