"In the last 10 years or so, people started suggesting you could be fit and fat-you could be obese and metabolically healthy and have no health risk," Dr. Archana Singh-Manoux, lead author of the study and research director at Inserm, the French National Institute of Health and Medical Research, told the Wall Street Journal.
Obese study participants lacking signs of metabolic troubles ahead still showed signs of cognitive decline at a faster weight than their healthy-weighted counterparts.
"All of these [obese] individuals, whether they were metabolically healthy or not healthy, had a poor cognitive profile," Singh-Manoux said.
The study spanned a decade and included 6,401 subjects, 53 percent of whom were normal weight, 38 percent were overweight and 9 percent were obese.
Participants underwent cognitive tests three times in the ten-year span, measuring reasoning, short-term memory and verbal fluency.
Of the 582 obese subjects, 60 percent had two metabolic risk factors or more, making them "metabolically abnormal." Of that group, 22.5 percent say a cognitive decline occur faster than for normal-weight participants.
"More research is needed to look at the effects of genetic factors and also to take into account how long people have been obese and how long they have had these metabolic risk factors and also to look at cognitive test scores spanning adulthood to give us a better understanding of the link between obesity and cognitive function, such as thinking, reasoning and memory," the researchers wrote.
Scientists have yet to link obesity to a drop in grey matter prowess, but heart disease and inflammation are believed the likely culprits. Fat tissue could lead to hormonal imbalances which affect the brain, visiting professor at SUNY-Downstate Medical Center Deborah Gustafson told WSJ.