"When older folks develop dementia, it's usually from a host of factors [including Alzheimer's disease and hardening of the arteries]. Our study has shown that when older people with diabetes have dementia, it's not through Alzheimer's disease pathology," said senior study author Dr. Richard O'Brien.
However, O'Brien was quick to note that these findings don't mean that people with diabetes don't need to be concerned about their blood sugar levels. They do. "People with diabetes need to be good about blood sugar control," he said. "Atherosclerosis is a very important contributor to dementia in the elderly, and diabetes increases the risk of atherosclerosis."
O'Brien is a professor of neurology at the Johns Hopkins University School of Medicine, and chair of neurology at the Johns Hopkins Bayview Medical Center, in Baltimore. Results of the study were published online July 29 in JAMA Neurology.
O'Brien and his colleagues initiated the study because other research has suggested that problems with maintaining blood sugar levels, such as glucose intolerance and diabetes, increase the risk of Alzheimer's disease. O'Brien said some experts have even gone so far as to refer to Alzheimer's disease as "type 3 diabetes." (There's no such condition.)
Alzheimer's disease is the most common, but not the only, type of dementia. It occurs when deposits of amyloid beta (also known as plaques) build up in the brain along with twisted fibers of another protein (tangles), according to the Alzheimer's Foundation. Many people develop some plaques and tangles with age, but those with Alzheimer's disease have many more than the average person. Experts aren't sure exactly how this build-up of plaques and tangles may cause the symptoms of Alzheimer's disease.
There are several types of diabetes, but the most predominant is type 2 diabetes. Before developing type 2 diabetes, people generally show signs of insulin resistance and glucose intolerance. Insulin is a hormone that's necessary for the body to metabolize the carbohydrates in food and use the sugar (glucose) from those carbohydrates as fuel for the cells in body and brain.
If someone has insulin resistance, his or her body isn't using insulin efficiently, which allows glucose to build up in the blood. This is evident when someone does an oral glucose tolerance test. This test requires you to drink a sugar liquid, and then have your blood sugar levels tests at periodic intervals, such as at the start of the test, after an hour and then again after two hours. If you're insulin-resistant, the results of your oral glucose tolerance test will show higher levels of glucose than expected.
Such a finding indicates a higher risk of type 2 diabetes, according to the American Diabetes Association. Or, if the results are high enough, that may even indicate existing type 2 diabetes.
In the current study, the researchers followed 197 people who'd had at least two oral glucose tolerance tests during their lifetime. Thirty of these people had diabetes and were taking medications to control their diabetes. When these study volunteers died, autopsies of their brains were done. The average age at autopsy was 88.
A smaller group, consisting of 53 people who'd had two or more oral glucose tolerance tests, was also involved in the study. This group underwent positron emission tomography (PET) scans of their brains while they were alive. The average age at which they underwent imaging was 79.
The researchers found no significant correlation between glucose intolerance, insulin resistance or diabetes and the amount of amyloid beta in the brain, according to the study. That means that blood glucose levels likely don't contribute to the development of Alzheimer's disease specifically. Though, as O'Brien pointed out, blood glucose levels may still play a role in other types of dementia.
One expert said that the study scope may not be wide enough to allow definitive conclusions.
"There has been a lot of debate about diabetes and dementia. Is there a cause-and-effect relationship, or is it just an association? This study suggests that Alzheimer's isn't really related to insulin resistance and impaired glucose tolerance, but the number of patients with true diabetes was very small," said Dr. Joel Zonszein, director of the clinical diabetes center at Montefiore Medical Center, in New York City.
Zonszein said the study also doesn't look at the length of time someone has had diabetes, and whether or not it's well-controlled.
"We see a lot of people who have [small strokes] who have hypertension and type 2 diabetes," he said, noting that this can cause a lot of the symptoms of dementia, such as loss of memory. "Clinically, it's very difficult to tell the difference between dementia and Alzheimer's disease. And, although it may not be true [for] Alzheimer's disease, this won't mean any difference in current clinical practice."
"Early and aggressive treatment to control the blood sugar is the best treatment right now," he added. "Hopefully, with more studies, we can find out what medications are better for people with diabetes to help prevent dementia."
Learn more about Alzheimer's disease from the Alzheimer's Foundation.