But what happens when you take a bunch of people with depression, encourage them to eat better and watch to see if their mental health improves? That’s what researchers at Victoria’s Deakin University are doing in a new study in which people with unipolar depression, but not bipolar depression, are encouraged  to eat a diet rich in vegetables, fruit and whole grains, oily fish, olive oil, legumes and raw unsalted nuts, with some lean red meat and reduced fat dairy.

If the results are positive, lead researcher Dr Felice Jacka hopes it will help build the case for health professionals to recommend better diets for patients with depression -  and for governments to do more to improve our food environment which she believes is undermining mental as well as physical health.

Although depression is influenced by many different factors, Jacka believes poor diet is having an effect.

There’s enough evidence to say that a food supply that includes as much highly processed denatured food as ours is contributing to levels of depression as well as other chronic diseases,” she says. “With obesity, heart disease, diabetes and some cancers alone we now have a tidal wave of preventable non-communicable diseases that the World Health Organisation estimates will have a global cost of $ 30 trillion by 2020 - and that’s without adding the cost of depression and dementia which is also influenced by diet.”

Research has often singled out single nutrients as being important for mental health – fish oil is one example and so is zinc.  But it looks like it’s more complicated than that.

“Not only does some research suggest that it’s a pattern of healthy eating that’s important, but we’re starting to understand that with fish, for example, it’s not just omega 3 fats that might make a difference but other components of fish too,” Jacka explains.

Diet may also influence the brain’s production of a protein called BDNF (brain derived neurotrophic factor) which acts like fertiliser for brain cells.

“We used to think that once brain cells died you couldn’t grow more but now we know that BDNF can help new brain cells to grow, especially in some areas such as the hippocampus, the part of the brain that’s important for learning and memory and also for psychiatric health.

“When people have depression the hippocampus shrinks but with treatment it gets bigger partly because of BDNF – and we think that anti-depressants help promote BDNF, “says Jacka. “But animal studies have found that diets high in fat, refined carbohydrates and sugar dramatically reduce BDNF.”

What do psychiatrists think about the diet-depression link?

“In the early 2000s there was a lot of scepticism but now research from across the globe from countries as diverse as Norway, Taiwan, Spain, Japan and the US is showing the same thing across all age groups and I think a lot of younger psychiatrists are taking notice,” she  says.

“Healthy diets are different in different countries. A healthy Spanish diet is different to a healthy Norwegian diet, but they all have something in common as far as brain health is concerned and the evidence points to diets rich in plant foods – vegetables, fruit, nuts legumes and tofu, and quality protein like fish and grass-fed meat. In other words foods that are unadulterated. With dairy and grains the jury is still out.”

As for which fats are good or bad for the brain that’s in the ‘more research is needed’ basket.

“While there’s some evidence for omega 3 fats as being protective in humans, it’s not clear which fats might be detrimental. Studies that have found harmful effects of other fats on animal brains have used different types of fat –sometimes saturated fat has been used and sometimes polyunsaturated fat or sometimes it’s not specified what the fat is,” she explains.

“It could be that saturated fat is fine as long as the diet is based on plant foods with quality protein but we need more research. However there is evidence that trans fats can be problematic for depression.”

SMH