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Diabetes can take a toll on the body, taxing the heart, circulation, the kidneys and even the eyes. Now it's becoming clear that the disease may affect the brain as well, contributing to a decline in mental functioning.

Studies have shown that diabetes may speed up aging-related deficits in mental function and lead to a twofold increase in the risk of dementia. Some researchers have speculated that diabetes could even boost the risk of developing Alzheimer's disease. Roger Dixon, a psychologist at the University of Alberta in Canada, wanted to learn whether this was true and set out to study exactly how uncontrolled blood sugar affected the brain.

Dixon and his colleagues studied 41 adults with diabetes and 424 healthy adults between the ages of 53 and 90, and reported their findings in the journal Neuropsychology. After testing the participants on memory, recall, verbal fluency, executive functions involving critical thinking and the speed of their mental faculties, researchers found the most significant deficits in diabetes patients on tasks of executive function and speed. These problems showed up in the youngest patients as well as the older ones, and once the cognitive symptoms appeared, they did not seem to worsen or change over time. Although Dixon's study failed to add new information on the question of diabetes and Alzheimer's disease, other experts view these results as useful fodder in the growing field of diabetes research.

"This study in general supports what we understand," says Dr. Alan Jacobson, chief of psychiatric services at the Joslin Diabetes Center in Boston. "It's another study adding to our recognition that Type 2 diabetes portends some type of problem in terms of cognitive function."

Earlier this week, another group of researchers, from Columbia University, reported in the journal Annals of Neurology that spikes in blood-glucose levels affect a region of the brain that forms memories and can lead to faster memory decline in people with diabetes.

Exactly how diabetes is associated with cognitive deficits isn't clear, but there is evidence suggesting that certain areas of the diabetic brain — such as the amygdala, which processes emotions, and the hippocampus, which is related to memory — are smaller than normal, a difference that may affect learning and recall of information. Early studies have even suggested that these physical differences may also predict Alzheimer's disease. While Dixon's study did not find a difference between the diabetes patients and controls on memory skills, Jacobson says the connection between the two diseases is an area of intense research.

As more people develop Type 2 diabetes in adulthood — diabetes has been diagnosed in 20% of American adults, and the vast majority have Type 2 — "more and more people are going to show significant cognitive problems," says Jacobson. "This whole area of research is going to be one of considerable importance in coming years." And studies like this one remind us that conditions like diabetes have wide-ranging effects throughout the body — and that we have only begun to pick apart some of these network connections.

• Alice Park
• More and more scientific evidence suggests that Type 2 diabetes may be linked to mental decline, memory loss and perhaps even Alzheimer's disease