The monounsaturated fats in olive oil are an important part of the diet
The Mediterranean diet, already thought to protect against heart disease and cancer, may also help to prevent depression, Spanish researchers say.
They found depression was more than 30% less likely to develop in people who followed a diet high in vegetables, fruit and cereals, and low in red meat.
They studied 10,094 healthy adults over four years, the Journal of the American Medical Association reports.
However, the team stressed additional, larger-scale studies were required.
Researchers at the Universities of Las Palmas and Navarra recruited university graduates to take part.
They completed questionnaires and the researchers calculated their adherence to the Mediterranean dietary pattern (MDP) for an average of four-and-a-half years.
MEDITERRANEAN DIET FEATURES
A high intake of monounsaturated fatty acids like olive oil
A moderate intake of alcohol and dairy products
A low intake of meat
A high intake of fruit and nuts, cereals, vegetables and fish
Participants who had a strong adherence to the MDP tended to be male, ex-smokers, married and older individuals.
They were more active physically and showed a higher total energy intake.
The researchers identified 480 new cases of depression during the follow-up period - 156 in men and 324 in women.
They found that those with the highest adherence to the MDP were more than 30% less likely to develop depression.
They took into account marital status, the number of children and factors associated with a healthy lifestyle and found the relationship did not change.
Even taking account of personality traits, such as competitiveness and anxiety, had no effect on the results.
'More research needed'
Professor Miguel Martinez-Gonzalez, of the University of Navarra, said the results would have to be confirmed in longer trials with more participants but they had found a strong inverse association between the Mediterranean diet and depression.
"Thirty per cent is a large reduction in the risk and this could be very important considering the large burden of disease represented by depression.
"We know how important the Mediterranean diet is in reducing cardiovascular risk factors and the same inflammatory proteins are also raised in patients with depression."
He said it was likely that the overall dietary pattern was more important than the effect of single components and "may exert a fair degree of protection against depression".
Dr Cecilia D'Felice, a clinical psychologist, said there was mounting evidence for the importance of diet in treating depression.
She said: "What we do know is that a diet high in olive oil will enhance the amount of serotonin or brain transmitter available to you.
"Most anti-depression drugs work to keep more serotonin available in the brain."