A nutritionist is an expert in nutrition. That is to say, what you eat or don't eat.
Dietitians are health professionals who specialize in human nutrition, meal planning, economics and preparation. Nutritionist is an accepted title but less so than a Dietitian.
A dietician - it's a recognised medical qualification.
They would be called a dietician or a nutritionist.
Nutritionist
nutritionist
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
An Expert Aviator is called an "Ace".
what ia an expert pilot called
A bully expert is called a bullnation
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.
Don't know the origin of the word, but a snake expert is called a herpetologist.
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 often referred to as a Sinologist or a China specialist.
A cultures expert is called an ethnologist.