The individuals with the highest levels of blood glucose, insulin, and insulin resistance were more likely to have brain plaques, a distinguishing feature of Alzheimer’s, compared to those with healthier levels. Although only 15% of the subjects developed Alzheimer’s-type dementia, 65% had brain plaques. Another marker of the disease, brain fiber tangles, did not appear to be linked to glucose or insulin.
The relationship was strongest among the 20% of participants who carried the ApoE4 gene variant, a known risk factor for Alzheimer’s. Compared to those without the gene variant who had low glucose, the people who had both high glucose and the ApoE4 variant had a 38-fold higher risk of plaques, for instance.
It’s far from certain that insulin resistance actually causes Alzheimer’s, however. The study wasn’t able to rule out the possibility that an unidentified factor may contribute independently to both brain plaques and insulin resistance.
Still, a link between insulin resistance and Alzheimer’s is plausible. Insulin resistance causes insulin levels to rise, which may interfere with enzymes that slow down the production of the protein found in brain plaques, says Ian Murray, PhD, an assistant professor of neuroscience at the Texas A&M Health Science Center College of Medicine, in College Station.
“The brakes are turned off,” Murray says.
The study had some important shortcomings. It was relatively small, for one, and the researchers did not track whether the participants were being treated for diabetes. And the rates of diabetes, insulin resistance, and obesity in the study were low overall, which makes comparisons to the U.S. difficult.
“It is likely that the levels [of insulin resistance] will be much higher in a Western society, where obesity and diabetes are more prevalent,” Murray says.
