Scientists have found that Alzheimer's - a neurodegenerative disorder - may actually be a late stage of Type 2 diabetes.
The findings also suggest that losing weight and exercising may ward off Alzheimer's, at least in the very early stages, researchers said.
The extra insulin produced by those with Type 2 diabetes also gets into the brain, disrupting its chemistry, which can lead to the formation of toxic clumps of amyloid proteins that poison brain cells, researchers said.
"The discovery could explain why people who develop T2 diabetes often show sharp declines in cognitive function, with an estimated 70 per cent developing Alzheimer's - far more than in the rest of the population," said Ewan McNay at Albany University in New York.
"People who develop diabetes have to realise this is about more than controlling their weight or diet. It's also the first step on the road to cognitive decline," McNay said.
The increased risk of Alzheimer's disease in Type 2 diabetics has been known for a long time.
McNay's research aimed at discovering the mechanism by which T2 diabetes might cause Alzheimer's.
He fed rats on a high-fat diet to induce T2 diabetes and then carried out memory tests, showing that the animals' cognitive skills deteriorated rapidly as the disease progressed, 'The Sunday Times' reported.
An examination of their brains showed clumps of amyloid protein had formed, of the kind found in the brains of Alzheimer's patients.
McNay suggests that, in people with Type 2 diabetes, the body becomes resistant to insulin, a hormone that controls blood-sugar levels - so the body produces more of it.
However, some of that insulin also makes its way into the brain, where its levels are meant to be controlled by the same enzyme that breaks down amyloid.
"High levels of insulin swamp this enzyme so that it stops breaking down amyloid. The latter then accumulates until it forms toxic clumps that poison brain cells. It's the same amyloid build-up to blame in both diseases - T2 diabetics really do have low-level Alzheimer's," McNay said.
The research was presented at the Society for Neuroscience meeting in San Diego.