Scientists from the University of Dusseldorf, in Germany, are looking at how variations of the fat mass and obesity associated (FMO) gene affects how fat or thin a person is.
A study, published online in the journal Nature, shows the gene affects not only food intake and appetite, but also how quickly cells burn up energy.
It was previously believed that FTO variants alter the amount people eat but made no difference to the way cells expended energy.
Experiments on mice found that those missing the FTO gene had retarded growth after birth because they were burning up energy faster than animals with a functioning FTO gene.
Studies found that people with a high risk version of the gene weighed around three kilograms more than those with a low risk version.
Metabolism expert Professor Stephen O'Rahilly, from Cambridge University, said: 'This finding will promote research into the development of drugs that modulate FTO activity.
'We strongly suspect that, in man, FTO might have more complex effects on both food intake and energy expenditure than has been so far suggested and that it is still not clear what the overall effect of inhibiting FTO in humans would be.'
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