Swedish researchers found that eating meals rich in fat, sugar and cholesterol triggered changes in the brain associated with the early stages of the debilitating Alzheimer's disease.
Tests have shown such 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 a key in storing memories.
Weight is also a key, with those overweight at 60 being more than twice as likely to have dementia by 75.
The researchers 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.
The Stockholm study adds to the growing evidence that eating healthily can cut the odds of developing Alzheimer's.
The number of people with the disease is forecast to double within a generation, so any method of cutting the increase would have a huge impact on public health.
Researcher Susanne Akterin of the Karolinska Institute said: "On examining the brains of these mice, we found a chemical change not unlike that found in the Alzheimer brain."
"We suspect that a high intake of fat and cholesterol, in combination with genetic factors . . . can be a contributory factor in the development of Alzheimer's," she said.