He was probably a jerk about the question, because questions like that sometimes trigger a "OMG it's a hypochondriac!" reaction in doctors.
A doctor's job, sadly, is to treat symptoms. Without symptoms of some illness with something obviously related to vitamins or minerals -- thinking vitamin A deficiency or too much iron -- of course that type of doctor is going to say "everything's fine", as there are no symptoms.
If the doctor dismissed symptoms you went to the doctor to address, then there's a real problem.
You do have the option of finding a doctor or other healthcare practitioner that is into preventative medicine -- they may be more amenable to your requests to satiate your curiosity about your own body.
There are labs you can submit your own blood work to to have a variety of tests done yourself. Don't believe the doctor is a gatekeeper to healthcare and health tests.