If you are interested in diet effect mood there are really interesting studies with parasites, like the mildew mushroom. It can produce human hormones like estrogen or melatonin. So the parasite can induce sleep or sleep disorders. It also likes to make hormones that increase hunger so you keep feeding it before it moves to the next target.
Of course, in this case this would be an effect of diet in conjunction with a parasite, but diet in general can have effect on mood too.
We can argue about the magnitude of the effect on the human psyche, nonetheless the positive or negative effects of diet on the physiological function of the human body are undeniable.
The better the foundation, the better your likelihood to stay healthy (physically and mentally). You are what you eat is a maxim that holds true, however, I won't claim that you'll get depression just because you eat a certain type of food. No one can proof that beyond a doubt. We can just show that consumption of certain macro (or micro-nutrients) correlates with certain symptoms of certain disorders.
Good luck actually finding the causal relations. That's a different beast to slay.