From what all I've read the over-looked (under-popularized) factor in depression is INFLAMMATION in the body. Diet can have a huge impact on this and in general the inflammatory foods (vegetable oils, sugar, processed carbs) can contribute to or cause depression, and the anti-inflammatory foods, like fatty fish (wild salmon), greens, etc can help. Lists of foods and their inflammation levels are easily available on the internet, so talk of "hiring a dietician" is overblown.