The finding has been published in the October issue of Archives of General Psychiatry, one of the JAMA/Archives journals.
To reach the conclusion, Almudena Sánchez-Villegas, B.Pharm., Ph.D., of University of Las Palmas de Gran Canaria and Clinic of the University of Navarra, Pamplona, Spain, and colleagues studied 10,094 healthy Spanish participants who completed an initial questionnaire between 1999 and 2005.
Participants reported their dietary intake on a food frequency questionnaire, and the researchers calculated their adherence to the Mediterranean diet based on nine components (high ratio of monounsaturated fatty acids to saturated fatty acids; moderate intake of alcohol and dairy products; low intake of meat; and high intake of legumes, fruit and nuts, cereals, vegetables and fish).
After a median (midpoint) of 4.4 years of follow-up, 480 new cases of depression were identified, including 156 in men and 324 in women. Individuals who followed the Mediterranean diet most closely had a greater than 30 percent reduction in the risk of depression than whose who had the lowest Mediterranean diet scores.
The association did not change when the results were adjusted for other markers of a healthy lifestyle, including marital status and use of seatbelts.
"The specific mechanisms by which a better adherence to the Mediterranean dietary pattern could help to prevent the occurrence of depression are not well known," the authors write.
Components of the diet may improve blood vessel function, fight inflammation, reduce risk for heart disease and repair oxygen-related cell damage, all of which may decrease the chances of developing depression.
"However, the role of the overall dietary pattern may be more important than the effect of single components. It is plausible that the synergistic combination of a sufficient provision of omega-three fatty acids together with other natural unsaturated fatty acids and antioxidants from olive oil and nuts, flavonoids and other phytochemicals from fruit and other plant foods and large amounts of natural folates and other B vitamins in the overall Mediterranean dietary pattern may exert a fair degree of protection against depression," the authors write.