The 10-year study into the lifestyles of thousands of middle-aged workers found obese people with underlying health problems such as high blood pressure scored nearly one-quarter worse in mental tests compared with the fit and healthy.
Nearly 6,500 Britons with an average age of 50 were given mental tests three times over the course of 10 years, the 'Daily Express' reported.
Some were classed as obese or overweight and nearly one-third (31 per cent) were found to be suffering from metabolic abnormalities such as high blood pressure, diabetes, or high cholesterol.
The results found that those who were both obese and metabolically abnormal experienced a 22.5 per cent decline in their mental test scores compared to those who were fit and healthy.
Medical experts had always suspected a link between poor diet and mental health but did not have the evidence to prove it.
They now hope the study will jolt millions into taking simple lifestyle action choices such as quitting smoking and taking regular exercise to prevent the onset of dementia.
The study, which used British civil servants working in Whitehall, shatters the myth that people can be so-called "metabolically healthy obese" and suffer no side effects such as cognitive problems.
Scientists at top French research institution Inserm spent thousands of hours painstakingly studying how well the civil servants, who were aged between 39 and 63, processed glucose and the levels of fat in their bloodstream before making the link between health and cognitive behaviour.
They concluded that obesity reduces blood flow to the brain, a factor which is linked to vascular dementia.
"We all know that piling on the pounds is bad for your physical health, but this robust study suggests that it is bad for the head as well as the heart," Jess Smith, the Society's research communications officer, said.
Earlier research in the US has shown that an increase of the fatty acid ceramide is linked to a higher risk of developing Alzheimer's.
