Obesity contributes to global warming, too.

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 the food shortage and higher energy prices, the school's researchers wrote in the journal Lancet on Friday. 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. 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 percent more than someone with a stable BMI.

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

This is also very important because 20 percent of greenhouse gas emission stem from agriculture. The next step is quantifying how much a heavier population is contributing to climate change, higher fuel prices and food shortage. "Promotion of a normal distribution of BMI would reduce the global demand for, and thus the price of, food," Edwards and Roberts wrote.