Their findings could also demonstrate the benefit of diet in reducing this risk.
Using human tissue, scientists at the University of California, Los Angeles identified a shortfall of insulin-degrading enzyme (IDE) in the brains of Alzheimer's disease patients.
This protein known to play a role in eliminating amyloid peptides that cause destructive plaques and tangles in the brains of Alzheimer's patients but until now, little has been known about the cellular and molecular regulation of IDE.
But the researchers found a cause-effect relationship between insulin signalling and increased production of IDE, they report in tomorrow's issue of the Journal of Neuroscience.
They also demonstrated in animal models that a low-fat diet high in fish and soy can increase production of this enzyme.
"The findings for the first time explain the association between insulin-resistant type 2 diabetes and Alzheimer's disease, and demonstrate the ability to manipulate levels of the protective IDE protein through diet," write the researchers.
Lead author Greg Cole told Reuters Health that an anti-diabetes drug Avandia, used to treat insulin resistance, is currently being tested in a clinical trial to see if it can help patients with Alzheimer's disease.
But lifestyle changes such as lowering saturated fats in the diet and increasing intake of omega-3 rich fish oil, can also modify insulin's effects and perhaps increase the clearance of amyloid beta, he said.