The Link between Diabetes and Alzheimer’s

Both type 2 diabetes and Alzheimer’s disease are increasing at epidemic rates throughout the world.  A link between the two – i.e. diabetes may predispose to an increased risk of Alzheimer’s – has been suggested for a number of years, without there being knowledge of the mechanism.  Now a Japanese neuropathology study has thrown some light on this.  It’s appeared online in Neurology.

The pathological hallmark of Alzheimer’s disease is the development of amyloid plaques and neuron tangles in the brain tissue.  135 autopsy brain samples were obtained from participants in a long-term prospective study of inhabitants of a Japanese town and examined for these features.  The subjects had had a glucose tolerance test, fasting insulin levels, and insulin resistance assessment in 1988, along with a full clinical exam.  Their average age was 67 at baseline in 1988 and 79.5 years at death.  21 (15.5%) developed Alzheimer’s disease during the 10- to 15-year study period.

After adjustments were made for differences in possibly disruption factors (age, gender, systolic blood pressure, cholesterol, BMI, smoking, exercise level, and cerebrovascular disease) it was found that higher levels of 2-hour postprandial glucose, fasting insulin, and insulin resistance were associated with increased numbers of amyloid plaques, but not neuron tangles, in the brains.  Thus 72% of the participants with increased insulin resistance at baseline were found to have amyloid plaques at autopsy, whereas 62% of those without increased insulin resistance had plaques, as did 65% of all those who died. 

This is not a very dramatic difference.  Commenting on the study findings, an editorialist wonders if they could reflect ‘reverse causality’, rather than a cause (insulin resistance) and effect (plaques).  Thus, a disturbance of amyloid metabolism might precede dementia diagnosis by decades, and insulin resistance and type 2 diabetes could be a consequence or correlate of Alzheimer’s rather than a cause.  Obviously there’s still a lot of work to be done on this relationship.  What’s important is that Alzheimer’s changes can begin many years before symptoms appear, so the time to reduce the risk of dementia is now, according to the Alzheimer Society.  Eat a balanced diet, keep a healthy weight, and your blood pressure and cholesterol under control; and exercise your body and your mind!