A defect in a gene that may affect food selection and impulse control could be why many people have problems keeping their weight at a healthy level as they get older, a study suggests.
Some variant of the so-called "obesity gene," also known as the FTO gene, may be present in around 45 percent of adults, bringing a risk of obesity almost double of that faced by people without the gene, researchers at the Laboratory of Behavioral Neuroscience, part of the National Institute on Aging, report.
Brain scans of older people with the variants showed evidence of reductions in the function of regions involved in impulse control and awareness of food taste and texture, the researchers discovered.
"Sure enough, people who carry one or two copies of the FTO variant show increased intake of high-calorie or fatty food as they age," said senior study author Dr. Madhav Thambisetty, the lab's chief of translational and clinical neuroscience.
More than a third of U.S. adults 65 years old and over are obese, the study authors point out.
"There may be a common biological factor underlying both the risk for obesity during aging as well as obesity-related behavior like your ability to resist impulse eating," Thambisetty said.
In the published study, almost 700 volunteer participants -- average age 46 at the start of the study -- agreed to let researchers conduct annual PET scans of brain function and structure, as part of the Baltimore Longitudinal Study of Aging, a long-term investigation of human aging.
People confirmed as having the gene variant were found to have reduced functioning in the medial prefrontal cortex, the brain region linked to impulsivity and food perception.
The number of the gene variants found in any one person magnified the risk of increasing obesity, the researchers found.
"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.
That increased genetic risk suggests many people may be faced with an uphill fight against obesity with advancing age, the researchers admit.
"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.
Still, such genetic risks can be overcome and obesity is not inescapable, says Ruth Loos, director of the genetics of obesity for the Icahn School of Medicine at Mount Sinai in New York City.
"You may be genetically susceptible, but by living a healthy lifestyle you can overcome your genetics," Loos says. "You are not destined to be obese."