Nutritionists and dietitians both work with groups and with individuals one on one, teaching about general nutrition and the effects of food choices on health. Different states set different regulations regarding what nutritionists can do, but in general, they can help clients learn about food and nutrition. However, nutritionists usually cannot provide medical nutritional counseling or diagnose or treat illnesses. In contrast, a registered dietitian is a licensed professional. In addition to doing everything a nutritionist can do, an RD can provide medical nutritional therapy and counseling. Dietitians can help diagnose and treat certain illnesses.