CHICAGO - A VARIANT of an obesity gene carried by more than a third of the US population also reduces brain volume, raising carriers' risk of Alzheimer's disease, US researchers said on Monday.

People with a specific variant of the fat mass and obesity gene, or FTO gene, have brain deficits that could make them more vulnerable to the mind-robbing disease.

'The basic result is that this very prevalent gene not only adds an inch to your waistline, but makes your brain look 16 years older,' said Paul Thompson, a professor of neurology at the University of California Los Angeles, who worked on the study published in Proceedings of the National Academy of Sciences.

Brains generally shrink with age. The study compared brain scans of more than 200 people and found consistently less tissue in the brains of people who carry the 'bad' version of the FTO gene compared to non-carriers.

On average, people with the obesity variant of the FTO gene had 8 per cent less tissue in their frontal lobes - sometimes referred to as the brain's 'command centre.' They also had 12 per cent less tissue in their occipital lobes, which is the part of the brain that processes vision and other perceptions.

Dr. Thompson said reduced brain volume raises a person's risk for Alzheimer's disease by reducing the amount of brain reserve a person has to compensate if the brain plaques linked to Alzheimer's form. Stroke can also reduce brain tissue, depleting the brain's reserve.