A nutritionist is someone who specializes in the study of nutrition, including nutritional deficiencies, sources of nutrition, and nutritional challenges which may face individuals or communities. The word is thrown about a bit loosely in many parts of the world, which can make it difficult to define. A person may take a few classes in nutrition and consider him or herself a nutritionist, or someone may have a PhD or MD in nutritional science.

There are several different aspects of the study of nutrition which interest nutritionists. Some nutritionists work directly on the study of nutrients and food, in labs and through field work. These nutritionists may also make nutritional recommendations for society as a whole, and they focus on learning as much as possible about nutritional requirements and sources of nutrition. Public health nutritionists may work to identify nutritional problems in various communities, and they also provide education and outreach to encourage people to improve their nutritional habits.

A dietitian is a nutritionist who provides services directly to patients or clients. The role of a dietitian varies, depending on where he or she practices. Some dietitians work in environments like hospitals and school, supervising large-scale food production and ensuring that everyone gets the foods that they need to do well. In other cases, a dietitian may work one-on-one with clients, providing personal advice to help people lose weight, train for sports competitions, or fight diseases like cancer. In this case, a nutritionist may be a medical doctor or work with a medical doctor to ensure that all of a client's needs are met.

The tasks of a nutritionist can get extremely complex, especially when working with sick patients in an environment like a hospital. A nutritionist may be consulted about the insertion of a feeding tube into a patient who does not appear to be gaining the nutrition that he or she needs, for example, or a nutritionist may evaluate a patient's diet to determine whether or not it needs to be changed for better health. This type of nutritional evaluation usually requires a medical degree, since it requires extensive knowledge of physiology and biology.

For consumers seeking advice from a nutritionist, the poorly defined boundaries of what a nutritionist is can be very frustrating. It is a good idea to ask about qualifications such as education and field study when consulting a nutritionist, to determine how much the nutritionist actually knows. Ideally, you should seek a recommendation from a doctor or another medical professional who may have worked directly with a particular nutritionist.