The combined weight of adult humans is 287 million tonnes, researchers estimate. Around 15 million tonnes of that from overweight adults and 3.5 million tonnes from obese adults, according to research from London School of Hygiene and Tropical Medicine.

The highest average body mass was North America with 80.7kg (12stone 10pounds) per person, where 6% of the world's population accounts for 34% of the world's obesity biomass figures.

The world's seven billion people population all need to be fed and extra mass requires more energy levels in order to move the heavier bodies, the study points out.

Prof Ian Roberts, who contributed to the report, says, "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."

The research, which calculates the combined mass of adult humans and the regional distribution, has just been published in BMC Public Health, which is a BioMed Central journal.

The study used information from the World Health Organization and United Nations to make their estimations. However, because it is based on the 2005 World Health Organisation figures, the true adult human biomass total is even higher than these figures.

The average body mass across the world is 62kg (9 stone 11 pounds). Asia accounts for 61% of the world's population but just 13% of the world's biomass from obesity.

If everyone had the same average Body Mass Index as in North America, the total world's biomass would rise by 58 million tonnes - the same as adding 935 million people with the world's average body mass.

Researcher Sarah C Walpole says the study findings drive home the fact that it is vital to look at biomass not just the number of people when calculating the ecological impact of humans.

The world's population is continuing to rise and the United Nations estimates that by 2050 the total of humans could rise to 8.9 billion.

As well as Professor Roberts and Sarah Walpole, the study, called Weight of nations: an estimation of adult human biomass, included research by David Prieto-Merino, John Cleland, Phil Edwards and Gretchen Stevens.

The London School of Hygiene & Tropical Medicine is one of the UK's highest-rated research institutions and has been recognised as one of the world's top universities for collaborative research. Its goal is to improve health and health equity. It has around 4,000 students and more than 1,300 staff working in over 100 countries.