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Excess blood sugar could harm cognition

Study links persistently high readings to poor performance in older people who have diabetes

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11:24am, January 30, 2009
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Chronically elevated blood levels of the simple sugar glucose may contribute to poor cognitive function in elderly people with diabetes, a study in the February Diabetes Care suggests. But whether these levels add to a person’s risk of developing dementia is unclear, the study authors say.

People with diabetes face a risk of old-age dementia that’s roughly 50 percent greater than those without diabetes, past studies have shown. Research has also hinted that surges in blood sugar might account for some of that added risk. Many previous studies have tested for elevated blood glucose by obtaining a snapshot blood sample taken after a person has fasted for a day.

In the new study, Tali Cukierman-Yaffe, an endocrinologist at Tel-Aviv University and McMaster University in Hamilton, Canada, teamed with an international group of colleagues to assess blood glucose levels in nearly 3,000 diabetes patients by measuring A1c, shorthand for HbA1c or glycosylated hemoglobin. Since sugar in the blood sticks to the hemoglobin protein in red blood cells, the A1c test reveals an average sugar level over two or three months.

In addition to collecting these blood glucose readings, the scientists also asked each volunteer to take a 30-minute battery of four standardized tests designed to assess memory, visual motor speed, capacity for learning and managing multiple tasks.

On average, the participants were 63 years old at the time they entered the study, had blood drawn and took the cognition tests. Advancing age is known to hamper performance on such tests. After accounting for patients’ ages, the researchers found that people with higher A1c levels fared slightly worse on the tests than those who had lower A1c scores and therefore lower blood sugar.

After further adjusting for several factors that might affect cognitive performance — including heart disease, education level, alcohol use and depression — a high A1c score was still associated with poorer performance on one of the tests, which measured a wide array of cognitive functions.

Cukierman-Yaffe cautions that this study shows an association between high A1c and poorer scores on cognition tests, but doesn’t prove that reducing A1c levels will slow the rate of cognitive decline in a person with diabetes.

Even so, says neuropsychologist Adam Brickman of Columbia University, “there is now converging literature that implicates uncontrolled blood glucose levels with poor cognitive aging. While the mechanisms underlying that are still unclear, there have been enough … studies now to really raise our eyebrows.”

A key problem in assessing blood sugar’s role in cognitive decline is sorting out the multiple other factors that might also affect such decline, says psychologist Lawrence Fisher of the University of California, San Francisco. “Alcohol use, depression and other things are thought to influence cognitive functioning as well,” he says. “It’s really hard to partition out what the exact effect of each is.”

The good news is that A1c levels can be lowered with exercise, better diet and use of medication. “These are lifestyle factors that can be modified probably more easily earlier in life than after a diagnosis of dementia,” Brickman says.

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