It seems like a fair request, that you should be able to do what you want, but that isn't what you are asking of the doctor in this situation. You are asking them to do something outside the bounds of their profession to do what you want.
An extreme example would be if a suicidal patient asked their therapist to put a bullet in their head rather than give them an antidepressant, should the patient be free to ask for this and should the therapist be obligated to fulfill this request? It's a question of what is ethical and that goes beyond your own agency. You are free to put a bullet in your own head, or find your own source of vitamin C, but you can't put that ethical dilemma on the doctor and expect a result.
An extreme example would be if a suicidal patient asked their therapist to put a bullet in their head rather than give them an antidepressant, should the patient be free to ask for this and should the therapist be obligated to fulfill this request? It's a question of what is ethical and that goes beyond your own agency. You are free to put a bullet in your own head, or find your own source of vitamin C, but you can't put that ethical dilemma on the doctor and expect a result.