Rise in carbon dioxide (CO2) in the atmosphere has been blamed as the reason behind weight gain in people.
Researcher Lars-Georg Hersoug, a post-doc at the Research Centre for Prevention and Health at Glostrup University Hospital, said that the increase in obese people in Denmark is roughly equivalent to the increase of carbon dioxide in the atmosphere.
Hersoug studied the weight of both fat and thin people over 22 years, and first started looking for explanations after noticing even the thin people were putting on the pounds.
"The normal theory is that fat people get fatter because they don't move as much as they should," the Daily Mail quoted Hersoug as telling Science Nordic.
"But the study showed that thin people also get fatter, and this happened over the whole of the 22-year period of the study," he explained.
When he looked around for other factors, he saw how the CO2 concentration of the atmosphere had also increased in correlation to the weight gain.
Hersoug now proposed that orexins, a type of hormone that reside in the brain and stimulate wakefulness and energy expenditure, may be affected by CO2.
The hormones regulate when we go to bed, as well as the stimulation of food intake.
Hersoug also suggested as evidence that obesity increases in the U.S. happened fastest in the period 1986-2010 on the East Coast, which is where CO2 concentrations are highest.
He also cites a 2010 study of 20,000 laboratory animals who all gained weight, despite being in controlled conditions.
Testing his hypothesis, a pilot study at university placed six men in special climate rooms, where some of them were exposed to increased amounts of CO2.
Seven hours later, the men were allowed to eat as much as they liked, and the men with more exposure to CO2 ate six per cent more food than the control group.