A dietitian nutritionist/registered dietitian is a food and nutrition expert who has met the academic and professional requirements to use the credential RDN/RD. The RDN credential is offered as an option for professionals who want to emphasize the nutrition aspect to the public and to other health practitioners. In addition to credentialing, many states have regulatory laws for practitioners. State requirements frequently are met through the same education and training required to become an RDN/RD.