NEW research linking a diet high in sugar and processed foods to brain shrinkage has prompted a rethink on safe blood sugar levels.

People with blood sugar levels of 4ml-6.1ml were found to have a greater risk of the brain atrophy that occurs with ageing and dementia, researchers at the Australian National University's College of Medicine found.

These blood sugar levels are not high enough to meet the clinical definition for type 2 diabetes and are at the upper end of the normal range, which has surprised researchers.

The research may force doctors to redefine what blood glucose levels are dangerous.

The study found that the loss of brain volume affected mostly memory and cognitive skills.

Dr Nicolas Cherbuin, who conducted the study, said the research suggested long-term exposure to foods like sugary drinks and white flour were responsible for the problem.

Previous medical research found higher levels of brain shrinkage among the obese and patients with type 2 diabetes.

"These findings suggest that even for people who do not have diabetes, higher blood sugar levels could have an impact on brain health,'' Dr Cherbuin said.

"More research is needed, but these findings may lead us to re-evaluate the concept of normal blood sugar levels and the definition of diabetes,'' he said.

After controlling for age, high blood pressure, smoking, alcohol use and other factors the study found blood sugar accounted for six to ten per cent of the brain shrinkage.

The study involved 249 Canberra residents aged 60-64 who had an MRI scan of the brain and a blood glucose test and were then retested four years later.

The shrinkage of the brain was measured by comparing brain size in the two MRIS.

Dr Cherbuin says he is now undertaking new research to see whether people aged in their 40's and 50's experience the same brain shrinkage.

The best way to avoid brain shrinkage was to eat low- glycaemic foods and avoid sugary soft drinks and highly processed foods that used sugar, white flour and fat, he said. Physical activity also helped lower blood sugar levels.