High blood sugar levels are known to be detrimental to the brain, but even levels that are on the upper end of the normal range may be harmful, according to a new study.

In the study, people whose blood sugar levels were on the high end of the normal range —not high enough to be categorized as having diabetes or prediabetes — were more likely to have brain shrinkage in certain areas compared with people with lower blood sugar levels.

"Numerous studies have shown a link between Type 2 diabetes and brain shrinkage and dementia, but we haven't known much about whether people with blood sugar on the high end of normal experience these same effects," said study researcher Nicolas Cherbuin, of Australian National University in Canberra.

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

The study involved 249 people ages 60 to 64, whose blood sugar levels were in the normal range as defined by the World Health Organization. The participants had brain scans at the start of the study, and again an average of four years later.

The study was published Sept. 4 in the journal of the American Academy of Neurology.