The tongue-in-cheek answer would be to say that they are not in their right mind.
But I'd rather ask: Is it worthwhile to consult a physician before a diet change? Since, from my lay understanding, dietary needs are highly individual I'd say yes. Except for the dietary change the vast majority of people need: To consume fewer calories and more vegetables.
Isn't individuality of dietary needs reason not to consult a physician? In most cases it's going to be impractical for a physician to study any individual patient's requirements. They may also be legally or professionally prohibited from the kind of experimentation needed to find those requirements.
Previous open thread
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