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A nutritionist is not subject to professional regulation and anyone can call themselves a nutrition expert even if they are self-taught.
Registered dietitians have met strict requirements: a four year Bachelor’s degree, 1200 hour accredited and supervised practice internship, and have passed a national exam that board certifies them. The rigorous training ensures that they have the depth of knowledge to provide medical nutrition therapy, assessment, nutrition diagnosis, planning, implementation and monitoring of nutritional programming. The only credential that legally allows someone to nutritionally diagnose and recommend diets and supplementation is the Registered Dietitian credential.