Men who are obese may have less brainpower than their trim counterparts, according to US research.
Half of all Americans are obese
Those with a "body mass index" of 30 or more - equivalent to a 5ft 8in person weighing over 14 stone performed significantly worse in tests of mental ability.
However, it is unclear whether the obesity is causing the lower IQ, or vice versa.
The scientists who did the research, from the University of Boston, suggested that the unhealthy diet which led to the weight gain might be also damaging blood vessels supplying the brain.
However, previous studies have suggested that people who already have a lower IQ are less likely to eat healthily, which could lead to them piling on the pounds.
The finding was uncovered by examining the health records from more than 1,400 men.
No similar link could be detected for overweight or obese women.
It was suggested that the different places that fat is distributed in men and women may offer a clue to this.
Fears for children
Professor Merrill Elias, the lead researcher, said: "The really frightening thing is the implication for obese children."
Obesity in on the increase in many Western countries - in the UK, one in five people is now said to be obese, and experts warned on Monday that this could rise to almost 50% in two decades unless unhealthy lifestyles are checked.
Obesity is already known to significantly increase the risk of strokes, cancer and heart disease.
However, the effects of obesity on the brain are less clear.
The Boston study found that men with a BMI of 30 or more scored on average 23% lower marks in tests of mental acuity.
Dr Richard Harvey, from the Alzheimer's Research Society, said that it was vital to maintain a good blood supply to the brain.
He said severe damage to blood supply, perhaps caused by a stroke, could lead to a progressive and fatal condition called "vascular dementia", in which brain cells die, leading to a rapid loss of cognitive ability.
However, he told BBC News Online: "No link has been proven between obesity and dementia.
"In this case, it is equally possible that people with a lower IQ are more likely to be obese because they are eating less healthily."