Younger age of type 2 diabetes onset is strongly linked to heightened odds of developing dementia, a new study finds. 

The researchers performed a longitudinal cohort study with a median follow-up of 32 years. Data spanning 35 to 75 years of age of diabetes onset showed that every five-year earlier onset of diabetes was significantly associated with higher hazard of dementia. At age 70, for example, the risk for dementia is 1.24 times greater for every five-year-earlier age of onset, according to Archana Singh-Manoux, Ph.D., of the Université de Paris and University College London, and colleagues.

Notably, however, late-onset diabetes was not significantly associated with subsequent dementia. And in individuals with diabetes, stroke comorbidity was tied to additional increased risk, the authors wrote.

There was also little evidence of a link between prediabetes and dementia. This may be because fasting glucose within the normal range does not adversely affect brain function, the authors surmised. In fact, fasting blood glucose in people without diabetes is only modestly associated with a risk of vascular disease, according to the literature.

“A certain threshold of high glucose levels may be necessary for hyperglycemia-induced brain injury,” Singh-Manoux wrote.

The mechanisms for the link between type 2 diabetes and dementia remain unclear, and past studies do not always show a consistent association between diabetes and Alzheimer’s disease hallmarks such as amyloid and tau pathology, they added. 

But “taken together, these findings highlight the importance of age at onset of diabetes and cardiovascular comorbidity in persons with diabetes for risk of dementia,” they concluded.

Full findings were published in JAMA.