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The effect was very subtle, however, suggesting that higher blood sugar levels may be more of a nudge toward memory loss than a shove.
"If I had diabetes and I read this study, my reaction would be relief," said Dr. Richard O'Brien, chair of neurology at Johns Hopkins Bayview Medical Center in Baltimore, who was not involved in the research. "The effect was small."
The risk increases tied to rising blood sugar (or blood glucose) levels ranged from 10 percent to 40 percent. O'Brien pointed out that other risks appear to have much greater impacts. Having a parent with dementia, for example, roughly doubles or triples a person's risk for developing the disease.
O'Brien recently conducted a different study that looked at a similar, but slightly different question: whether or not blood glucose levels were linked to brain changes of Alzheimer's disease. That study, published online July 29 in JAMA Neurology, concluded there was no connection.
But O'Brien's study had fewer participants than the current investigation, which means it may not have been large enough to detect the slight differences between people who did and did not have signs of Alzheimer's. And because his study was solely focused on Alzheimer's disease, it couldn't rule out the possibility that higher blood sugar levels might be contributing to other kinds of dementia, particularly when it's caused by damage to the small blood vessels of the brain.
"The studies are completely compatible with each other," he said.
The U.S. obesity epidemic has led to soaring rates of type 2 diabetes, which is characterized by higher than normal blood sugar. As the baby boom generation ages, Alzheimer's disease is also on the rise, and experts are trying to determine whether a connection exists between the two.
For the new study, published Aug. 8 in the New England Journal of Medicine, researchers followed more than 2,000 adults enrolled in the Group Health Cooperative, a nonprofit managed care collective in Washington State.
All study participants were aged 65 and older and free of dementia at the start of the study. Everyone had had at least five blood sugar checks in the two years prior to study enrollment.
At the start of the study, 232 people had diabetes, while 1,835 did not.
Through detailed health records kept on each participant, the researchers were able to estimate each person's average glucose levels.
Over the next seven years, on average, one-quarter of the participants developed dementia, including 450 who did not have diabetes and 74 with diabetes. About 20 percent of them had probable Alzheimer's disease, the most common form of dementia, while roughly 3 percent had dementia from vascular disease and slightly more than 3 percent were deemed to have dementia from other causes.
When researchers compared participants' average blood glucose levels to their risk of dementia, they found that for people without diabetes, as glucose levels rose above 100 milligrams per deciliter (mg/dL), dementia risk increased, too.
People with average daily blood sugars of 105 to 115 mg/dL in the previous five years saw a 10 percent to 18 percent increase in the risk of developing dementia.
For people with diabetes, the risk began to rise with average blood sugars above 160 mg/dL. People with diabetes had a 40 percent greater risk of developing dementia if their average blood sugars were above 190 mg/dL for the same time frame.
Study author Dr. Paul Crane, an associate professor of medicine at the University of Washington in Seattle, agreed the risk isn't monumental. "It's not explaining boatloads of dementia risk," he said.
And because the study only looked at the relationship between blood glucose and dementia, it can't definitively say that higher blood sugar levels lead to memory loss, or that lowering blood sugar can reduce a person's risk.
"People who had lower blood sugar had lower risk than people who had higher blood sugar," Crane said. "That's not the same thing as saying that lowering your own blood sugar through any means has any influence on your personal risk of dementia," he added.
Other studies will need to test that theory more directly. Until more is known, Crane said exercise appears to be a good way to lower your personal risk of dementia.
"There's a lot of observational data to suggest that exercise is good for your brain, and exercise is one means to lower your blood sugar," he said. "I tell my patients to exercise."
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