Your question is very broad, and you may get a number of responses depending on who answers you! Healthy food should come from the main food groups (fruits, vegetable, lean proteins, whole grains, and dairy–along with some healthy fats). A healthy, balanced diet provides our bodies with the right amount of nutrients, in the right quantities, to give us energy and enable us to lead a healthy lifestyle.
There are numerous resources for you to learn about nutrition and how it impacts your health (IDEA alone has numerous articles on nutrition). Or, you may refer to a Registered Dietician for yourself to learn about what is best for you, personally. Hope this helps…
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.