I’m interested in how nutrition relates to disease, and how dangerous food additives (like artificial sweeteners) contribute to disease. I’m also interested in alternative medicine. What careers best match my interests?

You can work for your state health department as a registered dietician. Most health departments have a need for these in their medical clinics. They usually work closely with doctors in order to assess a patient’s needs and then provide ongoing education through one-on-one appointments and group sessions.
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Kudos to you for having a desire to do this. If you don’t want to work for the health department, you may be able to join a practice that specializes in holistic health. Whatever you choose to do in this field, it should be very rewarding.