Scientists found that eating meals rich in fat, sugar and cholesterol triggered changes in the brain associated with the early stages of the debilitating disease.
The study adds to the growing evidence that eating healthily can cut the odds of developing Alzheimer's, a dementia affecting 400,000 Britons.
The number is forecast to double within a generation, so any method of cutting the number of cases would have a huge impact on public health.
Researchers in Sweden looked at the effect of a junk food diet on mice genetically altered to be prone to Alzheimer's.
The creatures' brains were tested after they were fed a diet laden in fat, sugar and cholesterol for nine months.
Researcher Susanne Akterin, of the Karolinska Institute in Stockholm, said, "On examining the brains of these mice, we found a chemical change not unlike that found in the Alzheimer brain."
Her tests showed the food altered the formation of a protein called tau which forms tangles inside the brain of Alzheimer's patients, causing brain cells to shrink and die.
The study also suggested that cholesterol cut levels of a brain protein called arc that is key in storing memories.
"We suspect that a high intake of fat and cholesterol, in combination with genetic factors, can adversely affect several brain substances, which can be a contributory factor in the development of Alzheimer's," said Akterin.
"All in all, the results give some indication of how Alzheimer's can be prevented, but more research in this field needs to be done before proper advice can be passed on to the public."
Neil Hunt, chief executive of the Alzheimer's Society, said, "Experts increasingly believe eating healthily is key to reducing your risk of dementia.
"It's important to eat less fatty food, salt and more fruit and vegetables which are high in antioxidants.
"A Mediterranean diet containing lots of fruits, vegetables, cereals, some fish, moderate amounts of alcohol, and little diary and meat has been shown to reduce your risk of dementia by as much as 30%.
"This is because it is low in fat, sugar and processed food, and high in protein, fibre and anti-oxidants."
Weight is also key, with those who are overweight at the age of 60 being more than twice as likely to have dementia by 75.
The warnings come as Britain fights rising levels of obesity: almost a quarter of men and women are so overweight that their health is at serious risk.
Research also shows that Britons are the world's biggest junk-food addicts, beating even the Americans in their appetite for fat and sugar-laden snacks.
The average Briton will get through 22,000 ready meals, sandwiches and sweet snacks in a lifetime -- nearly one a day.
However, he or she eats only three portions of fruit and vegetables a day, well below the recommended level of five a day.
From the December 5, 2008, Prepared Foods e-Flash