Obesity-inducing junk food could also give you dementia through high blood pressure and cholesterol, which interrupt blood supply to the brain, says a study.
Some animal studies have specifically implicated insulin, a hormone, suggesting that Alzheimer's could be 'diabetes of the brain'. But the latest theory also points to high levels of fatty and sugary food damaging the brain by interrupting its supply of insulin.
In type 2 diabetes, junk food prompts our cells in becoming resistant to the insulin they need to convert sugar into energy.
Insulin is required to regulate brain chemicals, key to memory and learning, to make and strengthen connections between brain cells and to maintain the blood vessels that supply the brain with blood and oxygen, the journal New Scientist reports.
The study suggests that something similar may be happening in Alzheimer's, with a bad diet preventing brain cells from responding properly to insulin.
Rats developed Alzheimer's after being fed a compound that prevented their brains from using insulin, according to the Daily Mail.
Suzanne de la Monte, study co-author from Brown University, US, said: "They were demented. They couldn't learn or remember." When researchers fed healthy men and women fatty and sugary foods for a month, levels of insulin and beta amyloid rose.
With rates of diabetes soaring, dementia could be reaching 'epidemic' proportions.
The Alzheimer's Society's director of research, professor Clive Ballard said: "One in three people over 65 will develop dementia. Research like this points us in new directions for treatment development."