High blood-sugar levels, even levels less than those found in people with diabetes, may increase the risk of developing dementia, according to a new study.

Higher levels of blood sugar were associated with almost a 20% increase in the risk of getting Alzheimer's, the researchers said.

They monitored blood sugar levels in people with and without diabetes to see what effect these levels might have on the risk of developing Alzheimer's Disease.

The study included 2,500 dementia-free people over 65 in Washington State in the US, with just over 2,000 monitored at follow up visits.

During an average follow-up period of 6.8 years, 524 people developed dementia - 74 with diabetes and 450 without the condition.

It was found that patients without diabetes and who subsequently dementia had a considerably higher blood glucose levels in the five years before diagnosis of dementia.

The study, carried out at the University of Seattle, is published in the New England Journal of Medicine.