A nutritionist is someone whose work is food and how it keeps you well. They may plan meals, read labels with you, or talk about vitamins and illness. The word is made in a simple way: nutrition + -ist, a person who belongs to that field. In English, though actually, the title is a little loose. In some countries almost anyone can put nutritionist on a card, while dietitian is the title you earn with a licence and hospital training. In Taiwan the licensed professional people meet in a hospital is 營養師, and that job is closer to the strict English dietitian. A nutritionist in a gym or on the internet may or may not have that training. The English word only says “someone who deals in nutrition”; it does not, by itself, promise a government licence. If you need medical advice, you want the trained person, not only the title.