What's eating you?

There have been several stories in the news regarding diet and one's risk for depression. This isn't surprising. Depression, at its most severe, is one of the most debilitating, intractable, and expensive conditions to treat. In its milder forms, it's often treated successfully with various therapies and/or medications.
Wouldn't it be great, then, if we could find ways to reduce its severity and duration or, even better, prevent depression before it starts. Adding or removing some foods from our diets, it would seem, might be an answer.
We've long heard that oils from fatty, typically cold-water fishes are rich in omega-3s which, it's believed, can relieve depressive symptoms. It turns out, though, that it's not quite that simple. There are, we learn (Nutrition and health: Food, glorious food | The Economist), two types of omega-3 oils - long-chain and short-chain. The long-chain, primarily from fish, are the ones that do us the most good and are also the more expensive. Short-chain omega-3 oils, derived from plant oils, aren't nearly as effective.
A study published in the British Journal of Psychiatry concludes

that people who ate a diet high in Mediterranean-style foods, such as pulses, fruits, nuts, cereals, fish and olive oil were 30% less likely to develop depression than those who ate less healthy foods.
via Mediterranean diet 'can help stave off depression' | Society | The Observer

(Pulses is another word for beans.)
It seems simple to conclude that a Mediterranean diet supplement with cold-water fish ought to be part of a depression prevention program.
Except that, olive oil includes omega-6 which can negate the beneficent effects of long-chain omega-3 oils. The Economist concludes " those who buy omega-3 supplements that also contain omega-6s are probably wasting their money."

Another study released this week shows how junk food diet 'can give you depression' | Mail Online. Well, it doesn't quite say that. The study says that people who eat more processed foods have a higher risk of depression. The researchers conclude that these dietary habits are predictive of depression and imply that the processed foods themselves are causing depression. I'm not sure. (The full text of the study is behind a subscription wall, so I can only rely on the news reports.)
For example, I'm having a heck of time parsing this item

Although the researchers cannot totally rule out the possibility that people with depression may eat a less healthy diet they believe it is unlikely to be the reason for the findings because there was no association with diet and previous diagnosis of depression.

via BBC NEWS | Health | Depression link to processed food

I suspect that the study's authors and certainly the reporters are confusing causation with correlation. It's not surprising that people with depression would have bad eating habits. When in the middle of depressive episodes, most folks are lucky to put on clean clothes, let alone make sure that they're getting enough veggies.
And, while we're at it, another study indicates that happiness is overrated anyway.

An Australian psychology expert who has been studying emotions has found being grumpy makes us think more clearly.

via BBC NEWS | Health | Feeling grumpy 'is good for you'