Diabetes increases the risk of dementia and Alzheimer’s disease

To the long list of complications that should make you want to avoid diabetes, Japanese researchers have added this: People with diabetes are more than twice as likely to develop Alzheimer’s disease and also have an increased risk of developing some kind of dementia.

Diabetes, of course, is a metabolic disorder in which the body can’t use insulin properly, causing a dangerous buildup of blood sugar (glucose). Nearly 11% of American adults have it, according to the National Institutes of Health. The disease can lead to complications including kidney failure, nerve damage, heart disease, stroke, bladder control problems and erectile dysfunction.

Some studies have found a link between diabetes and risk of dementia, including Alzheimer’s, but other studies found no association between the two conditions. So a group of Japanese researchers conducted an unusually thorough analysis to try to settle the question.

The researchers tapped into a long-term study of heart disease and stroke that has been going on since 1961 in a suburb of Fukuoka called Hisayama. (Think of it as the Japanese equivalent of the Framingham Heart Study.) In 1988, more than 1,000 Hisayama adults agreed to take a glucose tolerance test to see how well their bodies could process the sugar. Then researchers followed them over the next 15 years, during which time 232 developed dementia.

Compared to people without diabetes who had normal glucose tolerance, those who had been diagnosed with diabetes had a 74% increased risk of being diagnosed with some type of dementia over the course of the study -- even after taking account of other risk factors, including age, body mass index and a variety of lifestyle factors. People who were not classified as diabetic but had impaired glucose tolerance (and therefore might be considered prediabetic) were 35% more likely to develop dementia, the researchers found.

Alzheimer’s was the most common type of dementia diagnosed in the study. The Hisayama adults with diabetes had slightly more than double the odds of developing Alzheimer’s compared to their counterparts who passed the glucose tolerance test. In addition, those who were not diabetic but had impaired glucose tolerance were 60% more likely to get Alzheimer’s. These figures also took account of age, BMI and other risk factors.

The researchers also found an increased risk for vascular dementia, which is caused by impaired blood flow to the brain. But the link disappeared when researchers adjusted for lifestyle factors like drinking, smoking and exercise.

It’s not exactly clear why diabetes would make someone more susceptible to dementia, but the researchers offered four physiological explanations, including artherosclerosis (hardening of the arteries) and microvascular disease, both of which would limit blood flow to the brain. Glucose intolerance could also increase oxidative stress and metabolism problems in the brain, they wrote.

“Our findings emphasize the need to consider diabetes as a potential risk factor for all-cause dementia,” they concluded.

The study was published in Tuesday’s edition of the journal Neurology.