Is there an educational level needed to be considered an expert?

Yes, though I think licensing doesn't necessarily – depending on each state law – entail title protection for "nutritionist" : e.g., even if the state permits a trained dietitian to obtain a license as a "Registered Dietitian Nutritionist," it might also permit a non-dietitian to hang out a shingle as a "Nutritionist."

 

It appears the key term is "nutrition counseling." In some states, if you're counseling a patient, you need the RD or RDN. Just say that you're coaching, and you're good to go. That's what people are doing in every other field. lol