Jim Logajan writes:

I think he made the mistake of inverting the causality in the statement "If
you are thirsty then you are dehydrated," which appears to be generally
accepted as far as I can tell.

So is the inverse. Thirst is a reliable indicator of dehydration. An absence
of thirst is a reliable indicator of proper hydration.

While it appears to be true that by the time you experience thirst you are
dehydrated, there appears to be some physiologic lag time after other
dehydration symptoms have occurred but before thirst kicks in.

Only in extraordinarily acute dehydration, which is not a type of dehydration
that occurs when flying an aircraft.

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