A dietitian is a professional specializing in the application of the principles of nutrition when advising on the feeding of individuals or groups.
Dietitians can work in hospitals and will have to be sure certain patients are fed certain foods. They can work in nursing homes, or just for the general public. They talk to the patient to find out about their eating habits and what their problems are (diseases, anorexia, bulimia, etc.) and then will make up a diet to suit the patient and work with them on it.
The dietrist is the Doctor Who cared about the digestion and infections in the stomach.
The correct name is dietitian - expert in food and nutrition.
An expert on diet and nutrition