Here's a pop quiz for you. What is the adult population of North America?

A few years ago, the official number was around 260 million. But the more accurate figure may be some 30 per cent higher — closer to 340 million. To be brutal, it's because so many of them are so fat.

London researchers have put the world's population on the scales and worked out that so many people are overweight or obese, they are effectively making the planet much more crowded.

If every nation's population was as fat as America's, they reckon, it would be the same as having about an extra billion bodies on earth.

That's equivalent to the population of India. Bigger than Brazil. Three quarters of China.

For hundreds of years, we've been worried about how many people are on the Earth. In 1798, the British clergyman and economist Thomas Malthus warned that rapid population growth would eventually outstrip food supply, leading to widespread famine.

Graphic by Ella Rubeli/The Global Mail

By 1968, in the bestseller The Population Bomb, US biologist professor Paul R. Ehrlich was advocating compulsory birth control, if necessary.

More recently, one of the world's most prominent economists, US professor Jeffrey Sachs, gave a series of lectures called Bursting at the Seams. "Our planet is crowded to an unprecedented degree," he said. "It's bursting at the seams in human terms, in economic terms and in ecological terms."

But this new paper, Weight of Nations: An Estimation of Adult Human Biomass, just published in the London-based journal BMC Public Health suggests we should be more concerned about where the babies are born, not how many there are.

Because if they are born in the United States, they will end up, on average, nearly twice as big as someone from Bangladesh or Vietnam, greatly increasing their demands on the planet for food and fuel.

"Our main point from the paper is actually we've [not only] got to consider the number of mouths to feed on the planet, we've also got to consider the amount of flesh we're feeding," says Ian Roberts, Professor of Epidemiology and Public Health at the London School of Hygiene and Tropical Medicine.

"For years the discussion around population has been about, you know, people in Africa having too many babies. It's actually more nuanced than that because people who are fat have a disproportionately large ecological footprint," he says.

If we're fatter — like Americans and, let's face it, Australians, who ranked fifth-fattest — we need more energy just to move around. So an overweight person needs to eat more than a trim person to undertake the same level of physical activity.

In fact, overweight or obese people burn up more energy than skinny people even when they're just sitting around.

To prove the point, the researchers took United Nations and World Health Organisation data, dug out national health surveys and did some modelling to work out the total human biomass — an estimate of the height and weight of adult men and women in most nations on Earth.

It turns out that all 4.6 billion of us adults, together, tip the scales at 290 million tonnes.

Next, and this is the crucial bit, they wanted to work out how many extra people you could create — for no extra biomass — if so many of us weren't overweight or obese.

The answer is hundreds of millions of people.

Overweight people (for the scientifically minded, that's those with a body mass index, or BMI, greater than 25) carry around 15 million extra tonnes in excess fat, according to the report. The obese (BMI more than 30) are lugging around another 3.5 million tonnes. (Calculate your own BMI here.)

Now, if you somehow drained off all that excess fat from the overweight and obese and used it to create average-sized people, you'd have an extra 298 million humans.

That's about 6.2 per cent of the world's population. Roughly the population of the United States, as it happens. A phantom country. A whole New World.

Asians have the smallest body mass. Fewer than a quarter of them are overweight. The average Asian person — that's an average of male and female together — weighs 57.7 kilograms.

If every person in the world weighed what Japanese men and women weigh, the world's population would be 5 per cent lighter. That's the equivalent of 235 million fewer people in the world, the report says. More sushi for everyone!

However, the average North American, male and female together, weighs 80.7 kilograms. Nearly three-quarters of their population is overweight. About a third of all obese people on the planet live there, according to the report.

But if you readers in Australia, the Middle East or indeed Croatia are congratulating yourselves on being nowhere near as large as the Yanks, you could take a look at the graphic, showing the heaviest 10 countries and the lightest 10.

It puts Australia at number five in the heaviest countries in the world, if you take an average weight of men and women. Only Tongans, Samoans, Americans and Kuwaitis are fatter than Australians.

The report works those figures every which way. For example, it also calculates how many adults you could, theoretically, produce per tonne of biomass in each country. You can make 12.2 Americans with a tonne of flesh but many more — up to 20 — Vietnamese, Sri Lankans and most particularly Bangladeshis.

Illustration by Victoria Roberts

Here's a neat calculation: How many Bangladeshis does it take to make an American? Answer: nearly two.

As it stands, there are more than a billion overweight adults on the planet, and according to another study published in The Lancet last year, in most countries the fatness of the population is increasing.

Professor Roberts hopes this new report will "put fatness on the radar as an ecological concern", linking the public health efforts to combat rising obesity rates in the developed world to the broader debate about sustainability.

"Having a big body is like having a sort of gas-guzzling car," he says. "If your body is like a mini, you use less fuel than if your body is like a Range Rover. So being lean means we consume less food energy just to maintain a body mass. So having a lean body is good for your own personal health and it's good for planetary health."

But how do you put the developed world on a diet?

"It's no good blaming individuals for eating too much," says Australian public-health academic Garry Egger. "The problem is much more complicated than that."

Egger, professor of lifestyle medicine and applied health promotion at Southern Cross University in northern New South Wales, has been studying obesity for 25 years, has been a World Health Organisation and government advisor on obesity and chronic disease in Asia and the South Pacific, and was even moved to invent a popular Australian men's weight-loss program called GutBusters.

"Obesity," he writes in his bookPlanet Obesity, "is collateral damage in the battle for modernity; an unintended but unavoidable consequence of economic progress."

Obesity is not a disease, but a signal that a population has overshot what he calls the economic "sweet spot". He suggests that economic growth, in its early stages, drove improvements in our standard of living and in longevity, but it is now having an increasingly negative effect.

In happiness economics, it's called the Easterlin Paradox, in which national economic growth and increasing personal wealth does not necessarily lead to a corresponding increase in happiness or life satisfaction. That is, one motor car might make you very happy; 10 motor cars won't make you 10 times happier.

"It happened in the United States between 1970 and 1980," says Egger. "Rates of obesity suddenly increased. You could see it on the graphs, the line went zooming up."

Americans had overshot the sweet spot for health. At that point, infectious diseases such as tuberculosis, tetanus and measles had become much less common, and the big killers had become chronic ailments like heart disease, cancers and the classic fat disease, type 2 diabetes.

He says the same pattern has now started to emerge in Brazil, Russia, India and China.

This fatness, he says, "is a sign of economic excess. Economic growth has been fantastic for raising us out of poverty and away from disease, but there comes a point where it's too much of a good thing."

Egger says history proves his point.

"We got thinner in the Depression, fatter after, thinner during World War II, fatter afterwards. I'd be interested to see whether the continuing economic crisis in Europe has an effect on waistlines."