Final answer:
A fad diet is an unsustainable diet that offers quick fixes or rapid weight loss but lacks essential nutrients for long-term health.
Step-by-step explanation:
A fad diet can be best described as unsustainable. Fad diets usually offer quick fixes or promise rapid weight loss through extreme and restrictive eating patterns. However, these diets are often based on unsubstantiated claims and lack essential nutrients necessary for long-term health.
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