A dietitian. The American English versions of Oxford]and Collins]list the spelling with "t" first. is an expert in dietetics; that is, human nutrition and the regulation of diet. A dietitian alters their patient's nutrition based upon their medical condition and individual needs. Dietitians are the only healthcare professionals licenced to assess, diagnose, and treat nutritional problems. For example, safely regulating the diet of a patient with Crohn's disease is out of medical doctors' scope of expertise, thus a dietitian must be called to permit any changes based upon their knowledge of nutritional biochemistry. Dietitians work in a variety of settings from clinical to community and public policy to media communications.