Being overweight is not just bad for waistlines but for brains too, say researchers who have linked obesity to declining mental performance.
Experts are not sure why this might be, but say metabolic changes such as high blood sugar and raised cholesterol are likely to be involved.
Obesity has already been tipped as a risk factor for dementia.
The work, published in Neurology
, tracked the health of more than 6,000 British people over a decade.
The participants, who were aged between 35 and 55, took tests on memory and other cognitive skills three times over a 10-year period.
People who were both obese and who had unhealthy metabolic changes showed a much faster decline on their cognitive test scores compared to others in the study.
The experts stress that they only looked at cognitive function, not dementia.
The boundary between normal aging, mild cognitive impairment and dementia is blurred - not all impairment leads to dementia.
All of the study participants came from one group of civil service workers, which may mean the findings may not apply more generally to other populations.