It is bad for your blood pressure, knocks years off your life and is a strain on your heart. Now scientists have discovered that gaining weight lowers your intelligence.

The findings follow last week’s government figures that show Britain as the “fat man” of Europe, with nearly a quarter of adults and more than 14 per cent of children under 16 classified as obese.

The new five-year study of more than 2,200 adults claims to have found a link between obesity and the decline in a person’s cognitive function. The research, conducted by French scientists, which is published in this month’s Neurology journal, involved men and women aged between 32 and 62 taking four mental ability tests that were then repeated five years later.

The researchers found that people with a Body Mass Index – a measure of body fat – of 20 or less could recall 56 per cent of words in a vocabulary test, while those who were obese, with a BMI of 30 or higher, could remember only 44 per cent.

The fatter subjects also showed a higher rate of cognitive decline when they were retested five years later: their recall dropped to 37.5 per cent, whereas those with a healthy weight retained their level of recall.

According to British guidelines, a person with a BMI of between 18.5 and 25 is considered to be at an ideal weight, while 25 is overweight and 30 or more is regarded as clinically obese.

Dr Maxime Cournot, who headed the study, suggested that hormones secreted from fats could have a damaging effect on cerebral cells, resulting in decreased brain function. “Another explanation could be that since obesity is a widely known cardiovascular risk factor, due to the thickening and hardening of the blood vessels, that the same happens with the arteries in the brain,” said Dr Cournot, an assistant professor in clinical epidemiology at Toulouse University Hospital.

Dr David Haslam, the clinical director of the National Obesity Forum, said the research was alarming. “It goes to show obesity affects every single organ in the human body,” said Dr Haslam.

But Ann Widdecombe, the former Tory minister, said that the research seemed unsustainable. “You just need to look around the world and you will see hundreds of thin nitwits and clever fat people,” said Ms Widdecombe, who lost two stone when taking part in the television show Celebrity Fit Club.

“When I lost weight it was my waistline that improved, not my cerebellum.”

Nina Goswami/The Telegraph