Carbohydrate Intake and Depression

It's hard to know where to start on this one.  Everywhere you look, if you look, you see surprisingly consistent correlations between carbohydrate intake and depression.  Some studies draw a direct link.  Others find strong correlations with certain long-term results of high carbohydrate consumption, such as insulin resistance (see here also), diabetes (and here), vitamin D deficiency (and here), obesity and so on.  Some of these, and many others, make the leap to causation and successfully attempt to treat depression by correcting specific deficiencies or excesses.  Still others observe a relationship between low-fat diets (independent of carbohydrate intake) and depression.

After a lot of searching, I managed to find a single study investigating the effects of a ketogenic diet on depression in humans.  Its loneliness in the literature was not lost on its author:

It is surprising, after so much clinical experience spanning a period of two thousand years, that this paper is the first [and last] by a psychiatrist describing the applications of the ketogenic diet in the treatment of anxiety, depression, and dysperception. Meanwhile, throughout this most advanced society of ours, in every modern psychiatric facility patients are exposed to an overdose of carbohydrates... It is time that the application of available knowledge in this field should be the rule rather than the exception. Ignorance and fear of controversy are no longer an excuse to withhold this basic and physiologically-oriented treatment from our patients.