Study leader Caroline Graff of the Karolinska Institute in Stockholm says the study subjects with the genotype were not necessarily obese during the study, but they may have been previously overweight.
"In a way, the finding that surprises us most is that when we correct for body mass index, we still see the effect. We thought the effect would work through a higher BMI," Graff told The Local.
"One explanation may be that those we examined were already way into the disease. That could explain why their BMI was already lower, but we don't know whether this effect is independent of BMI or not."
The study involved 1,003 individuals age 75 and older -- half who had the FTO genotype -- who did not have or had not yet been diagnosed with dementia at the beginning of the study.
The findings are published in the Journal of Alzheimer's Disease.