The hippocampus and amygdala are involved in memory, among other things, and researcher Nicolas Cherbuin, PhD, says shrinkage in these areas could worsen memory.
"It has been generally assumed that blood glucose in the normal range is not a risk factor for brain health in non-diabetics," Cherbuin says. "If the present results are replicated in other studies the definition of normal fasting blood glucose levels and of diabetes may need to be re-evaluated."
Results Are 'Robust'
For the study, Cherbuin, a neuroscientist at Australian National University in Canberra, and his colleagues studied 249 people in their early 60s. Each of them had blood sugar levels in the normal range. At the beginning of the study, and again four years later, the researchers scanned their brains.
Comparing the before and after images, they found significant brain shrinkage among those whose blood sugar levels were high but still below the World Health Organization's threshold for pre-diabetes. The researchers report that those high levels may account for a 6% to 10% decrease in the volume of the hippocampus and amygdala.
Cherbuin and his team then excluded people who were overweight or obese, and substituted the American Diabetes Association's stricter normal range for that endorsed by the WHO. The results were virtually the same.
Cherbuin says they did not take their conclusion "lightly," but the association between these higher blood sugar levels and brain shrinkage was "robust."
Next, he plans to study the impact that such brain changes may have.
More Research Required
Cherbuin's results suggest a need to reassess what's considered a healthy blood sugar level, but more research must be done before any changes to recommendations are made, says neurologist Marc Gordon, MD.
"The research is too preliminary, and the association shown here does not establish a cause or mechanism," says Gordon, chief of neurology at Zucker Hillside Hospital in Glen Oaks, N.Y., who describes the study as thoughtful and carefully considered. "To speak as a clinician and tell patients that they better cut out all candy because it will shrink their brain is a leap of faith."
Cherbuin says that we still do not fully understand all the factors involved in regulating blood sugar levels. We do know enough to say that poor diet, lack of exercise, and constant stress likely play a leading role in maintaining unhealthily high levels, he says.
"It is this chronic exposure to high glucose levels that is more likely to lead to poorer brain health," he says.
For Gordon, who says that there is good evidence that such factors all likely play some role in thinking and memory decline, the message is a simple one.
"It's just what all of our mothers told us: Eat well and exercise," says Gordon. "That's a principle we would all do well to live by."