The "RD" credential is a legally protected title that can only be used by practitioners who are authorized by the Commission on Dietetic Registration of the Academy of Nutrition and Dietetics.
Some RDNs may call themselves "nutritionists," but not all nutritionists are registered dietitian nutritionists.
The definition and requirements for the term "nutritionist" vary. Some states have licensure laws that define the range of practice for someone using the designation "nutritionist," but in other states, virtually anyone can call him- or herself a "nutritionist" regardless of education or training.
Individuals with the RDN credential have fulfilled specific requirements, including having earned at least a bachelor's degree (about half of RDns hold advanced degrees), completed a supervised practice program and passed a registration examination — in addition to maintaining continuing education requirements for recertification.