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

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

Depending on its result, the trial - run by Deakin University, St Vincent's Hospital and Barwon Health - could mean sufferers around the world would 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 critically important,'' said lead researcher, Associate Professor Felice Jacka from Deakin University's 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 a dietary group - which will receive weekly nutritional support and counselling - or to a control group. The groups will be monitored for three months at sites in Collingwood and Geelong.

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.

Sisto Lucarelli has followed a Mediterranean diet his whole life, having moved to Australia from Italy in 1955. He has always grown his own vegetables and lives by the maxim that ''you are what you eat''.

''You feel a lot better when you eat well. It makes a real difference to your happiness if you eat natural foods that you grow yourself,'' he said. At his Oakleigh South home, Mr Lucarelli grows veggies including broccoli, cauliflowers, tomatoes, lettuce, garlic, cabbage, onions and beans. ''This is the way I was taught to eat as a child. We eat a lot of salad, grilled meat, we have our own chickens, lots of legumes and veggies and of course, spaghetti.''

■For more information on the trial email: diet@barwonhealth.org.au or call (03) 4215 3304.