The study, by researchers at INSERM, the French National Institute for Health and Medical Research, in Bordeaux, France, also discovered that consuming omega-6 rich oils could increase chances of developing memory problems.
Omega-3 oils are thought to be beneficial because the outer membranes of nerve cells are largely made up of them.
Previous population studies have suggested that a diet rich in fatty fish such as tuna, mackerel and herring is beneficial in older people at risk of dementia, but not all studies have found the same beneficial effects.
For the new study, researchers examined the diets of 8,085 men and women over the age of 65 who did not have dementia at the beginning of the study. Over four years of follow-up, 183 of the participants developed Alzheimer's disease and 98 developed another type of dementia.
The study found people who regularly consumed omega-3 rich oils, such as canola oil, flaxseed oil and walnut oil, lowered their risk of dementia by 60 percent compared to people who did not regularly consume such oils.
They also showed that people who ate fruits and vegetables daily also reduced their risk of dementia by 30 percent compared to those who didn't regularly eat fruits and vegetables. They also discovered that regular fish consumption reduced the risk of Alzheimers' by 35 percent, and dementia by 40 percent, but only if they did not carry the gene that increases the risk of Alzheimer's, called apolipoprotein E4, or ApoE4.
"Given that most people do not carry the ApoE4 gene, these results could have considerable implications in terms of public health. However, more research is needed to identify the optimal quantity and combination of nutrients which could be protective before implementing nutritional recommendations," said study author Pascale Barberger-Gateau, PhD, of INSERM.
In addition, the study found people who did not carry the ApoE4 gene and consumed an unbalanced diet characterized by regular use of omega-6 rich oils, but not omega-3 rich oils or fish were twice as likely to develop dementia compared to those who didn't eat omega-6 rich oils, which include sunflower or grape seed oil.
"While we've identified dietary patterns associated with lowering a person's risk of dementia or Alzheimer's, more research is needed to better understand the mechanisms of these nutrients involved in these apparently protective foods," said Barberger-Gateau.
The study is published in the November 13, 2007, issue of Neurology, the medical journal of the American Academy of Neurology.