Diet rich in nuts, fish and vegetables cuts the chance of developing Alzheimer's significantly, says the experts in Archives of Neurology.
Experts believe it is a combination of nutrients in foods that is important.
But they stressed that diet was not the sole cause or solution where dementia is concerned.
Dr Yian Gu and colleagues at Columbia University Medical Centre in the US over the four years study shown that out of 2,148 retirement-age adults living in New york, 253 of these older adults developed Alzheimer's disease.
When the researchers scrutinised the diets of all of the individuals in the study, a pattern emerged.
Adults whose diets included more salad dressing, nuts, fish, poultry, fruits and green leafy vegetables, and less high-fat dairy, red meat and butter, were far less likely to develop dementia.
But it is the varying levels of specific nutrients that these food combinations offer that is important, say the researchers.
Diets rich in omega 3 and 6 fatty acids, vitamin E and folate but low in saturated fat and vitamin B12 appear to be best.
Experts have long suspected that nutrients might modify dementia risk.
Folate reduces circulating levels of the blood amino acid homocysteine which has been linked to Alzheimer's.
Similarly, vitamin E might be protective via its strong antioxidant effect, while monosaturated and saturated fatty acids could increase dementia risk by encouraging blood clot formation, say the researchers.
Rebecca Wood, chief executive of the Alzheimer's Research Trust, said: Understanding the connection between diet and dementia risk may help prevent the development of diseases like Alzheimer's for some people.
Adapting our lifestyles as we get older - by exercising regularly, watching what we eat and maintaining an active social life - can reduce dementia risk.
Unfortunately, no diet or lifestyle factor can eliminate dementia risk entirely.
With 35 million people worldwide living with dementia, she said it was important to focus efforts on research to develop new treatments.