fad diet

noun2

1. •
   A restrictive diet whose main goal is short-term weight loss.

   • The diet revives old food friends unjustly scorned by fad diets: breads, cereals […] — 1979
2. •
   A diet that is popular only for a short time, a novelty diet; a fad in food.

   • Usually a fad diet will take one idea that research has shown to be somewhat beneficial and take it to the extreme. A good example of this is the statement “Eat seaweed at every meal.” — 2009