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NEW YORK (Reuters Health) - Elderly people with high blood sugar - but not high enough to be diabetic - face a slightly greater risk of developing dementia, according to a new study of over 2,000 volunteers.
The research, published in the New England Journal of Medicine, does not prove that high glucose levels directly cause dementia.
But having an average glucose reading of 105 to 120 milligrams per deciliter (mg/dL) was tied to an increase in dementia risk by 10 percent to 20 percent in non-diabetics. If the reading was below 100, the risk was lower.
A fasting blood glucose of 126 mg/dL and above is used to diagnose diabetes.
Diabetics also faced a higher risk if their average blood sugar levels remained high.
"These data suggest that higher levels of glucose may have deleterious effects on the aging brain," writes the research team, led by Dr. Paul Crane of the University of Washington and the Group Health Research Institute in Seattle.
But Crane cautioned in a telephone interview with Reuters Health that "the magnitude of risk for any individual is low. I don't want anyone to read this and get alarmed. It just shows that what we had thought was normal for blood sugar is appropriate for the heart and the kidney, but the brain seems to have a different idea. There is no threshold where you're safe from dementia."
"The extra risk is modest," Dr. Richard O'Brien of Johns Hopkins University in Baltimore, who also studies diabetes and Alzheimer's but was unconnected with the new research, told Reuters Health.
"If your mom has Alzheimer's, you increase the risk two- to three-fold. That risk is gigantic compared to this."
O'Brien is part of the Baltimore Longitudinal Study of Aging, which reported last month in another journal, JAMA Neurology, that glucose intolerance, diabetes or insulin resistance were not tied to key features of Alzheimer's. That test followed fewer people and didn't look at other forms of dementia.
Elderly volunteers in the new study - 839 men and 1,228 women - were 76 years old at the outset, on average. After nearly seven years of follow-up, 32 percent of the people who were diabetic at the start of the study and 25 percent of the non-diabetics had developed some form of dementia.
Suspected Alzheimer's cases accounted for 77 percent of the instances of dementia.
Non-diabetics who had an average glucose reading of 115 mg/dL over the previous five years were 18 percent more likely to develop dementia than volunteers whose average reading was 100 mg/dL, the researchers found.
Among people with diabetes, there was a graduated risk as well. Compared to those whose blood sugar was typically 160 mg/dL, people with a much higher average of 190 mg/dL were 40 percent more likely to develop dementia.
The researchers said they took gender, education level, exercise, blood pressure, smoking and other dementia risk factors into consideration in their findings.
Doctors have suspected for years that diabetes, known to affect blood vessels, is linked to an increase in dementia risk.
"Dementia is not just one thing, especially in older folks," said O'Brien. "It's usually a combination of different disease processes - usually Alzheimer's, but vascular disease is probably number two."
"Because this was an observational study, we don't know if any changes a person would make to lower their blood sugar would be associated with a decreased risk for dementia," said Crane.
The result "changes how we think, not what anyone does or how much fear somebody should have."
SOURCE: bit.ly/147HPuP New England Journal of Medicine, online August 7, 2013.