Scientists found a close association between glucose levels and dementia risk in a group of 2,000 older patients aged 65 and over.
Even in apparently healthy participants, a blood sugar level increase from 100 to 115 milligrams per decilitre (mg/dl) raised the risk of dementia by 18%.
In those with diabetes, whose blood sugar tends to be higher than average, the risk was higher. A diabetes patient with a glucose reading of 190 mg/dl was 40% more at risk than one with a level of 160 mg/dl.
Lead researcher Paul Crane, from the University of Washington in Seattle, US, said: "The most interesting finding was that every incrementally higher glucose level was associated with a higher risk of dementia in people who did not have diabetes.
"There was no threshold value for lower glucose values where risk levelled off."
Blood sugar levels depend on individual metabolism as well as what you eat, the scientists pointed out.
"We have no data to suggest that people who make changes to lower their glucose improve their dementia risk," said Dr Crane.
The findings appear in the New England Journal Of Medicine.
More research is planned to uncover what underlies the link between blood sugar and dementia.