Having diabetes increases the risk of getting dementia. The widely referenced Rotterdam Study published in 1999, reported that having diabetes almost doubled the risk of dementia. Subsequent studies have backed up that finding.

Diabetes and dementia may have a cyclical link.

The brains of people with diabetes often show changes that are consistent with what is seen in dementia patients’ brains. Some researchers think that diabetes fuels the damage caused by dementia, and vice versa.

Duration of diabetes may be important and it may shrink the brain.

One study reported a strong and consistent association between diabetes in late life and subsequent onset of dementia, and an equivalent or even greater impact on dementia if diabetes is diagnosed earlier, in mid life. The researchers suggest that the duration of diabetes, i.e., how long someone has the condition, is an important risk determinant. Another study backs this up; they found that brain scans showed cognitive impairment happens over a long period of time, and the earlier someone develops type 2 diabetes, the more likely they are to have damage. In particular, diabetes appears to shrink the brain over time, reducing the size of important areas like the hippocampus, which plays an important role in memory. The brains of people who had developed diabetes in middle age were found to be, on average, 2.9 percent smaller than the brains of people without diabetes. Their hippocampi were affected even more dramatically, averaging four percent smaller in volume than non-diabetics.

Alzheimer’s disease may be “type 3” diabetes.

In many ways, Alzheimer’s disease is like a brain form of diabetes in that the brain experiences an impaired ability to take in glucose from the blood. It’s like the brain is in a diabetic state, perhaps due to decreased insulin levels or to insulin-insensitivity. This has led some researchers to refer to Alzheimer’s disease as “type 3 diabetes,” although at least one researcher is not completely behind using that term; the concern is that calling it “type 3 diabetes” may place too much of a focus on diabetes and neglect exploration of other important risk and protective factors.