Previous studies have shown that a gene called FTO is vital in the development of obesity, a major worldwide health problem.
Now scientists have found a new piece of the puzzle. Rats with the so-called IRX3 gene don't get as fat when eating junk food, a study just published in the science journal Nature shows.
"Our data strongly suggest IRX3 controls body mass and regulates body composition," said study author Marcelo Nóbrega.
"Any association between FTO and obesity appears due to the influence of IRX3," he added.
In their study, the researchers found that rats with the IRX3 gene had 25 to 30 percent less fat because of a loss of white fat and increased metabolic activity. The same link was also found in humans and zebrafish.
The researchers including a member of Seville's CABD research centre believe the new discovery could help in the development of drugs to fight obesity and diabetes.
Global rates of obesity, or a body mass index of 30 or more, have nearly doubled since 1980, according to the World Health Organization. Some 1.4 billion adults were affected in 2008.
Obesity can lead to cardiovascular problems, diabetes, arthritis, and some cancers.
Spain is the thirteenth most obese country in the world according to the WHO while an OECD update in 2012 reported one in six Spanish adults were obese.