A midlife diagnosis of diabetes increases the risk of developing Alzheimer's disease and vascular dementia, based on results of a twin study including more than 13,000 individuals.
Previous studies have shown that people with diabetes are at increased risk for dementia, but little is known about the mechanism of action, wrote Dr. Weili Xu of the Karolinska Institutet, Stockholm, and the Stockholm Gerontology Research Center.
Dr. Xu and colleagues conducted this twin study to verify the effect of diabetes on dementia, examine whether this effect varied based on the age of diabetes onset, and assess the possible role of genetics (Diabetes 2009;58:71-7). Data were taken from a national registry of Swedish twins who were at least 65 years old when they entered the study between 1998 and 2001.
Of 13,693 participants, 13,056 had no dementia, 467 had dementia, and 170 had questionable dementia, based on DSM-IV criteria. Midlife diabetes was defined as the onset of type 2 diabetes before age 65 years.
A total of 1,396 individuals had type 2 diabetes; 643 developed diabetes before age 65, and 753 developed diabetes at age 65 or older.
Overall, diabetes was significantly associated with a higher risk of dementia, and patients whose diabetes was diagnosed at midlife were more than twice as likely to develop dementia as those diagnosed later in life, even after controlling for diabetes duration and twin factors. In an analysis using a generalized estimating equation model (to measure all subjects at a common set of times) midlife and late-life diabetes diagnoses were significantly associated with increased dementia risks of 176% and 63%, respectively.
In addition, data from co-twin matched case-control analyses showed that the effect of midlife diabetes on dementia remained significant while the effect of later-life diabetes diagnosis on dementia did not.
These data suggest that adult lifestyle traits such as diet, exercise, smoking, and diabetes control may have a substantial impact on the link between midlife diabetes and dementia.
But “unmeasured familial factors” including genetic factors and environmental influences in early life might contribute to the association between late-life diabetes diagnosis and dementia, the researchers noted.
The study's limitations include the prevalence of dementia cases, the use of self-reports, and the lack of information about genes and environmental factors.
However, the findings “add to the growing evidence of a link between diabetes, vascular damage, and neurodegenerative changes in the brain,” they wrote, adding more studies are needed to identify environmental factors and genetic influences.
The study was supported in part by research grants from sources including the National Institute on Aging and the American Alzheimer's Association. The researchers disclosed no financial conflicts of interest.