Final answer:
Fad diets are popular for a while but not sustainable in the long run, as they rely on extreme restrictions and lack scientific evidence.
Step-by-step explanation:
The best summary of fad diets is that they are popular for a while but not able to stand the test of time. They often promise quick and easy weight loss solutions, but they are not well-balanced or nutritious in the long run. Fad diets usually rely on extreme restrictions, such as eliminating entire food groups, and lack scientific evidence to support their claims.
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