nutritionist
A nutritionist is an expert in nutrition. That is to say, what you eat or don't eat.
An Expert Aviator is called an "Ace".
A cultures expert is called an ethnologist.
what ia an expert pilot called
A bully expert is called a bullnation
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 grammar expert is called a grammatician or a grammarian.
Expert advice or expert guidance.
An expert in his field is often called a specialist or an authority.
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
an expert on China is called a sinologist.