If an 'expert' promotes a diet, does that mean it's legitimate?

If an 'expert' promotes a diet, does that mean it's legitimate?

Andrea Giancoli (Registered Dietician ) gives expert video advice on: What are some popular types of fad diets?; What do all fad diets have in common?; What are some of the popular past fad diets? and more...

Usually on a lot of these fad diets, the calories are so restricted and some of them, if you're doing a fast or if you're doing just a liquid diet, your gut doesn't have a chance to do what it needs to do, and so it could stop working properly. You could suffer constipation, and you could suffer pain. When you do start eating again, you know all kinds of different gastrointestinal problems can happen. Also, you may have bad breath which may not be so great for the people around you because when you restrict calories like that, your body goes into something called ketosis and produces acidic bodies that give you bad breath.