Obese and overweight people require more fuel to transport them and the food they eat, and the problem will worsen as the population literally swells in size, a team at the London School of Hygiene & Tropical Medicine says.

This adds to food shortages and higher energy prices, the school's researchers Phil Edwards and Ian Roberts wrote in the journal Lancet on Friday.

"We are all becoming heavier and it is a global responsibility," Edwards said in a telephone interview. "Obesity is a key part of the big picture."

At least 400 million adults worldwide are obese. The World Health Organization (WHO) projects by 2015, 2.3 billion adults will be overweight and more than 700 million will be obese.

In their model, the researchers pegged 40 percent of the global population as obese with a body mass index of near 30. Many nations are fast approaching or have surpassed this level, Edwards said.

BMI is a calculation of height to weight, and the normal range is usually considered to be 18 to 25, with more than 25 considered overweight and above 30 obese.

The researchers found that obese people require 1,680 daily calories to sustain normal energy and another 1,280 calories to maintain daily activities, 18 per cent more than someone with a stable BMI.

Because thinner people eat less and are more likely to walk than rely on cars, a slimmer population would lower demand for fuel for transportation and for agriculture, Edwards said.

This is also important because 20 per cent of greenhouse gas emissions stem from agriculture, he added.

The next step is quantifying how much a heavier population is contributing to climate change, higher fuel prices and food shortages, he added.