Participants who have been diagnosed with depression will follow a Mediterranean diet high in fruits, vegetables, fish and legumes and low in processed foods, sugar, salt and saturated fats.

Previous studies have shown a link between the healthy diet and a reduced risk of the condition but this will be the first full-scale clinical trial to examine whether it can be used to alleviate symptoms after diagnosis.

If successful, the trial - run by Victoria's Deakin University, St Vincent's Hospital and Barwon Health - could mean sufferers around the world will rely less on psychiatric drugs to manage their illness.

Up to a third of depressed people are non-responsive to antidepressants or psychotherapy and researchers say food may be the key to helping this group.

''Depression now counts for the largest burden of disability in the developed world and, by 2020, it will account for the second-largest burden right across the world. We're really limited in effective treatments, so finding something that is helpful for people with depression, something that is under their own volition, this is really critically important,'' said the lead researcher, Felice Jacka, from the Deakin University school of medicine.

To be eligible, participants must be over 18 and have a diagnosis of major depression and be non-responsive to other treatments.

A quarter of the 200 volunteers have already been recruited and are being assigned to either a dietary group, which will receive weekly nutritional support and counselling, or a control group. The groups will be monitored for three months.

The nutritional regime will vary slightly from the traditional Mediterranean diet in that it will also include moderate amounts of red meat, which has been linked to reducing risk of mental illness, particularly in women.

''We want to emphasise the foods that have been shown to be particularly important in mental health. One of those is fish - food that's rich in omega 3 fatty acids and higher in good fats - such as salmon. Leafy green vegetables are very high in folate, which has also been shown to be important in mental health, along with legumes such as lentils and beans, and wholegrains and nuts,'' Associate Professor Jacka said.

John Ventouris, who owns one of Sydney's oldest Greek restaurants, Diethnes, has enjoyed the benefits of a Mediterranean diet his entire life. He believes a simple combination of fresh seafood, fruit and vegetables is a recipe for a healthy and happy life.

''I grew up eating a lot of fresh produce, fresh seafood lightly cooked in olive oil, a lot of fresh vegetables, lots of different herbs, lots of fresh beans and dried beans. It's traditional real food, not packaged food,'' he said.

Mr Ventouris, whose family-run restaurant in the city has operated since 1952, believes happiness comes from physical health.

with Rachel Browne