I do believe that this includes most illness, including mental illness. It also requires supplementing to make up for the deficiencies we are bound to have. This is a complex subject that should not be tossed off lightly. So many people that I talk to about vitamin C scoff or say that it didn't work for them. Nutrition is not something you should try for a week and then draw conclusions.
Every individual is different.
Different foods have very different consequences depending on who you are. The amount of supplementation depends on your biochemical individuality.