3 Water Balance: We're Soup
Though Scripture does not point this out, when the Lord God formed Man
out of the dust of the earth, He added water, most likely before
breathing the Breath of Life into his nostrils, as the nostrils would
otherwise not have held together. Since then, the American Heart
Association has wrapped its hands tightly around jurisdiction of
mouth-to-mouth resuscitation, but has not required of God that He
annually re-certify to maintain His competence.
Hundreds of eminent biologists have devoted entire careers to working
in windowless laboratories to prove in tiny detail the exact manner in
which we are constructed of various solutes in a water solvent,
combined in an elaborate framework of enzymatically-controlled
reactants. While we are thankful for their detail, which makes modern
medicine possible, among other blessings like chicken soup and fried
eggs, we can boil down the requirements for optimal functioning of the
organism to just a few elements:
- Solvent: Water, in the right amount.
- Solute: Electrolytes, in proper proportion.
- Reactants: Fuel in various forms, ideally delicious.
- Electrons: Oxygen is our electron donor, for oxidative metabolism.
- Temperature: The proper ambient temperature is maintained, subject
to foolish interference, by oxidation of fuel, homeostatic
mechanisms, insulation, and a nice fireplace.
3.1 Hydration Status
The fact that we're about 85% solvent does not signify the extent of
the problem we face in maintaining optimal hydration status. The
important facts are that our expired air is fully humidified, that we
are required to make a certain amount of urine daily, and are covered
with millions of pores, each containing a little water pump that is
more responsive to our core temperature than to our hydration status.
We are little more than elaborately leaky sponges that must be
saturated with water to function well.
The bottom line is that we must steadily consume beverages in order to
keep the inhabitant of the sponge -- ourselves -- happy and well
coordinated; exactly what we consume and how we manage our
relationship to the energy status of our environment determines how
successful we are in maintaining our performance status. More simply,
dehydration hinders brain function, overhydration is inconvenient, but
safer.
Having said this, I do need to caution you that overhydration can be
taken too far: it's not only that you might flood the cockpit; your
kidneys can get rid of extra water only so fast. Under normal
circumstances your kidneys and sweat glands can get rid of water
faster than your stomach and intestines can absorb it. But sometimes
folks develop "defective water excretion" on account of disease or
medication, and then excess water can accumulate, diluting the solute
and eventually hindering brain function. This causes irrationality,
hallucinations, or even seizures (convulsions). This is called "water
intoxication," a bad name because it's much more subtle than
drunkenness and very difficult to recognize in ourselves or others.
3.2 Volume status
Hydration and fluid volume are related but different features of the
water-management problem.
- "Hydration" refers to how much water is in the soup;
- "volume" refers to how much soup there is.
This is important to the pilot and to the walker, because volume is
what maintains blood pressure, and without blood pressure we faint.
If you become dehydrated, your blood volume contracts and you will
tend to grey out when pulling g's, but you can quickly restore this
volume with water. If you become volume depleted, you must replace
solute (salts, essentially) as well as water in order to restore blood
volume. This is what made Gatorade a household word. But if Gatorade
were the only way to restore electrolytes they would not have to
promote it. (In fact, Gatorade contains too much salt.)
There are a number of things you can do to make management of your
fluid balance easier and yourself a safer pilot.
First, look for and recognize the signs in your body of proper fluid
balance. Mental mistakes and incoordination are late signs, not early
signs, of dehydration. Thirst is the best clue you have: if you are
thirsty, drink. Thirst is very imprecise; runners, for example, think
they are drinking about ten times as much water as they actually are.
On the other hand, people get in serious trouble only when they don't
respond to thirst or are forbidden access to water, such as by macho
football or wrestling coaches.
It is impossible to know exactly how much water and salt you are
losing. You can measure your urine easily, although no one does, but
we can't measure sweat, especially the part that evaporates. On the
other hand, it is possible to estimate your total water loss during
any period of time. All you need is a reliable (not necessarily
accurate) scale. A pint is a pound; a liter is a kilogram. With an
empty bladder, weigh yourself before flight, and weigh yourself
afterward. If you have lost weight, you have failed to maintain
normal hydration. If you have gained weight, you have overhydrated
(probably by taking in salt and water). Loss of more than 3% to 5% of
your body weight is associated with performance impairment. Three
percent for a 120-pound woman is 3.6 pounds; for a 210-pound man, it's
6.3 pounds.
If you have experienced dehydration, you will be aware of other, more
subtle symptoms that are partly specific to yourself.
Overhydration is mostly inconvenience: In a normal, 70-kg healthy
human, the kidneys are able to get rid of about 16 ml (.5 ounce) of
extra water per minute. In other words, if conditions are cool and
you are not working hard, you should not drink more than 2 pints of
fluid (1 quart, 1 liter) an hour above your losses. If you are
working hard in hot conditions, up to 16 liters a day of sweat can be
lost; this is more than one can keep up with by drinking. Eventually
you'll weaken and lie down to drink...
Frequent urination is a clue to overhydration. If we consume extra
water, our kidneys begin excreting it in about 15 minutes, and are
fully ramped up in about 45. After the body has adapted to having too
much water, it takes about four hours to revert to water conservation,
so that if you overhydrate and then stop drinking, you may become
mildly dehydrated while you shift into water-conservation mode. It's
just another reason to listen to your body's hints in order to stay
reasonably well hydrated and avoid extremes.
3.3 Water Balance
We ooze water: our sweat glands are never quite completely at rest,
our kidneys are obligated to make at least some urine to rid us of
soluble wastes, our exhaled breath is completely saturated with water,
our nose may run, we may weep. We're unaware of this constant water
loss; physiologists call it "insensible loss." We're pretty well
aware of making pee, and this plus the sweat that doesn't evaporate
are the "sensible" losses.
We need to replace the sensible plus the insensible losses in order to
maintain proper water balance and hydration. This is a guessing game,
because we can't measure sweat, and can't even estimate it when it
evaporates quickly in a dry, hot climate; and we don't bother to
measure urine volumes, as this would provoke gossip.
3.4 Clues: Thirst
Fortunately we have this little gizmo built into our brain, the
osmostat, that detects when we've lost water and generates an appetite
called "thirst." If you're overloaded with water, there's no thirst;
if you lack water, you gradually get more and more thirsty. You'll
seldom get dehydrated if you drink water when thirsty.
Can we be fooled? Yes, we can. Glad you asked. First, a dry mouth
often accompanies thirst, so in very dry air, those of us who are
mouth-breathers due to stress, allergies or colds get dry mouths long
before we need water. And folks taking medications that cause dry
mouth--none of which you should take prior to flight--are rarely
tricked by this into a condition of water intoxication. For this
reason elderly people with defective salivary secretion sometimes
become seriously over-hydrated, especially in the northern states in
winter. But dehydration does result in thick spit, so this is a clue
unless you're a dedicated mouth-breather.
Also, when it's very hot and you're working very hard--before or after
a flight, perhaps, but probably not during one--thirst is delayed. So
thirst does not perfectly reflect our hydration status and water
needs, but it is a reliable clue you can use to decide to drink.
People get in trouble from water deficits only when they don't have or
aren't allowed access to water when thirsty.
Thirst is a fairly reliable guide to dehydration, but responds late to
volume depletion. An injured person who bleeds heavily becomes
intensely thirsty, but we don't like to wait for this degree of volume
depletion before re-hydrating. The more sensitive part of your thirst
mechanism detects dehydration--the relative excess of sodium in the
blood that accompanies water loss--and generates a thirst sensation.
One reason that "thirst" is unreliable in adults is that we have
learned to suppress our visceral sensations and functions. It's worth
getting reacquainted with what thirst really is by experiencing it,
and watching it go away with drinking. Hunger, thirst, and dry mouth
are all closely related, but in my experience can be differentiated
adequately.
If you eat excess salt, you become thirsty until you take in enough
water to restore a normal electrolyte balance (normal osmolality), and
you are in a state of volume excess until your kidneys dispose of the
extra salt.
Moral: if you're thirsty, drink. If you're sweating, drink. If you
feel hot, drink. Trust your body on this.
3.5 What to Drink
Water works well. It's what you need, unless you've been exercising
hard. With vigorous exercise in hot weather, it may be impossible to
keep up with fluid and electrolyte loss by drinking. But while
pushing your glider to the line may qualify as "hard," glider flying
is mostly just "hot," and only while you're low. If we have good
weather, as soon as we find lift we can get into the air-conditioned
heights. So water is sufficient to preserve volume most of the time.
But there are alternatives to water, especially after the flight.
Let's briefly review a few. Let's call fluids that are equivalent to
water, "beverages."
Milk: Milk is food, not "hydration," as it contains a lot
of salt (sodium, actually). With 125 mg sodium per
1-cup serving (290 mEq per liter), it is nearly
"isotonic" with body fluids. So it will replace
"volume" but will not make up a water deficit. It
tastes good, a cold glass feels refreshing, but don't
try to hydrate with it. Milk is not a beverage as
we've defined it here.
Coffee: Caffeine is a diuretic, meaning that all caffeinated
beverages, including coffee, tea, and soda get rid of
most of the fluid in which you drink it. "Decaf"
coffee is not caffeine-free, but contains 17-25% as
much caffeine as regular coffee. Caffeinated drinks
are stimulants, not "beverages."
Pop: Before flight, the fizz will fill your stomach with
carbon dioxide gas just before you take flight. Gas
expands at altitude. If you have any trouble burping
this up, you will get bloated or airsick.
Many soft drinks have quite a bit of sodium, even
though they no longer are made with "soda." Sodium,
in large amounts is a diuretic. This is not a
concern if you've been sweating heavily, as you may
need the sodium.
Sport drinks: These are divided into high-sodium (Gatorade) and
medium-sodium/high-potassium versions. Gatorade has
110 mg sodium per 8 oz, about half as much as milk,
and a lot more than is needed. Extra sodium just
increases the amount of urine you have to make.
Other sport drinks have about 50 mg sodium per 8 oz,
a better level for most people. But they do have
about 50 mg potassium also. If you have kidney
disease and have been warned by your doctor to reduce
potassium, or if you have high blood pressure and
take a "potassium-sparing diuretic," you should avoid
these.
Juice: Fruit juices contain sugar and moderate amounts of
potassium. Because of this, they aren't quite as
effective as water, and do have some food value. The
electrolytes in juice are not as precisely measured
as sport drinks. But fruit juices are beverages that
can replace water.
Tomato juice and V8 juice are very salty, are not
fruit juices, and should not be used for hydration.
3.6 Sweat
Sweat (sudoresis, diaphoresis, or perspiration) is a clue to the
success of your hydration efforts and the aggressiveness with which
you need to hydrate. Whether you are acclimated influences what type
of solution you should use to rehydrate.
Sweating is annoying and inconvenient. It runs onto our glasses and
down our chin. When we close the canopy, it condenses and fogs both it
and our glasses. It corrodes metal. And it makes us itch. We're
grateful when it stops. Unfortunately this gratitude may be
misplaced, as on a hot day sweating may stop (or seem to stop because
it's diminished) simply because the tank is low.
So on a hot day, to sweat is a sign that you're well hydrated. And if
you feel a little warm and are not sweating, you're getting in
trouble!
You lose a little salt along with sweat, especially when sweating
profusely or if you, like me, spend a lot of time in an air-
conditioned office and have not become acclimated to heat.
Acclimatization involves several slow adjustments of our physiology,
one of which is for the sweat glands to conserve salt. On a dry, warm
day, when sweat evaporates rapidly, the non-acclimated person will get
a little crusty from the salt left behind, and the dog will be happier
when he licks your hand.
A non-acclimatized person's sweat is about 0.3% salt. (Blood is 0.9%
salt.) After 3 or 4 days of continual exposure to heat, a person's
sweat is about 0.03% salt -- 1/10th as salty -- and the acclimatized
person is able to produce a greater volume of sweat as well. So the
non-acclimatized person shouldn't eat a low-sodium diet while
acclimatizing. In these days of air-conditioned cars and offices and
homes, nearly every pilot is non-acclimatized. And acclimatization is
lost in just two days.
When sweat is salty, you lose volume, not just water. You may not get
normally thirsty because the osmolality of your blood changes little,
and instead you simply feel worn out. Which you might expect at the
end of a long flight anyway, and fail to realize that your chief need
is not sleep or rest but a long draught. The amounts of salt actually
lost by a person who's not acclimated is small -- there's no need to
eat potato chips and corned beef or bologna.
The sweat of an acclimated person, or the person sweating slightly, is
not very salty. Thirst tends to occur more appropriately, and the
chief effect of such sweating is dehydration. But in acclimatized
people thirst is actually delayed, for unkown reasons, until about 3%
dehydration occurs.
In addition, if people try to keep ahead of sweating by drinking water
avidly, their stomach tends not to empty properly, and they just get
bloated or nauseated. It's better to wait for a little thirst,
perhaps an hour or two.
So if you are thirsty, drink water. If you are drenched with sweat or
your skin is getting crusty, drink sport drinks or have some chips
with your water. Or dilute tomato juice of V8 half and half with
water.
Our bodies lose about 50 ml/hr of sweat as an obligatory minimal
amount; just enough, I suppose, to keep the pumps in shape and the
pores open. On a hot day and with vigorous physical activity up to
1600 ml/hr can be produced. That's right, folks. 1.6 liters/hour.
Fortunately piloting aircraft is not a vigorous physical activity, but
sometimes it's quite a warm one. So bear this range in mind.
Now, let's combine this with the fact that the kidneys can't get rid
of more than 16 ml of extra water each minute (.9 liter/hr). This
implies that 2.5 liters of fluid per hour should be a reasonable upper
limit for fluid consumption in hot weather. This is about a pint
every 12 minutes, more than the stomach can absorb. I recommend that
you measure what you bring or buy so that, in hot weather, you have a
couple of liters for every hour you will be working in hot conditions,
and then respond to your thirst.
What is the minimum water consumption? We must replace the water
vapor lost in breath and obligatory sweating, and the kidneys must
excrete a minimal amount of waste solute that must be carried out with
water. Altogether, this is somewhere between 750 and 1000 ml daily:
three to four cups of water a day.
Beyond this, we must replace what's lost in sweat, and the difficulty
of accurately estimating this has led to all sorts of anxiety, mostly
about other people, and various oversimplified rules on how much fluid
pilots should drink. I repeat my advice: have more water available, in
some form, than you think you'll need, and let thirst be your guide.
Leeway
Sometimes people talk as if staying properly hydrated is difficult.
The reality is that not only do we have thirst to protect us from
ignorance, we also have the luxury of very adaptable kidneys.
Ignoring sweating, we can survive in vibrant good health on .5 to 20
liters of water daily, a huge range. There's lots of leeway, and you
really aren't likely to get in trouble unless you skimp on your water
supply and become unable to slake your thirst.
3.7 Water conservation
Your body does have several ways in which it tries to conserve water.
The most obvious is that all body secretions diminish as you dry up.
One clue of mild dehydration, for example, is whether you need to take
a sip of your beverage in order to chew your toast comfortably.
Ardent spit production means you're well hydrated.
The colon, an emissions-control device, has as one of its main
functions the removal of water from its contents. It receives twenty
liters a day of watery post-digestion fluid, nearly all the nutrients
removed, and reabsorbs the water and most of the electrolytes.
Cholera prevents this reabsorption, and causes death in hours through
volume depletion. (And as it causes loss of water in excess of sodium
is associated with weakness more than thirst.) "Food poisoning" and
other causes of diarrhea are conditions in which actually the colon is
not working rather than being overactive.
One clear sign of dehydration is firm stools. In my part of the
country, hot weather arrives in June. People don't hydrate until they
start sweating or feel hot and thirsty. But before that happens, the
weather turns comfortably warm and they perspire insensibly. They
don't get dehydrated or volume depleted because the colon faithfully
extracts every last drop of water from the stool to protect life. The
first firm clue that the weather has turned warm comes the next
morning when they try to expel the brick that was manufactured to
prevent thirst. For the pilot this provides an annoying check
regarding whether, on the average, your hydration attempts have been
adequate. If your stools are soft, your water balance is OK; if they
are firm, drink more, not harder.
Your kidneys are designed to regulate blood volume. It is their job
to get rid of extra stuff and conserve scarce stuff. Sometimes water
is the "extra stuff" and sometimes it's the scarce stuff. Healthy
kidneys can conserve water to the extent of making only about 1/4
liter of urine daily or can get rid of extra by making about 20 liters
a day.
A biological mystery not yet solved is the origin and purpose of
urochrome, the chemical that makes urine yellow. As a constant amount
of this stuff is made, the intensity of color varies with how
concentrated the urine is. This provides another clue to the success
of your hydration efforts: if your urine is dark yellow, drink
harder; if it is pale, you're doing fine. The kidneys react very
quickly to your volume status, so if you pee just before a flight you
can judge immediately whether you've hydrated well enough.