They looked at the diets of 8 085 people older than 65 who did not have dementia at the start of the study.
Over the following four years, 183 of the participants developed Alzheimer's disease, and 98 developed another form of dementia.
People who regularly consumed omega-3 rich oils, such as canola, flaxseed, and walnut oil, were 60 percent less likely to develop dementia than those who did not regularly consume such oils.
The study also found that regular consumption of fruits and vegetables lowered dementia risk by 30 percent.
What the study revealed
People who ate fish at least once a week were 40 percent less likely to develop dementia and 35 percent less likely to develop Alzheimer's, but only if they didn't carry a gene (ApoE4) known to increase the risk of Alzheimer's.
"Given that most people do not carry the ApoE4 gene, these results could have considerable implications in terms of public health," said study author Pascale Barberger-Gateau, of the French National Institute for Health and Medical Research.
"However, more research is needed to identify the optimal quantity and combination of nutrients which could be predictive before implementing nutritional recommendations."
The researchers also found that people without the ApoE4 gene who regularly consumed omega-6 rich oils - such as sunflower or grape seed oil - but not omega-3 rich oils or fish, were twice as likely to develop dementia as people who didn't eat omega-6 rich oils.
The study is published in the November 13 issue of the journal Neurology. – (HealthDay)