Researchers in Sweden looked at more than 8,500 twins over several decades. Nearly 30 percent of the subjects were overweight or obese in their 40s and 50s.
By the age of 65, 350 or about 4 percent of the twins were diagnosed with dementia or Alzheimer's disease.
Despite the findings, researchers said further studies on weight and mental decline are needed.
The study was published in the May issue of Neurology.