^ SOARING RxBY DR. DANIEL L.JOHNSON

DehydrationWithout Thirst

When we cool off in climb, we lose bodyfluid: we lose volume, not merely water.Volume loss does not cause thirst until it'ssevere. With volume loss, we feel weak-ness and fatigue. Taking only water doesnot restore volume or the lost electrolytes(usually salt).

Summary

• Water/salt balance drives thirst• Most dehydration involves volume

loss. This is generally water + salt• Both water and salt must be

replaced• Dark urine

o Means that you could use morewater, because kidneys areconserving water

o Hard stools mean that you'rewater deficient

o Light urine means you haveenough water^ but no answer onvolume

• "Thirst" reflects only water deficit,but usually it's volume that'sneededo You'll feel bad until the volume

is replacedo Water-only rehydration is

dangerous with heavy sweating• A low-salt diet results in much

less urination, a convenience tothe pilot.

An ordinary soaring pilot, a malenamed Leroy, went to the airport ona lovely day. // would not matter if thepilot were Leticia because the physiologyis the same.

The story is true, but names arechanged.

It was a pleasant morning, so hewore light clothes. He spent about anhour-and-a-half assembling his gliderand doing light maintenance. Withthis activity, he sweated a little, justenough to make the band of his capwet. When he was ready to tow hisglider to the runway, he realized hewas thirsty, so he drank two tall glasses- about a pint - of water.

A technically-aware pilot, hechecked the temperature-dew pointspread. On one hand, he was keen-ly aware that he could estimate thecumulus cloud bases in thousands offeet above ground by subtracting thedewpoint, 35 °F, from the day's pre-dicted maximum temperature anddividing by 5.

He was not so keenly aware that,while he was flying near cloud base inthe shade, the temperature in his cock-pit would be roughly the temperatureof the ground dewpoint. He did real-ize that it would be cool aloft, so heput on a light jacket. He immediatelyfelt too warm, so felt satisfied. He hadno clear recollection of how it felt towear this jacket when it was 35 °F onthe ground.

It was indeed a fine day. Oncelaunched, it was not too difficult tofind good thermals. Cloud base wasactually about 2,000 ft higher thanforecast, and he idly noted that hisglider's thermometer showed thetemperature to be below freezing. Heclosed the vents.

When near cloud base, in cloudshadow, he felt quite cool, though hedid not feel cold and did not shiver.

The cockpit was pleasantly warm inthe sun between thermals and a littletoo warm when he got low and had toscratch for lift.

After about an hour and a half,while zipping along near cloud base,he realized that his feet felt a littlebit cold, and he also realized that hislower abdomen would feel much morecomfortable if he used the Depend-able cockpit-emissions garment thathe often wore but seldom used.

He had brought water, but didnt feelthirsty and so he didnt use it.

It was a fine day. He was able tofly about 4 hr, was able to completea modest triangle, and enjoyed a fastfinal glide back home.

He felt pretty good about the day,and in celebration made a beautifullyexecuted steep turn from downwindstraight to final. At the apex of theturn, he felt a little bit strange, butwasn't sure why.

There was a light crosswind, slight-ly gusty. Though he didn't bounce,he whacked the ground a little moresharply than he preferred, and thewings waggled a little bit more thanhe would've preferred in front of thegliderport spectators.

He drank some water, then disas-sembled his glider, put it in the box,and went home. All evening he felt

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quite thirsty and had drunk 6 or 8glasses of water by the time he wentto bed.

Have you lost water? Or volume?

• Thirst reflects water deficiencysensitivelyo A 1% increase of osmolality —

electrolyte concentration -results in thirst

o In practical terms, this is a lossof about 1% of body weight inwater. A pint is a pound, so 1%for a 200 Ib person is a 2 pintwater loss, almost a liter.

• Thirst reflects volume deficiencyless wello -10% decrease of blood volume

results in thirsto The average blood volume is

about 5 liters, so that's aboutone "unit" of blood (525 ml)

o Loss of volume causesweakness before thirst

o Salt lost with sweatingdelays thirst because itdecreases the osmolality dropthat would occur purely fromthe water loss.

In the morning, he passed a turdthat was somewhat firmer than usual.

What Happened to Leroy?First, we must clear up a common

misunderstanding.The ancient Greek word for water,

l)5cop (hydor), is the root of all thingshydro- in English and many otherlanguages. But in thinking about ourbodies, it's better to consider it tomean "liquid," "juice," or "soup."

In other words, we seldom lackpurely water because we often loseboth water and the stuff that's in itwithin our bodies. Our bodies consistof a variety of different soups that areencapsulated by membranes, in whichare proteins, salts, and cells, and heldtogether by sinews and bone.

We generally lose proteins and cellsalong with juice only when we bleed.We may lose protein with juice if wehave kidney disease. Otherwise we

always lose some salts (electrolytes)along with water.

Thirst is exquisitely sensitive to waterloss. There is a "salt appetite" that isrelatively insensitive.

When we lose bodily juice of anykind, and replace only water, we failto replenish volume, so drinking untilthirst is quenched fails to replace thevolume we've lost.

Quenching thirst shuts off the drivefor water when the proportion of wa-ter to salt in the blood has becomeideal - but if we've lost volume, sim-ply quenching thirst wrongly shuts offpart of the volume-preserving mecha-nism prematurely.

Salt-Water Balance:One-Directional

• With water loss or salt intake,fluids are more concentrated(higher osmolality), shrinkingcells

• Separately, this causes thirst:water-seeking and

• Release of: vasopressin -hormone

o Raises blood pressureo Increases water retention by

moving aquaporins into thecell membranes of the kidney'scollecting ducts

• There is no un-thirst when water isexcessive (salt is never deficient).

Types of body juices.Urine\e kidney excretes any salt/

electrolyte that's present in excess, andtransforms into electrolyte many wasteproducts to make them water-soluble.The kidney does excrete excess water,but must excrete some water in order toget rid of the electrolytes it produces.This is called the obligatory water loss.

Cooling off causes volume lossthrough urination.

Cold Diuresis: As the body cools, theveins under our skin in arms and legsconstrict to stop radiating heat, andthe arteries to our extremities constrictto reduce outward blood flow to pro-tect our core temperature.

This reduces the capacity of ourcirculatory system and increases thepressure in central veins. This is inter-preted by our body as "too much juice"and a signal is sent to our kidneys:"Too much juice onboard; get rid ofthe extra."

This is why we need to pee an houror two after getting cool (we don'thave to feel uncomfortably cold).

Important — no thirst here: This isbalanced water-electrolyte loss, sowe lose volume without thirst. It evenhelps to be a little volume-depletedwhen we cool down because there'sless inconvenient urination.

In case it's not obvious, we get introuble when we rewarm, because ourvasculature expands and we don't haveenough juice to fill it.

The time to drink sport drinks is whilewe are warming up — during descentin hot weather, in the recovery-saunain cold weather.

Years ago, I received a nice letterfrom a paraglider: "You saved my life!I didn't understand why my landingswere so sloppy in Death Valley afterflying at 17,000 ft. I lost one of myfriends in a bad landing there."

After-Chill: I received an email withanother aftermath of flying in cooltemps that said, "I spent most of the5 hr flying with Outside Air Temper-ature (OAT) ~8 °C (45 °F). I pulledthe glider about 30 ft (9 m) to thestaging area and walked to the bath-room. When I came out, I was shak-ing with cold. Why? It was obviouslymuch warmer on the ground than inthe cockpit."

Medically, this is called "reperfu-sion hypothermia."The pilot feels coolbut not cold lying in the sunny cock-pit, wiggling ankles and wrists for fivehours. During this time, the musclesreceive little blood flow and eventu-ally contain a fair amount of chilledblood. The pilot's core temperature hasbeen protected by the normal mecha-nisms that restrict circulation andso he merely feels cool, not realizinghow much his core temperature hasdecreased.

Then he hops out of glider and im-mediately uses his large muscles tomove about. This flushes the chilledblood out of the muscles of his legsand lower back, which circulates intohis core and drops his core tempera-ture just enough to trigger shivering.

This pilot then warmed up by usinghis muscles to disassemble his glider.This muscular metabolism generatedenough heat to lessen his hypother-mia. He did not become weak or faintbecause he had hydrated well duringthe flight.

Sweat: There are two kinds of sweat,the smelly kind (apocrine) that devel-ops with puberty and aids social dis-tancing, and the salty kind (eccrine)that exudes from skin everywhere forevaporative cooling when we get toowarm.

Sweat is a distillate, as it were, ofblood: Sweat glands exude water andelectrolytes, and the sweat gland'stubules recover much of the electro-lytes (sodium, potassium, and chlo-ride) as the sweat migrates along thetubule to the skin, for evaporativecooling.

How much salt we lose depends onthree things:

• The rate of sweat flow. When we'resweating heavily, there's less time torecover electrolytes.

• Our dietary salt intake: The sweatglands go into a salt-conserving stateif we follow an un-American low-sodium diet.

• Heat acclimation. With severaldays of continual exposure to heat,sweat glands go into a salt-conservingstate. (It takes only 24 hr in an air-conditioned environment to lose heatacclimation.)

The Sum of All Sweating is that wedo lose sodium and chloride when wesweat. If we Ve been sweating heavily,we need to replace both salt and water.

If we only take water, our thirst isquenched, but the lost volume isn't re-stored - so we feel good, but haven'tregained g-tolerance (for example).

Pounding water in response to heavysweating dilutes the blood and bodyelectrolytes severely, and has killedathletes.

Gastric juice: If you're sick and vom-iting, you won't be flying, because whowants to clean the cockpit, eh?

If you were sick yesterday, one of thereasons that you still feel crummy to-day is that you lost more than water.

Key to this point is that stomachjuice contains both protons and chlo-ride (hydrochloric acid). If we vomit,we lose acid. Read your favorite medi-cal physiology textbook to find outwhy the kidneys then excrete potas-

sium and bicarbonate.The loss of volume and acid increas-

es and prolongs the nausea, so the bestway to recover is to sip on fruit juicefor a few hours. 4 ounces an hour isenough to keep you safe and usuallycan be tolerated even with a queasystomach. Orange juice and grapefruitjuice are the two commonly availablefruit juices high in potassium.

The point here is to help you recoveras fast as possible so that you'll be safewhen you drive or fly.

Diarrhea: The colon (large bowel) isan emissions control device. The smallintestine, having absorbed the use-ful nutrients, sends a steady stream offluid into the colon.

The colon absorbs all the extra waterand electrolytes, the bacteria processthe indigestibles, and the result is asoft, compact mass, mostly bacteriawith food fiber.

When we have diarrhea, the colon isill and impaired, failing to absorb bothwater and electrolytes. We lose volume,and must replace volume.

We experience thirst with diarrheaonly if the volume loss is severe orwhen water is lost disproportionate tothe electrolytes.

The electrolytes lost are sodium,potassium, chloride, and bicarbon-ate. The proportions depend on the

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cause of diarrhea; the best treatmentfor this is to sip oral rehydration drinks,available at your drugstore, or to adda quarter-teaspoon of salt to a pint ormore of fruit juice.

Stool consistency is the most effec-tive way to judge your hydration strat-egy. If stools are firm, you simply didn'ttake enough water yesterday.

What Happened to Leroy?• He was sweating for an hour

before he took off and replaced onlywater, so he was a little short onvolume.

• His mild volume deficiency wasinvisible because his blood vessels con-stricted as he became cool after takeoff(also diminishing the cold diuresis).

• When he became cool aloft, heurinated frequently as his blood ves-sels contracted. This represented a bal-

anced volume loss of water and elec-trolytes, so he was not thirsty. He wasnot volume deficient while cool, and ifhe drank extra fluid in flight he wouldsimply have peed it out quickly.

• He was correct to not drink aloft:his vasculature was contracted, andhe would simply have urinated more.If we become thirsty, we should takewater, for thirst is a reliable sign thatthe proportion of water to electrolytesin the blood is awry. (Thirst does notreflect volume status reliably.)

• When he warmed up at loweraltitudes, his vasculature relaxed, andhe did not replace the volume he hadlost while urinating during flight. Thisreduced his g-tolerance, which madehim feel weird in a steep turn fromdownwind to final, itself perhaps asign that his volume loss had impairedjudgment. His awkward landing may

have been related to this.• He only drank water after he ex-

ited the glider. This relieved thirst, butdid not replace volume. He thoughthe was tired because he had had along flight. In fact, he had been lyingin a chaise longue wiggling his anklesand wrists, not considered a form ofexercise. He was fatigued because hewas volume depleted. Some peopleget headaches with volume depletion.Leroy is headache-immune.

• Significant volume depletion isassociated with thirst. In addition,Leroy had some salty snacks and saltyfood for supper. These reliably causethirst; no surprise he was thirsty allevening.

The fact that he did not pass a brickthe next morning means that theevening's thirst and avid fluid intakebrought his volume almost to normal.

Usefulness of Salt

If you've lost volume, you'd bestreplace volume (salt + water)If you drink water, when you needvolume:o Thirst is quenched quickly, buto Volume is not replaced, so that

- Antidiuretic hormonecontinues to be produced, and

- Excess water is retained,outbalances body salts,dilutes body water, andhinders brain cell function,etc.

If you eat salt, you gain volumeo Water must be imbibed to

maintain osmotic balance(concentration)

Use sport drink to replace sweatpound for poundo Replace 1/2 the loss immediately,

1/2 over the next half hour.

AcknowledgementsThank you to Christina Larson for

review and helpful suggestions on themanuscript, and to Doug Tarmichaelfor a helpful story.

Drawbacks

• 20% of people develophypertension from extra salt

• Salt in the diet replaces thepotassium that would protect forthe effects of high blood pressure

• Excessive need to urinate occurso All that extra salt is gotten rid of

only one way: peeo -50 milligrams of salt daily are

enougho Americans eat -5,000 milligrams

of salt dailyo Salt is slowly losto Increased volume is harmless to

healthy persons.• Geezers with bad hearts, kidneys, or

livers retain the volume and getedema, shortness of breath, andmuch more.

About the author: Daniel L. Johnsonis a retired internist (pediatrician foradults) who wrote the monthly column,SoaringRxyfrom 2010 to 2015, when heran out of gas. The collection, in pdffor-

Only eat fresh food; add no salto Minimize sweet peas, celery, and

milk - each has about 200 mgsodium per cup

Don't eat:o Cheese (it's preserved with salt)o Pickled stuff (pickling is soaking

in brine)o Processed meats - some shops

use sodium nitrite to keepmeat red

o Canned stuff (brine)o Snack foods and candy

(yes, candy)o Restaurants (unless you can

negotiate with the chef)

mat, is at tinyurl.com/drdanscolumnsHe is a mediocre soaring pilot with a

fine old sailplane, a Ventus Cm, with 27years, 1,200 flight hr, and 66 engine hron the clock.