Tom L. writes:

Check out the three articles below. The third one is more technical
and explains the delay between dehydration and thirst.

Sports and the military have nothing to do with aviation. I've already
explained the distinction of acute dehydration; but that does not occur in
pilots, as a general rule.

Additionally, of the three references, only one is even moderately trustworthy
(the Army link), and even that must be taken with a grain of salt. Anyone can
write for About.com or a random association. And one cannot be sure who is
writing for the Army, unfortunately.

Finally, these references don't even support your assertions to begin with.

From personal experience: I used to live in humid areas (Europe and
East Coast of the US) and never felt any simptomps of dehydration
there besides thirst.

That's because you were not significantly dehydrated. Thirst is one of the
earliest and most reliable indicators.

Now I spend most of my time in the West and
Southwest US and sometimes have a particular headache that goes away
immediately after drinking some fluids. This is more likely to happen
when the air is dry and especially if I'm at higher elevation, e.g.
while flying. I do not experience thirst, the headache is my first
indicator of dehydration.

You may be better hydrated than you are when thirsty.

Dehydration is extremely difficult to quantify. Whenever anyone says that you
are 1.5 litres low or anything as specific as that, you can usually assume
that he doesn't know what he's talking about. It's pretty much impossible to
prove that someone is properly hydrated or mildly dehydrated; only severe
dehydration produces unambiguous clinical signs.

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