Some people need help to learn what foods to eat properly because their parents never taught them how or they just have bad eating habits.

You don't need a nutritionist to tell you to stop eating junk/processed food. All other diet advice are pure b.s. as there's no scientific answer to what "a proper diet" is. What is "healthy" and what is not changes on a similar rate as weather broadcast i.e. see a recent turn on fat.

Which vitamin or mineral deficiency leads to psychosis? (With a link to this scientific proof please)

Both can cause mental symptoms (retardation for example). But both are rare.

2. You don't seem to understand what ADD and depression are. They're vastly more serious and prolonged than the sort of candy-induced, momentary fit-throwing you're suggesting.

Here's an example regarding hyperactivity and sugar: http://jamanetwork.com/journals/jama/article-abstract/391812