I cannot see from your profile what fitness certifications you have done? Generally training for a fitness or wellness certification will include work on nutrition, and on what the fitness professional’s scope of practice is with respect to disseminating that information.
The question is one that cannot be well answered in a few paragraphs in a format like this….. and one on which there is not universal agreement.
I would like to recommend a book I like ‘The Omnivore’s Dilemma’ as something you might enjoy as you think about this topic. I would also recommend booking a few appointments with a certified nutritionist to talk about it as it relates to your life. And if you are wondering about how to understand and use this information within your professional life you might start with some of the materials such as the ACE personal trainer Manual, or some other materials for whatever fitness certification you have or are interested in taking.