A nutritionist is a person who advises people on matters of nutrition. In jurisdictions such as the UK and most US states the term "nutritionist" is not legally protected. As a result, people who refer to themselves as "nutritionists" may have no officially recognized credentials.

Although some may confuse the terms "nutritionist" and "dietitian", there is a difference in training and expertise. The term "registered dietitian" is legally protected. A registered dietitian is a food and nutrition expert who has met the minimum academic and professional requirements to qualify for the credential “RD.” In addition to RD credentialing through the American Dietetic Association, many states have regulatory laws for dietitians and nutrition practitioners. State requirements frequently are met through the same education and training required to become an RD.

The title "nutritionist" is protected and designated by only some states in the United States. It is important that a person seeking the counsel of a nutritionist check with their local state's licensing agency to find out if prospective practitioners are duly licensed.

The majority of registered dietitians work in the treatment and prevention of disease (administering medical nutrition therapy, often part of medical teams), in hospitals, HMOs, private practice or other health-care facilities. In addition, a large number of dietitians work in community and public health settings and academia and research. A growing number of registered dietitians work in the food and nutrition industry, in business, journalism, sports nutrition, corporate wellness programs and other non-traditional work settings. Dietitians also frequently work in private practice providing one-on-one nutrition counseling.