Eating more greens may be a good way to chase away the blues.

More and more research is showing how a Mediterranean-style diet with plenty of whole grains, fresh fruit and vegetables along with olive oil, nuts fish and some red meat can help people with depression.

At the forefront of research into psychiatric nutrition is Professor Felica Jacka from Deakin University in Melbourne.

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She runs the Food and Mood Centre, a collaborative research centre led by Deakin University in Australia, and the only centre in the world focusing on mental health and diet.

She says her team is showing how a new approach to treating depression and other mental illnesses may offer hope to millions.

Professor Jacka says it's been long understood that depression is linked to the immune system, and we know diet is very important in governing the immune system.

"Our genes are not set in stone - you can switch them on and off - and diet is an important determinant of what genes get switched on and off."

So in the early 2000s she started to formally investigate whether such a link existed.

"I thought there might be a link, but it hadn't been investigated properly before."

Last year the Food and Mood Centre carried out a direct intervention study.

The SMILES study recruited adults with major depressive disorders and randomly assigned them to either face-to-face social support - which is known to be helpful for those with depression and acted as an active control condition - or to dietary support with a clinical dietician over 12 weeks.

"People in the dietary support group, about 30 percent of them, went on to have full remission of their depression compared to 8 percent in social support group," Prof Jacka says.

She says the gut is the key to all this.

"You can change your gut microbiota and its activity within just a few days of changing your diet. What we showed was, yes, if you improve your diet, it will have a measurable effect on your depressive symptoms."

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And the diet that helped people was a balance of fibre, legumes, whole grains, vegetables and meat.

Prof Jacka says the dieticians working with the depressed cohort kept it simple.

"We were saying, get your crockpot from the op shop for 20 bucks and on a Sunday cook up a big pot of vegetable and bean soup with some whole grains, some barley, with a bit of quality grass fed beef, and have that for the whole week."

Fast foods and processed food are a problem for two reasons, she says.

"One is we're getting too much food we know is problematic for our body and brain (such as salt, fat and simple carbohydrates) the other is not getting enough of the good stuff we know our body needs to be able to function properly."

Another advantage was the people on a new diet were spending less.

"We reduced their intake of sweets, cakes and donuts and the diet we were advocating was cheaper than the junk food diet people were eating when they first came in to the study."

She says the study has been formally evaluated and found to be highly cost-effective as a treatment option for depression.