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According to the study, published Thursday in the New England Journal of Medicine, people with diabetes, or a heightened level of blood glucose, are more likely to develop Alzheimer's and other forms of dementia.
Dr. Paul Crane, lead author and associate professor of medicine at the University of Washington, said the research "may have influence on the way we think about blood sugar and the brain."
The researchers took blood sugar levels of 2,067 members of the nonprofit HMO Group Health. Each was studied over about seven years and at the beginning, while some had Type 2 diabetes and the rest did not, none had dementia.
Blood glucose tests, in addition to Group Health doctor's visits, are "a common test in routine clinical practice," Dr. Crane said. "We had an amazing opportunity with all this data. All the lab results since 1988 were available to us."
While getting the regular blood sugar tests, the participants aged 76 on average at the start of the study, visited a Group Health doctor every other year for cognitive tests. About one-fourth of the patients developed dementia, primarily Alzheimer's, over the course of the study.
Compared with the blood glucose testing, the researchers also considered factors known to contribute to dementia, like smoking and high blood pressure.
"We found a steadily increasing risk associated with ever-higher blood glucose levels, even in people who didn't have diabetes," Dr. Crane said. "There's no threshold, no place where the risk doesn't go up any further or down any further."
The risk of dementia of some sort was 18 percent higher for those with blood glucose levels of 115 milligrams per deciliter than those with 110 mg/dL. Patients with an average of 190mg/dL were 40 percent more likely than those with a level of 160 mg/dL.
The study's results were not conclusive enough to declare high blood sugar leads to dementia and also does not suggest an improved diet could lower one's risk of Alzheimer's.
"People shouldn't run for the hills or try crazy diets," Dr. Crane cautioned. "This doesn't show that changes in behavior that lower your individual blood sugar would decrease your individual risk of dementia."
Dr. Medha Munshi, a geriatrician and endocrinologist who directs the geriatric diabetes program at the Joslin Diabetes Center in Boston and who was not involved in the study, said she would not caution her patients whose blood glucose levels were 115 mg/dL of a higher risk of dementia.
This research "offers more evidence that the brain is a target organ for damage by high blood sugar," said Dr. Munshi. "And everyone is still working on the 'why.'"