Scientists at the Kaiser Permanente Division of Research in Oakland, California, found that people who were overweight and had a large belly between the ages of 40 and 45 were 2.3 times more likely to develop dementia than people with a normal weight and belly size.
People who were obese and had a large belly were 3.6 times more likely to develop dementia, but the researchers noted that having a large abdomen increased the risk of dementia even if a person was not overweight and regardless of whether or not they suffered from diabetes or cardiovascular disease.
The findings are based on a study of more than 6,500 people and are published in the journal Neurology.
Study author Dr Rachel Whitmer commented: "Considering that 50 per cent of adults in this country have abdominal obesity, this is a disturbing finding.
"It is well known that being overweight in midlife and beyond increases risk factors for disease. However, where one carries the weight - especially in midlife - appears to be an important predictor for dementia risk," she concluded.