It has already been nicknamed the "Homer Simpson effect" - being overweight could affect your intelligence, a study suggests.

A five-year study of more than 2,000 middle-aged people in France has found a possible link between weight and brain function.

Research published in Neurology, the journal of the American Academy of Neurology, found that people with a higher Body Mass Index (BMI) scored lower on average in cognitive tests within a sample.

The findings came in the week that Britain was dubbed the "fat man of Europe" following new figures.

The research led by Dr Maxime Cournot, of Toulouse University Hospital in France, used 2,223 healthy people aged 32 to 62 who sat four cognitive tests including word learning in 1996 and again in 2001.

Results from a word memory test showed that people with a BMI of 20 - considered to be a healthy level - remembered an average of nine out of 16 words.

Meanwhile, people with a BMI of 30 - inside the obese range - remembered an average of just seven out of 16 words.

While those whose BMI changed over the five years did not appear to see any change in their cognitive function, those who started out with a higher BMI did appear to show higher levels of "cognitive decline", Dr Cournot said.

"The study's findings may be due to a host of factors including the thickening and hardening of cerebral vessels because of obesity or possibly the development of insulin resistance," she suggested.

The apparent phenomenon has already been dubbed the "Homer Simpson effect" by some North American media.

BMI is calculated by setting a person's weight against their height to come up with a single figure - the higher the score the more overweight someone is.

According to World Health Organisation ranges, a BMI below 18.4 means that a person is considered underweight for their height, 18.5 to 24.9 is considered ideal, 25 to 29.9 is classed as overweight and a score above 30 signals obesity.

Meanwhile, a separate paper published in the same journal suggested a link between physical and mental fitness.

A group of 460 Scottish people who took part in a mental health survey in 1932 when they were 11 years old were re-tested at 79.

Report author Professor Ian Deary, of Edinburgh University, said: "Fitness contributes to better cognitive ability in old age.

"Thus, two people starting out with the same IQ at age 11, the fitter person at age 79 will, on average, have better cognitive function."