When I was diagnosed with severe Major Depressive Disorder I (at the suggestion of my psychiatrist) spent about a month IP. The first week of that entailed a very detailed clinical examination, including investigation of my nutritional status. Nothing of clinical interest emerged from that examination.

That is just evidence-based medicine in action. With respect, I think that you are making a false dichotomy between "functional medicine" and "traditional medicine" - there is just evidence-based medicine.

Making a more general point, eating in a healthy way (minimal junk food, lots of fresh fruit, vegetables, whole-grain products, animal protein in moderation, etc) is good for both physical and mental health.

Dietitians are evidence-based health professionals who can offer their clients advice about healthy eating. That might (if indicated) include taking nutritional supplements.

The contrast is with self-medication (buying nutritional supplements from Web sites like Dr Hyman's among many others). I don't think that self-medication is a good idea.