WEDNESDAY, Aug. 25 (HealthDay News) -- Two of the most common and dreaded illnesses in America may share a connection, with new research suggesting that having insulin resistance or type 2 diabetes raises your risk of developing the brain plaques associated with Alzheimer's disease.

After adjusting for other risk factors, the Japanese study found that people with the highest levels of fasting insulin had nearly six times the odds of having plaque deposits between nerves in the brain, compared to people with the lowest levels of fasting insulin.

Those with the highest scores on a measure of insulin resistance (where cells become less able to use insulin effectively) had about five times the odds of having brain plaques vs. those with the lowest scores on the insulin-resistance test, the study found.

In fact, "the risk of plaque-type Alzheimer's disease pathology increases in a linear relationship with diabetes-related factors," according to one study author, Dr. Kensuke Sasaki, an assistant professor in the department of neuropathology at Kyushu University in Fukuoka, Japan.

Results of the study appear in the Aug. 25 online issue of Neurology.

Both type 2 diabetes and Alzheimer's disease have been rapidly increasing in incidence, so much so that experts worry the illnesses may overwhelm the health-care system in the coming years if nothing is done.

While numerous studies have found a link between cognitive decline and dementia in people with type 2 diabetes, the current study sought to determine the reason for that link.

Using autopsies from 135 Japanese adults, the researchers were able to compare if different indicators of insulin resistance or type 2 diabetes correlated with the development of plaque deposits between the nerves in the brain (neuritic plaques) or neurofibrillary tangles, which are found in dying cells in the brain. Plaques and tangles are thought by many to be the two main causes of the destruction of brain tissue seen in Alzheimer's disease.

All of those autopsied died between 1998 and 2003. In 1988, they had undergone numerous tests as part of an ongoing study on brain and heart health. The tests included an oral 2-hour glucose tolerance test, fasting blood sugar and insulin levels, and a measurement of insulin resistance using a test called homeostasis assessment of insulin resistance (HOMA-IR).

The researchers adjusted the data to control for age, sex, blood pressure, cholesterol, body-mass index, smoking, exercise and cerebrovascular disease.

They found no association between diabetes risk factors and the development of tangles. However, higher levels of blood sugar two hours after eating, high fasting insulin levels and an elevated HOMA-IR score were associated with an increased risk of developing plaques. Fasting blood sugar levels were not associated with an increased risk of plaques, according to the study.

When the researchers compared varying levels of diabetes risk factors, such as fasting insulin, they found a linear association with the development of plaques. For example, fasting insulin was broken into three groups: low, medium and high. The low group didn't have an increased risk of plaques, while the medium group had more than twice the risk of brain plaques, and those in the high group had a six times higher risk of plaques than those in the low group.

The researchers also performed a separate analysis to see if the presence of a gene long implicated in Alzheimer's disease (ApoE4) would have an effect on the association between diabetes risk factors and the development of plaques. It did: Those with the ApoE4 gene has the strongest association between high blood sugar levels, insulin resistance and fasting insulin levels and the development of plaques.

"Research has been linking diabetes to dementia, and probably to Alzheimer's, and this study is one more bit of evidence to say that we'd better get a handle on this," said Dr. Richard Bergenstal, president of medicine and science for the American Diabetes Association.

Bergenstal said this study's findings are likely applicable to people with both type 2 and type 1 diabetes, and possibly to those with pre-diabetes, as well.

"This study fits into a body of literature looking at the relationship between diabetes and Alzheimer's disease. This area is being pretty aggressively researched for a number of reasons. Would better control of type 2 diabetes improve the cognitive fate of those with the disease, and is there some way we can intervene in glucose metabolism that might affect Alzheimer's?" said William Thies, chief medical and scientific officer for the Alzheimer's Association.

"If you have diabetes, it's certainly a good idea to keep it under control while we're sorting out the research," Bergenstal said.

"Although we don't know anything that can prevent Alzheimer's disease right now, I do think there are a lot of good reasons for people to try to prevent type 2 diabetes, much of which can potentially be avoided with regular physical activity and weight maintenance," said Thies. "Preventing or controlling diabetes is good for all kinds of reasons, and also because it might contribute to your risk of Alzheimer's disease."

More information

Learn more about what steps you can take to help maintain your brain health from the Alzheimer's Association.