A nutritionist is an expert in nutrition. That is to say, what you eat or don't eat.
Such a person is a dietician, being an expert on diet and nutrition.
Some nutrition is just one of those issues that holds crucial distinction that is going to require expert advisement about
A bully expert is called a bullnation
what ia an expert pilot called
An Expert Aviator is called an "Ace".
A cultures expert is called an ethnologist.
The organisms that get their food from dead organic matter are called saprobes and this type of nutrition is called saprozoic nutrition.
A grammar expert is called a grammatician or a grammarian.
Don't know the origin of the word, but a snake expert is called a herpetologist.
Expert advice or expert guidance.
A specialist in the study of nutrition; Source: webnetweb A dietitian (also 'dietician', though 'dietitian' is used consistently by professionals) is an expert in food and nutrition. Source: wikipedia