They first confirmed that body mass index increased in those with one or two copies of the FTO gene variant. They then compared brain PET scans of patients with the FTO variant with scans of non-carriers, looking for differences in brain function over time.
They found people with the gene variant had reduced function in their medial prefrontal cortex, a region thought to be important in controlling impulses and response to the taste and texture of food.
In a final step, the team reviewed data gathered on participants' personality and diet. The group at increased genetic risk for obesity showed a greater tendency to impulsivity as well as a greater intake of fatty foods during aging.
The effect appears to increase with the number of copies. "We see a dose effect, where these changes in impulsivity or a preference for fatty foods increase with multiple copies of the gene," Thambisetty said.
The findings are published May 27 in the journal Molecular Psychiatry.
If these results pan out in additional studies, they mean that people who have a greater genetic risk of obesity face an uphill battle to maintain a healthy weight.
"This should not be an excuse, but it has to be a partial explanation why intelligent and motivated individuals struggle so much, because they are fighting their biology and it's uncomfortable to fight your own biology," said Dr. Steven Lamm, medical director of the Tisch Center for Men's Health at NYU Langone Medical Center in New York City.
However, a genetic predisposition to obesity does not mean one is doomed to obesity.
"You may be genetically susceptible, but by living a healthy lifestyle you can overcome your genetics," Loos said. "You are not destined to be obese."
Thambisetty agreed, noting that previous studies have shown that people can overcome the obesity risk posed by the FTO gene through regular exercise.