4  Diuresis


There are several ways in which pilots make it hard for themselves to
stay hydrated.	These all involve "diuresis" of one kind or another.
Diuresis is the process of making extra urine.	Physicians produce
diuresis deliberately when medical conditions hinder the kidneys from
getting rid of extra salt or water.  Pilots produce diuresis
unintentionally, a problem only when the main goal is to conserve
water, and a nuisance if no waste-disposal plans have been made for a
long flight.


4.1  Sugar

For most pilots, sugar is not a problem, and medical certification is
difficult for diabetics.  But if you are, especially you who are and
don't know it, or you who think their diabetes is mild and requires
little attention, listen up.

Sugar in the blood above a certain amount, which is different for
everyone, spills into the urine.  The level at which sugar begins to
"spill" is called the "renal threshold."  This can be fairly close to
normal, as low as 150 mg/dl.  This is significant because a normal
fasting blood sugar is 70 - 110 mg/dl, and a normal after-meal blood
sugar is up to about 150 (some would say 180).	A person can have mild
diabetes with no symptoms at all, and even a normal fasting blood
sugar, and after eating (or more significantly, after over-eating) can
run the blood sugar up to 250 mg/dl or more and have it stay there for
hours.

When sugar spills into the urine, it creates an "osmotic gradient"
that pulls water into the urine (or hinders its reabsorption).	This
results in excess loss of water.  As the water is lost through an
osmotic process, you feel thirst as a result and drink more water, so
generally people are able to maintain their blood volume easily.  But
it does require that you drink a lot more water than might otherwise
seem reasonable and sufficient, possibly a problem during a long
flight in a warm cockpit on a hot day.

And people age, people who haven't had diabetes but might have older
relatives who have had it, people who might be a little more portly
than they used to be or intended to become.  These folks, not you or
me, but some other folks, might develop diabetes without ever being
aware that it is happening, without ever feeling badly.  It can sneak
up on a person, just as old age tends to do.  Think about it.

While we're thinking about sugar, let me mention that it sometimes
causes fatigue.  Everyone handles sugar a little differently, and if
you happen to be one of those folks susceptible to "hypoglycemia,"
it's worth thinking about.  If you fast, your blood sugar drops
gradually.  Some folks run through their stored sugar--liver
glycogen--faster than others; other folks (more often the portly ones)
tend to have higher than desirable levels of insulin hanging around,
which hinders release of glucose from storage.

The result, in either case, is fatigue, usually coupled with hunger,
or perhaps shakiness or weakness.  If you happen to be unlucky enough
to have diabetes, either known or undiscovered (most people with
adult-onset diabetes don't know it), the hyperglycemia that follows a
hearty meal can last two to four hours, and itself can bring about a
sense of fatigue and subtly impairs cognition.


4.2  Caffeine


Caffeine is, plain and simple, "a diuretic to the kidneys" as the
Doane's Pills ads used to say.	It is relatively harmless, it is a
useful stimulant, but it is a diuretic, and if you drink it, you will
get rid through your kidney most of the beverage in which it is
imbibed.

The moral here is, if you don't want to be inconvenienced by copious
urination during a long flight, don't drink caffeinated beverages
before flight.	And if you are trying to hydrate on a warm day, either
avoid caffeinated beverages or double the amount you might otherwise
drink.

Soda is the source of caffeine by which you might be caught unawares.
In my area, for example, Barq's root beer is caffeinated, but Barq's
diet root beer is not.	The only clue is the fine print on the label.
In addition, the word "soda" is contracted from "sodium," the cation
of table salt.	See "salt," page 2.

And "de-caf" coffee is not caffeine free.  Depending on how it's
brewed, decaf has 16-25% as much caffeine as a cup of regular coffee.
Less, but not none.


4.3  Alcohol


Well, none of us would ever drink alcohol before flying any more than
we would take a sleeping pill before flying.  But I mention it in this
list because of its effect on post-flight hydration.  Alcohol is a
diuretic.  If you drink alcoholic beverages while you are re-
hydrating, you must over-hydrate in order to compensate for the
diuresis.

As long as we're on the topic, let me remind you that last night's
party will affect today's flight.  Modest alcohol use--1 to 3 ounces
for a man, 1 or 2 for a woman--is not likely to cause trouble.	But
alcohol's metabolites, chiefly aldehydes, stick around for many hours,
well into the next day, and do cause fatigue, decreased tolerance to
metabolic stresses like hypoxia, and decreased performance.  So if
you're in the least bit hung over, don't do a task.


4.4  Water


This is a surprise, isn't it?  Water is a diuretic?  Here's how:

Mark McMurray, eager to avoid the dangers of dehydration, avidly and
steadily drinks water in various forms.  His stools are soft, his
mouth moist, he sweats easily, his urine is as pale as water and he's
hourly in the rest room or irrigating a bush.

What Mark doesn't know is that his kidneys must establish an osmotic
gradient within their substance, that body water conservation requires
a metabolic and hormonal shift that requires an hour or more to become
complete.  By keeping himself in a persistently over-hydrated state he
causes himself no injury, but after a few days puts himself in a
situation in which his biological momentum is entirely in the
direction of getting rid of excess water, requiring that he continue
to drink ardently.  And if he runs out of water, he'll have to become
somewhat volume depleted in order for his kidneys and endocrine system
to shift quickly into water-conservation mode.

So the risk to Mark is small, but by over-hydrating he does make
adjustment a little harder when he makes that off-field landing and
begins working to conserve his water supply.  Better that he should
keep a little color in his urine by not hydrating quite so single-
mindedly.  Remember what we said about water intoxication up above,
page two


4.5  Salt



Americans love salt!  Restauranteurs and food processors love it even
more, because salt enhances flavor, and flavor sells food.

This is a problem for the pilot trying to hydrate because the kidneys
are obligated to get rid of excess solute.  If they do not, our
obligation to preserve osmotic balance (via thirst) causes
hypervolemia.  Not a big problem to the healthy young pilot, but
possibly a troublesome one to the older pilot with a gristly heart and
stiff arteries.  Excess volume for these folks can cause high blood
pressure, swollen ankles, and even pulmonary edema.

Even for a healthy young pilot, excess salt can take two or three days
to dissipate.  Meanwhile you're drinking extra water to maintain
normal osmolality and peeing twice as much as usual to get rid of the
excess salt (obligatory solute excretion, if you care to know).

Other solutes can contribute to this problem, but salt is the main
one. The average American consumes about 10 grams of sodium daily; the
minimum daily requirement is more like 10 milligrams.  It's hard to
follow a diet, even with diligent effort, containing less that 2
grams.	So sodium deficiency is definitely not one of our worries.  As
we've observed earlier, water follows salt, so if we get faint from
excessive sweating, eating a little salty food and drinking water
guarantees that we'll restore lost volume.

But how is salt a diuretic?  Well, extra salt must be gotten rid of.
And the body uses water to carry away this salt.  The brick that you
passed while on the commode this morning was almost salt-free, I'm
sorry to say.  So for every dollop of extra salt that you eat, you
must also excrete an aliquot of water that you might have otherwise
used as sweat to keep yourself cool or to make spit to keep your mouth
moist, or to keep mucus moist so that the inside of your nose doesn't
crust up.

The summary is that eating extra salt causes extra urination, an
inconvenience during flight; it wastes water (as urine) that might be
better spent as sweat. It thus creates a requirement that you drink
extra water, making it harder to hydrate adequately just as caffeine
does.

Conclusion: you're best off avoiding salty food unless you, the non-
acclimated pilot, have been sweating heavily.  Salt creates a need for
excess water that is swiftly excreted to get rid of the extra salt and
it does nothing to protect your hydration status.


4.6  What to drink?


In a nutshell, water is the best beverage.  It doesn't contain
anything detrimental or unnecessary.

Juices have extra sugar, which is fine if you need the nutrition, and
have potassium, which may help solve the electrolyte problem.

Soda pop may contain sodium, which decreases its effectiveness as
hydration, though it might help replenish volume.  And it contains
carbon dioxide, which may result in a lot of bloating or burping
aloft, and may magnify the nausea of motion sickness.  Not a good
choice, don't you think?

Milk is nutrition, not effective hydration because the sodium content
is relatively high -- nearly the same as blood.

Soup is rarely useful for hydration; canned soups are very salty, and
most soups made in restaurants are essentially brine.  If you make
your own, without salty foods or salt, then the liquid might add to
your hydration status.

Beer and other alcoholic beverages should never be used before flight
because alcohol degrades brain performance; after flight their
diuretic effect will delay effective rehydration.  First quench your
thirst with water; then relax with wine or beer.


4.7  Fatigue as a Clue


Fatigue is one of those symptoms that drives doctors crazy, because
everything causes fatigue.  Here's the deal.  The important causes of
fatigue to the pilot are:
 -  hypoxia
 -  dehydration
 -  volume depletion
 -  hypoglycemia
 -  (hyperglycemia)
 -  exhaustion
The message is simple: if you're feeling tired, think very seriously
about getting out of the sky.  Real men know when to quit.  Then try
to fix it.  Check out the obvious:
 -  Is your oxygen system working?  If you're between 8,000 and 12,000
    ft msl without O2 (5000 ft msl or above if you're a smoker),
    consider hypoxia as a possible cause of your fatigue.
 -  Are you even a little thirsty? Take a drink of water, to make sure
    you aren't dehydrated.
 -  Eat a snack.  Calories provide fuel, and the electrolytes in food,
    plus water, provide volume.