MONDAY, Oct. 5 (HealthDay News) -- People who followed the Mediterranean diet, an eating regimen that is rich in fruits, vegetables, whole grains, fish and nuts, were less likely to develop depression in a Spanish study.

"We are speaking of a relative reduction in risk of 42 percent to 51 percent," said study co-author Dr. Miguel A. Martinez-Gonzalez, chair of preventive medicine at the University of Navarra. "This is a strong association."

The Mediterranean diet usually is recommended to reduce the risk of heart attack, stroke and other cardiovascular problems. This study, reported in the October issue of the Archives of General Psychiatry, is one of only a few to assess its effect on mental function.

The Spanish researchers followed more than 10,000 healthy adults who filled out questionnaires between 1999 and 2005. All were free of depression when the trial started. Their adherence to the Mediterranean diet was measured by looking at nine components, such as low intake of meat, moderate intake of alcohol and dairy products, and high intake of fruits, nuts, cereals, vegetables and fish.

After an average follow-up of 4.4 years, the overall incidence of depression for those who most followed the diet was 30 percent lower than for those who most ignored the dietary rules. Even lower rates of depression were associated with intake of specific elements of the Mediterranean diet, such as fruits, vegetables and olive oil.

There are several possible explanations for the reported protective effect, Martinez-Gonzales said. The Mediterranean diet improves the function of the endothelium, the delicate inner lining of blood vessels, which is involved in the production of brain-derived neurotrophic factor (BDNF), a molecule that is responsible for the growth and function of nerve cells, he said. "Dysfunction of BDNF is thought to be responsible for some depression cases," Martinez-Gonzalez said.

In addition, olive oil improves the binding of serotonin to its receptors "and serotonin is a key neurotransmitter in depression," he said. "In fact, Prozac acts by increasing the availability of serotonin in the brain."

And the omega-3 fatty acids found in some fish might help improve the function of the central nervous system, Martinez-Gonzalez said. "All these mechanisms may lead to an improved brain function and a greater resilience to better face the frustrations of every day, to control stress, and so on," he said.

But Martinez-Gonzalez does not recommend that people diagnosed with depression try to treat it by adopting this diet. "The Mediterranean diet might be ideal for the prevention of depression, but not for its treatment," he said. "For those patients who already have depression, the best thing they can do is to seek the proper medical treatment by a psychiatrist."

"It's not surprising to see these results," said Dr. David Mischoulon, an associate professor of psychiatry at Harvard Medical School. "They are what we would have expected to see on the basis of previous information. There is a very large body of work in the psychiatric literature saying that components of the Mediterranean diet when looked at separately have such an effect."

Mischoulon agreed that he would not recommend the diet as a treatment for existing depression. "A person in an episode of depression needs more direct and more targeted intervention," he said.

But the study adds one more reason for adopting the Mediterranean diet, Mischoulon said. "If you have a family history of depression and you are concerned about it, a diet like this probably would be a good place to start," he said.

More information

The Mediterranean diet is explained by the American Heart Association.