I would say an optimal diet for mental health has plenty of fat, a good amount of protein, and a sparse amount of carbohydrates.
I've been diagnosed with clinical depression and Bipolar II disorder, and had been taking an anti-depressant plus an anti-seizure medication that's often prescribed for bipolar. When I first went very low-carb (<30 grams carb/day) in April 2007, I felt different immediately, and after a couple of months, decided to try to wean myself off my meds. Every week I reduced my daily dosage by one-eighth, until I was down to zero. I've never gone back.
The one time I saw a return of depressive symptoms, I had been experimenting with Kwasniewski's Optimal Diet, and had added in a slice of rye bread every day to meet the carb requirement (IIRC, it was 60g or so a day, much higher than I'd been eating). I immediately began to show symptoms. When I realized what was happening (after 5 days), I substituted a boiled potato for the bread, and like magic, the symptoms disappeared. In my case, it's clear that something in the bread -- lectins? gluten? -- was triggering depressive episodes.
There's a fair amount of research, much of it still rather "squishy," on the mental health benefits of a ketogenic diet. It's certainly worked wonders for me.
ETA: Specific food recommendations? Plenty of fats, including medium-chain triglycerides (coconut oil, for example). Minimal sugars, and no bread whatsoever. Your choice of proteins; I tend to eat on the high-fat, low-protein side.
There are lots of good "brain" foods, mostly protein and fat-based foods help, but I won't write about that today as it is fairly common language and I'm positive someone has mentioned it already, or will soon.
I want to talk about the fact that the "second brain" is the gut - the GI tract. It is also the main immune system. Many people have no idea how much our bodies rely on our guts. I am researching the GAPS diet - Gut And Psychology/Physiology Syndrome that Dr. Natasha Campbell MacBride has extensively researched - I will provide a link to a video of her speaking at the Wise Traditions conference.
Youtube series with guide: http://www.youtube.com/watch?v=HQi2ieuJQBc
Just her on the stage: http://vimeo.com/10507542
Lots of saturated fats, fish oils, and adequate amounts of carbohydrates (say 80-100g). This is pretty basic as an opinion; but I'm afraid I'm not an expert!
As far as Im aware protein is non-essential for the brain, but obviously its a good idea to have some :P
With the qualifier that diet cannot overcome sleep deprivation, I've been amazed at the calming effect of water kefir by which I mean the calming effect of improved gut status.
I should say that the water kefir was added to a menu already containing ample protein/fat and moderate veggie and fruits so I don't think of it as a singular remedy.
After 64 years of a fidgety, restless mind and nervous, temperamental digestion I find myself calm, with a GI tract so quiet it's spooking me out. My pulse has dropped at least 10 beats per minute. My mind is following my body--I'll never be a placid person but I am more "level" than I've ever experienced.
Since the only menu change during the time my physical/mental improvements occurred was the water kefir I'm thinking that my gut was out of whack and the kefir has helped it significantly.
I continue to feel the best meals for mental health are the ones that come with little toys. I was temporarily impressed with the dietary proposals of Wolf, Sisson, and Cordain. But nothing they advocate comes with toys that make me happy. Happy Meals. Back to the Golden Arches, for yours truly.
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