Doug, I'm a little confused about your term "fad diet" because a diet developed and studied scientifically and approved by medical doctors is not a fad diet as I understand it.

Fad diets tend to be promoted by celebrities in the media or on social media and tend to have been invented by someone with little or no expertise in nutritional science who is motivated to become famous and rich. The TV show called The Diet Testers which appears to be aka How to Lose Weight Well, tried out a range of diets on some willing participants and any of the diets which rely on an extremely limited range of foods e.g. cabbage soup, or the potato diet and not healthy balanced eating, counts as a fad diet in my definition.

If a diet has been scientifically proven to work for a significant number of people after rigorous, evidence-based studies, then it's not a fad diet in my definition. It might be popular because it works so people are talking about it but, in my opinion, it doesn't make it a fad diet.