Water and Electrolytes - ACS Symposium Series (ACS Publications)

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8 Water and Electrolytes John E. Greenleaf and Michael H . Harrison Laboratory for Human Environmental Physiology, Biomedical Research Division, NASA Ames Research Center, Moffett Field, CA 94035

Under optimal conditions humans have been able to survive about 10 minutes without oxygen, up to 18 days without water, but nearly 60 days without food. Despite the fact that oxidation of nutrients produces water, there is a longer lasting supply of foodstuffs within the body than water. Part of the fatigue mechanism occurring during physical exercise can be attributed to fluid loss (dehydration) from the body and possibly to fluid-electrolyte shifts within the body. There is no adaptation to successive periods of dehydration and the performance of the strongest and fittest people will deteriorate rapidly with dehydration. When compared with the daily variability of many physico-chemical parameters, the least variability is found in body temperature, and plasma sodium, chloride, calcium, and osmolality (1). There is a close association between thermoregulation and the sodium, calcium, and osmotic concentration of the extracellular fluid; increases in plasma sodium concentration (hypernatremia) and plasma osmotic concentration (hyperosmotemia) tend to increase body temperature while hypercalcemia tends to decrease body temperature (2-4). The precise control of the concentration of these ions suggests that their functions are of major importance for optimal physiological homeostasis and s u r v i v a l of the organism. In t h i s paper we w i l l discuss the anatomy of the f l u i d spaces i n the body, the f l u i d s h i f t s and losses during exercise and their effects on performance, and t h i r s t and drinking during exercise with comments on carbohydrate ingestion. Anatomy of Body F l u i d Compartments Total body water i s a r b i t r a r i l y divided into that contained within c e l l s ( c e l l u l a r ) and that located outside the c e l l s ( e x t r a c e l l u l a r ) . The e x t r a c e l l u l a r water i s further divided into that contained within the vascular system excluding the erythrocytes (plasma), and that located outside the vascular system and outside the c e l l s ( i n t e r s t i t i a l f l u i d ) (Figure 1 ) . This chapter not subject to U.S. copyright. Published 1986, American Chemical Society

Layman; Nutrition and Aerobic Exercise ACS Symposium Series; American Chemical Society: Washington, DC, 1986.


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NUTRT IO IN AND AEROBC I EXERCS IE

Approximate volumes and p e r c e n t o f body weight o f t h e v a r i o u s f l u i d compartments, and a d a i l y water b a l a n c e o f a r e s t i n g 80-kg man ( 5 ) , a r e g i v e n i n T a b l e s I and I I . W i t h e x e r c i s e , t h e sweat l o s s and beverage ( f l u i d ) i n t a k e would be g r e a t e r . O x i d a t i o n o f 1 gram o f v a r i o u s f o o d s t u f f s would y i e l d t h e f o l l o w i n g approximate water p r o d u c t i o n ( 5 ) : monosaccharides ( g l u c o s e ) = 0.6 g, d i s a c c h a r i d e s ( s u c r o s e ) = 0.6 g, s t a r c h = 0.6 g, f a t ( l a r d ) = 1.1 g, and p r o t e i n = 0.4 g. The preformed volume o f water would v a r y w i t h t h e q u a n t i t y and t y p e o f f o o d s t u f f m e t a b o l i z e d ( 5_). I t s sources are the p o l y m e r i z a t i o n o f g l u c o s e , c o n d e n s a t i o n o f amino a c i d s , e s t e r i f i c a t i o n o f f a t s ( g l y c e r o l ) , h y d r a t i o n o f p r o t e i n , and bound water (water of a s s o c i a t i o n ) ; t h e l a t t e r s o u r c e i s 1 g o f p r o t e i n a s s o c i a t e d w i t h 3 g H 0 , 1 g n e u t r a l f a t w i t h 0.1 g H 0 , and 1 g g l y c o g e n w i t h 2.7 g 2

2

H2O.

Table I .

F l u i d Compartment Volumes o f a R e s t i n g 80 kg Man*

Compartment Extracellular : Plasma Interstitial Cellular Total

Volume, l i t e r s

Body w e i g h t , %

4 19 30

5 24 37

53

66

^ M o d i f i e d from ( 5) . Table I I . D a i l y Water Balance o f a R e s t i n g 80 kg Man* Weight, grams

Percent

Input , Beverage Food water ( l i q u i d ) O x i d a t i o n water ( m e t a b o l i c ) Preformed water ( m e t a b o l i c )

1200 1000 250 50

48 40 10 2

Total

2500

100

Output U r i n e water I n s e n s i b l e water (vapor) F e c a l water Sweat

1400 900 200 0

56 36 8 0

Total

2500

100

Water b a l a n c e ( i n p u t - o u t p u t )

0

* M o d i f i e d from ( 5 ) . Depending upon t h e q u a n t i t y o f f a t i n t h e body, body water i n normal h e a l t h y p e o p l e comprises 50$ t o 70% o f t h e body w e i g h t . The h i g h e r t h e p e r c e n t a g e o f l e a n body mass, t h e h i g h e r t h e p e r c e n t a g e o f


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109

water because l e a n mass (muscle) c o n t a i n s more water than f a t t i s s u e (Table I I I ) . I n the g e n e r a l p o p u l a t i o n , t h e body water c o n t e n t a v e r ages about βΐ % (_6_). The h a l f - l i f e o f body water m o l e c u l e s i s about 12 days (1); t h e t o t a l water volume i s r e g u l a t e d d a i l y t o w i t h i n ±0.22% (±150 g) o f the body weight (8), and plasma volume t o w i t h i n ±0.7% (±25 g) (9_).


Table I I I .

Weight and Water Content o f Body T i s s u e From a 70.6 kg Man*

Tissue S t r i a t e d muscle Skeleton Adipose t i s s u e Skin Lungs Liver B r a i n and s p i n a l c o r d Alimentary t r a c t Alimentary t r a c t contents Heart Kidneys Spleen Pancreas Bile Teeth Hair Remaining t i s s u e s Liquid Solid T o t a l Body

Percent of body weight 31.6 14.8 13.6 7.8 4.2 3.4 2.5 2.1 0.8 0.7 0.5 0.2 0.2 0.2 0.1 0.1

Percent water content 79.5 31.8 50.1 64.7 83.7 71.5 73.3 79.1

3.7 13.5

93.3 70.4

100.0

67.2

73.7 79.5 78.7 73.1 5.0

* M o d i f i e d from ( 6 ) . Except f o r r e s p i r a t o r y and dermal i n s e n s i b l e water-vapor l o s s e s , a l l r e m a i n i n g water l o s t by the body c o n t a i n s e l e c t r o l y t e s , m a i n l y sodium and c h l o r i d e . The normal c a t i o n and a n i o n c o n s t i t u e n t compo s i t i o n of the f l u i d spaces i s g i v e n i n T a b l e IV. I n the e x t r a c e l l u l a r f l u i d space, sodium i s the major c a t i o n and c h l o r i d e the major a n i o n . Those two i o n s c o n s t i t u t e 95% o f the e x t r a c e l l u l a r f l u i d o s m o l a l i t y . Changes i n plasma sodium c o n c e n t r a t i o n r e f l e c t changes i n e x t r a c e l l u l a r f l u i d volume. Potassium i s t h e major c e l l u l a r c a t i o n and phosphates and p r o t e i n s comprise the major a n i o n s . The t o t a l c e l l u l a r o s m o l a l i t y (175 + 135 = 310 mosmol/kg H 0) i s e q u a l t o the t o t a l e x t r a c e l l u l a r o s m o l a l i t y (155 + 155 = 310 mosmol/kg H 0 ) ; t h e r e f o r e , e q u a l t o t a l o s m o t i c c o n c e n t r a t i o n s a r e m a i n t a i n e d between two f l u i d compartments o f w i d e l y d i f f e r e n t i o n i c c o n t e n t s ( T a b l e I V ) . 2

2

E x e r c i s e and Body Water E x e r c i s e has two s p e c i f i c e f f e c t s on body w a t e r . F i r s t , i t a l t e r s the d i s t r i b u t i o n of w a t e r , c o l l o i d s ( p r o t e i n ) , and c r y s t a l l o i d s


110

NUTRT IO IN AND AEROBC I EXERCS IE T a b l e IV.

Normal C o m p o s i t i o n o f F l u i d Spaces i n Man

Other


F l u i d space

Extracellular Cellular Total

Na

+

K

+

Ca

+ 2

Other"

+

• Mg

+ 2

0smols

+

Cl"

HC0~

3

P0jj

3

+ PRO"

Osmols"

mEq/1

mEq/1

mEq/1

mosmol/kg

mEq/1

mEq/1

mEq/1

mosmol/kg

142 10 152

5 145 150

8 20 28

155 175 330

103 2 105

27 8 35

25 190 215

155 135 290

(ions) within the body f l u i d compartments, Secondly, i f sweat i s l o s t that i s not replaced by f l u i d intake, the r e s u l t i s a decrease in t o t a l body water content which, i n hot environments, may be of s u f f i c i e n t magnitude to reduce markedly the capacity for prolonged exercise (10,11, Figure 2). F l u i d S h i f t s with Exercise. During exercise some plasma water i s l o s t (shifted) from the vascular compartment to the i n t e r s t i t i a l compartment and to the c e l l u l a r compartment of the active muscle (12,13); at the same time f l u i d i s s h i f t e d at a lower rate from the i n t e r s t i t i a l compartment of inactive muscle to the vascular space (14). The r e s u l t i s an absolute loss of plasma water (and e l e c t r o lytes) that i s d i r e c t l y proportional to the i n t e n s i t y of the exercise (Figure 2). These transcompartmental f l u i d s h i f t s occur as a r e s u l t of alterations i n the balance of osmotic and hydrostatic forces acting along and across the c a p i l l a r y networks of a l l tissues whose blood flow i s altered by exercise; e.g., muscle, skin, kidney, gut, and l i v e r . For exercise (cycling) performed i n a seated position, the f l u i d balance favors net c a p i l l a r y f i l t r a t i o n which r e s u l t s i n a reduction of plasma volume or hemoconcentration (15). For exercise performed i n an upright position such as running, the hemoconcentration i s often minimal (16) because the act of standing causes a subs t a n t i a l hemoconcentration; edema-preventing mechanisms, such as increased i n t e r s t i t i a l f l u i d pressure, act to reduce the potential for further hemoconcentration (17,18). Exercise at an i n t e n s i t y above 50% of peak working capacity i s usually accompanied by increased concentrations of plasma e l e c t r o l y t e s ; sodium, chloride, and especially potassium, with an accompanying increase i n osmolality. There i s , however, l i t t l e change i n plasma e l e c t r o l y t e and osmotic concentrations at exercise l e v e l s below 50? of the peak working capacity because the plasma f i l t r a t e i s isotonic with respect to e x i s t i n g plasma t o n i c i t y (19-21). At exercise levels above 50? of peak capacity, there i s an exponential increase i n plasma sodium and osmolality that i s associated with the linear decrease i n plasma volume (Figure 3) (20). Apart from an increase i n plasma-potassium concentration induced by the moderate muscular contraction, which may be augmented by the breakdown of glycogen to glucose (glycogenolytic a c t i v i t y ) i n active muscle, the


Water and

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11


CELLULAR FLUID

INTERSTITIAL FLUID

ONCOTIC-OSMOTIC PRESSURE HYDROSTATIC PRESSURE

F i g u r e 1. C e l l u l a r and e x t r a c e l l u l a r (plasma and f l u i d compartments.

PLASMA FLUID

interstitial)

F i g u r e 2. E f f e c t o f d r i n k i n g water on r e c t a l temperature l e v e l s d u r i n g t r e a d m i l l e x e r c i s e (37.7°C d r y - b u l b temperature and 35-45? r e l a t i v e h u m i d i t y ) i n one s u b j e c t . From r e f . 49 w i t h permission.



112


e l e v a t i o n s i n plasma e l e c t r o l y t e and o s m o t i c c o n c e n t r a t i o n s appear t o be a r e s u l t of the t r a n s c o m p a r t m e n t a l f l u i d s h i f t s . I f moderate d e h y d r a t i o n i s added t o the s t r e s s of moderate e x e r c i s e , e s p e c i a l l y when performed i n a hot environment, the e l e v a t i o n i n plasma sodium and o s m o t i c c o n c e n t r a t i o n s can be s u f f i c i e n t t o i m p a i r t e m p e r a t u r e r e g u l a t i o n (2,4). F l u i d s h i f t s d u r i n g e x e r c i s e o c c u r m a i n l y as a r e s u l t of i n c r e a s e d c a p i l l a r y f i l t r a t i o n from the v a s c u l a r compartment t o the i n t e r s t i t i a l space caused by the i n c r e a s e s i n h y d r o s t a t i c and s y s t e m i c b l o o d p r e s s u r e s (13,21,22), w i t h a s s i s t a n c e from the i n c r e a s e d t i s s u e o s m o l a l i t y (14,23) r e s u l t i n g from e l e v a t e d muscle metabol i s m . The l a t t e r would t e n d t o draw i n t e r s t i t i a l f l u i d i n t o the muscle c e l l s (13) and, i n c o n j u n c t i o n w i t h the s h i f t of water from i n a c t i v e muscle, would i n c r e a s e t i s s u e t o t a l p r e s s u r e ( F i g u r e 1 ) . The r e v e r s e f l u x of f l u i d from the i n t e r s t i t i a l t o the v a s c u l a r space (14) i s caused by i n c r e a s e d i n t e r s t i t i a l f l u i d p r e s s u r e (12) and i n c r e a s e d plasma p r o t e i n c o n c e n t r a t i o n ( o n c o t i c p r e s s u r e ) , hyperosmotemia, or b o t h depending upon the i n t e n s i t y (above or below 50?-peak c a p a c i t y ) and d u r a t i o n o f the e x e r c i s e . I n c r e a s e d i n t e r s t i t i a l h y d r o s t a t i c p r e s s u r e and i n c r e a s e d plasma o s m o t i c p r e s s u r e s r e t a r d the f l u i d s h i f t from plasma t o the i n t e r s t i t i u m . Equilibrium i s r e a c h e d when i n t e r s t i t i a l p r e s s u r e b a l a n c e s c a p i l l a r y f i l t r a t i o n p r e s s u r e ( 2 4 ) . A f t e r c e s s a t i o n o f e x e r c i s e , r e s t i t u t i o n of plasma volume t a k e s 40-60 minutes (21,22) u n l e s s s i g n i f i c a n t d e h y d r a t i o n i s present. The immediate p o s t - e x e r c i s e hyperosmotemia, the r e l a t i v e h y p e r p r o t e i n e m i a , and the r e d u c t i o n i n s y s t e m i c b l o o d p r e s s u r e cont r i b u t e t o the r e s t o r a t i o n o f plasma volume. The r e d u c t i o n i n b l o o d p r e s s u r e , which produces a f a l l i n l o c a l h y d r o s t a t i c p r e s s u r e w i t h i n the c a p i l l a r i e s of the p r e v i o u s l y a c t i v e muscle, i s p r o b a b l y the s i n g l e most i m p o r t a n t f a c t o r . Consequences of Sweating. Sweating o c c u r s d u r i n g moderate e x e r c i s e l e v e l s i n the c o l d as w e l l as a t h i g h e r e n v i r o n m e n t a l t e m p e r a t u r e s . At low ambient t e m p e r a t u r e s a g r e a t e r p o r t i o n of the m e t a b o l i c heat p r o d u c t i o n (depending upon e x e r c i s e i n t e n s i t y and c l o t h i n g ) i s d i s s i pated by c o n v e c t i o n and r a d i a t i o n and a minor p o r t i o n by e v a p o r a t i o n of sweat and r e s p i r a t o r y w a t e r . As ambient temperature r i s e s , the p o r t i o n o f heat d i s s i p a t e d by c o n v e c t i o n and r a d i a t i o n d e c r e a s e s p r o g r e s s i v e l y i n c o n c e r t w i t h a p r o p o r t i o n a l i n c r e a s e i n the r a t e of s w e a t i n g and e v a p o r a t i v e heat l o s s . The c o o r d i n a t i o n o f the r a t e of heat l o s s between c o n d u c t i o n , r a d i a t i o n , and e v a p o r a t i o n i s so p r e c i s e t h a t , f o r ambient d r y - b u l b t e m p e r a t u r e s between 5°C and 29°C, the e q u i l i b r i u m l e v e l of c o r e ( r e c t a l ) temperature i s r e l a t e d d i r e c t l y t o the i n t e n s i t y of the e x e r c i s e l o a d and i s independent of e n v i r o n m e n t a l temperature ( 2 5 ) . I n c o o l environments the i n c r e a s e i n m e t a b o l i c heat p r o d u c t i o n and c o r e temperature d u r i n g e x e r c i s e can be c o n s i d e r e d as an i n t e r n a l t h e r m a l s t r e s s . The f l u i d s h i f t s t h a t o c c u r d u r i n g e x e r c i s e i n warm and hot environments are m o d i f i e d by what can be c o n s i d e r e d as an a d d i t i o n a l e x t e r n a l t h e r m a l s t r e s s . Even under i d e a l ( c o o l ) c l i m a t i c c o n d i t i o n s e x e r c i s e i s a n t i h o m e o s t a t i c , h a v i n g the c a p a b i l i t y of imposing s i m u l t a n e o u s s t r e s s e s upon n e a r l y a l l the body's r e g u l a t o r y systems. P r o l o n g e d e x e r c i s e performed i n hot c o n d i t i o n s imposes a p a r t i c u l a r l y s e v e r e s t r a i n on the c a r d i o v a s c u l a r system which must p r o v i d e not o n l y f o r t h e m e t a b o l i c r e q u i r e m e n t s of the w o r k i n g



8.


113 Water and

Electrolytes

m u s c l e s , but a l s o f o r t h e d i s s i p a t i o n o f m e t a b o l i c and e n v i r o n m e n t a l heat v i a r e g u l a t i o n o f t h e cutaneous c i r c u l a t i o n . Under t h e c i r c u m s t a n c e o f g r e a t i n t e r n a l and e x t e r n a l t h e r m a l s t r e s s , t h e dominant t h e r m o r e g u l a t o r y mechanism i s t h e p r o d u c t i o n and e v a p o r a t i o n o f sweat. For each m i l l i l i t e r o f sweat e v a p o r a t e d , 2.4 k i l o j o u l e s (0.58 k c a l ) o f heat a r e l o s t from t h e body. D u r i n g marathon runs i n the h e a t , sweat r a t e s may approach 2 l i t e r s per hour ( 2 6 ) ; i f t h e body water l o s t i s n o t r e p l a c e d , d e h y d r a t i o n o c c u r s . Even i n c o o l environments, where c o n v e c t i o n and r a d i a t i o n a r e t h e main avenues o f heat d i s s i p a t i o n , s w e a t i n g may s t i l l r e s u l t i n s i g n i f i c a n t dehydra t i o n because o f t h e i n c r e a s e i n c o r e t e m p e r a t u r e . T h e r e f o r e , appro p r i a t e f l u i d i n t a k e i s an i m p o r t a n t r e q u i r e m e n t d u r i n g p r o l o n g e d e x e r c i s e performed i n c o o l environments as w e l l as i n t h e h e a t . Q u a n t i f y i n g " p r o l o n g e d " and " h e a t " i s d i f f i c u l t , s i n c e t h e two a r e i n t e r a c t i v e . The heat produced by metabolism d u r i n g i n t e n s e e x e r c i s e can exceed t h e a b s o r p t i o n o f extreme e n v i r o n m e n t a l heat by a f a c t o r of seven ( 2 7 ) , but t h i s l e v e l o f metabolism can be s u s t a i n e d f o r o n l y s h o r t p e r i o d s o f t i m e . Rates o f m e t a b o l i c heat p r o d u c t i o n e x c e e d i n g 600 W a t t s , which i s t h r e e times a s e v e r e e n v i r o n m e n t a l heat l o a d , can be s u s t a i n e d f o r s e v e r a l hours by endurance a t h l e t e s . A wet-bulb g l o b e - t e m p e r a t u r e (WBGT) i n d e x [ ( 0 . 7 χ wet-bulb temp.) + (0.2 χ b l a c k g l o b e temp.) + (0.1 χ d r y - b u l b temp.)] g r e a t e r than 18°C (65°F) p r o v i d e s a c o n d i t i o n f o r a p o t e n t i a l r i s k o f heat i n j u r y d u r i n g e x e r c i s e . Thus, t h e more i n t e n s e and p r o l o n g e d t h e e x e r c i s e , t h e lower the s a f e WBGT i n d e x . Sweat i s composed o f water and many s o l i d s u b s t a n c e s , m a i n l y t h e e l e c t r o l y t e s sodium, p o t a s s i u m , and c h l o r i d e ( 2 8 ) . W h i l e l o s s o f water and t h e e n s u i n g i n c r e a s e i n t o t a l body d e h y d r a t i o n may become a m e d i c a l problem, c o n t r a r y t o p o p u l a r b e l i e f , t h e accompanying l o s s o f e l e c t r o l y t e s does not c o n s t i t u t e a problem under most e x e r c i s e and e n v i r o n m e n t a l s i t u a t i o n s as l o n g as f o o d consumption i s normal. Sweat i s much more d i l u t e ( h y p o t o n i c ) than plasma (sweat = 0 . 4 ? s o l u t e , plasma = 0.9? s o l u t e ) . T h i s h y p o t o n i c i t y i n c r e a s e s i n sub j e c t s who have undergone e x e r c i s e t r a i n i n g i n t h e heat ( a c c l i m a t i z a t i o n ) (29). Consequently, sweating d u r i n g e x e r c i s e r e s u l t s i n i n c r e a s e s i n plasma e l e c t r o l y t e and o s m o t i c c o n c e n t r a t i o n s s i n c e p r o p o r t i o n a l l y more water t h a n s a l t ( e l e c t r o l y t e ) i s b e i n g l o s t i n the sweat. But i n t r a v a s c u l a r e l e c t r o l y t e c o n t e n t i s a l s o b e i n g decreased by l o s s e s i n sweat. Once s w e a t i n g c e a s e s , and any body water d e f i c i t i n c u r r e d i s r e p l a c e d by d r i n k i n g pure w a t e r , t h e r e s u l t i n g i n t r a v a s c u l a r e l e c t r o l y t e c o n c e n t r a t i o n w i l l be decreased from t h e p r e s w e a t i n g l e v e l u n l e s s a d d i t i o n a l e l e c t r o l y t e s a r e con sumed. The b e s t time t o r e p l a c e e l e c t r o l y t e s l o s t d u r i n g e x e r c i s e i s a f t e r e x e r c i s e ceases because i n g e s t i o n o f e l e c t r o l y t e s d u r i n g e x e r c i s e w i l l add t o t h e e x i s t i n g e x e r c i s e - i n d u c e d h y p e r o s m o l a l i t y . D r i n k i n g c o l d o r warm water d u r i n g e x e r c i s e i s more e f f e c t i v e i n a t t e n u a t i n g t h e r i s e i n c o r e temperature than d r i n k i n g an e q u a l volume b e f o r e e x e r c i s e o r p r o v i d i n g a r t i f i c i a l sweat by sponging t h e body w i t h water d u r i n g e x e r c i s e ( 3 0 ) . I n a d d i t i o n , h y p e r n a t r e m i a and h y p e r o s m o l a l i t y tend t o i n h i b i t s w e a t i n g and e v a p o r a t i v e heat l o s s (as does wet s k i n ) and a c c e n t u a t e t h e a l r e a d y e l e v a t e d core tempera t u r e (2,4,31). There a r e c i r c u m s t a n c e s when some e l e c t r o l y t e replacement i s n e c e s s a r y , f o r example d u r i n g r e p e a t e d bouts o f s t r e n u o u s e x e r c i s e



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performed d a i l y for many consecutive days, or when physical work or exercise i s performed continuously over a 12- to 24-hour period and adequate rest periods and meals are not a v a i l a b l e . Muscle cramping i s a common response to s a l t (NaCl) depletion which usually can be prevented by increasing s a l t intake. Heat cramp and the syndrome of s a l t and/or water depletion heat exhaustion are commonly the result of inappropriate l e v e l s of heat acclimatization and physical f i t n e s s (32). The enhanced a b i l i t y to conserve body sodium i n sweat and urine are major adaptive responses to heat acclimatization, and excessive e l e c t r o l y t e depletion i s usually a problem only during the f i r s t few days of work i n the heat. Further, i t should be noted that s i g n i f i c a n t l y increased s a l t intake during the f i r s t few days of heat stress can i n h i b i t the secretion of aldosterone (29), a hormone that aids s a l t conservation by f a c i l i t a t i n g reabsorption of sodium i n the sweat glands and kidney tubules. Provided that the diet i s adequate, there i s no substantial evidence to suggest that loss of trace elements i n sweat a f f e c t s n u t r i t i o n a l status or exercise performance adversely (28). A p o s s i ble exception i s iron; the low l e v e l of bone marrow iron content observed i n some endurance-trained athletes (33) may be due to excessive losses of as much as 40 micrograms of iron per 100 m i l l i l i t e r s of sweat (34). Iron metabolism i s discussed i n more d e t a i l i n the chapter by McDonald and Saltman. Dehydration and Exercise. Water loss corresponding to as l i t t l e as 1? of the body weight leads to accentuated increases i n body temperature and heart rate during exercise (Figure 4) (35). If water loss approaches 4 to 5% of the body weight, the capacity for prolonged work may be reduced by 20 to 30? (36). The adverse cardiovascular and thermoregulatory e f f e c t s of dehydration are p a r t l y a r e s u l t of reduction i n plasma volume (hypovolemia) and an increase i n plasma osmolality. Hypovolemia also reduces stroke volume and cardiac output, and reduces the rate of heat loss by r a i s i n g temperature thresholds for cutaneous vasodilatation and sweating (18). Thermal dehydration also elevates blood e l e c t r o l y t e s and osmolality, and t h i s too reduces the s e n s i t i v i t y of heat-dissipation mechanisms independently of any dehydration-induced reduction i n plasma volume (3)· Therefore, any factor which reduces plasma osmolality can only be advantageous to the endurance athlete or worker performing i n the h e a t — another strong argument for not adding e l e c t r o l y t e s to l i q u i d s consumed during exercise. Dehydration also a f f e c t s the plasma volume response to exerc i s e . For example, during cycling i n the heat, the magnitude of the exercise hemoconcentration i s greater when the c y c l i s t i s dehydrated than when dehydration i s prevented by drinking water (3). During walking, the prevention of dehydration by water consumption increases the tendency for hemodilution rather than for hemoconcentration (37). Thus, preventing or minimizing dehydration improves performance during exercise through s p e c i f i c b e n e f i c i a l e f f e c t s on both the cardiovascular and thermoregulatory systems. Heat Acclimatization and Endurance Training. Primary adaptive responses to repeated intermittent exposure to exercise i n the heat are (1) chronic expansion of the plasma volume, (2) increased retent i o n of body sodium, (3) increased capacity for sweating and, hence,



8.


115 Water and

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THIRST THRESHOLD, AND THRESHOLD FOR IMPAIRED EXERCISE THERMOREGULATION LEADING TO DECREMENT IN PHYSICAL WORK CAPACITY STRONGER THIRST, VAGUE DISCOMFORT AND SENSE OF OPPRESSION, LOSS OF APPETITE DRY MOUTH, INCREASING HEMOCONCENTRATION, REDUCTION IN URINARY OUTPUT DECREMENT OF 20-30% IN PHYSICAL WORK CAPACITY DIFFICULTY IN CONCENTRATING, HEADACHE, IMPATIENCE, SLEEPINESS SEVERE IMPAIRMENT IN EXERCISE TEMPERATURE REGULATION, INCREASED RESPIRATORY RATE LEADING TO TINGLING AND NUMBNESS OF EXTREMITIES LIKELY COLLAPSE IF COMBINED WITH HEAT AND EXERCISE

F i g u r e 4.

Adverse e f f e c t s of d e h y d r a t i o n .


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f o r e v a p o r a t i v e heat l o s s , and (4) some r e s i d u a l i n c r e a s e i n cutaneous b l o o d f l o w ( 3 8 , 3 9 ) . These r e s p o n s e s p r o v i d e f o r i n c r e a s e d heat d i s s i p a t i o n d u r i n g e x e r c i s e , which a l l o w s c o r e temperature t o be r e g u l a t e d a t a l o w e r l e v e l , and f o r reduced s t r e s s on t h e c a r d i o v a s c u l a r system a f t e r a c c l i m a t i z a t i o n . The p r a c t i c a l importance o f e x p a n s i o n o f t h e plasma volume w h i c h o c c u r s w i t h heat a c c l i m a t i z a t i o n and endurance t r a i n i n g remains c o n t r o v e r s i a l . C e r t a i n l y i t i n c r e a s e s t h e c a p a c i t y o f t h e c a r d i o v a s c u l a r system t o m a i n t a i n an adequate b l o o d f l o w t o muscle and s k i n d u r i n g e x e r c i s e i n t h e heat w i t h o u t compromising e i t h e r t h e r m o r e g u l a t i o n o r r e g u l a t i o n o f b l o o d p r e s s u r e . On t h e o t h e r hand, t h e magnitude o f t h e h e m o c o n c e n t r a t i o n (hypovolemia) i n d u c e d d u r i n g c y c l i n g i s g r e a t e r a f t e r heat a c c l i m a t i z a t i o n . Corresponding data f o r running e x e r c i s e are not a v a i l a b l e . Even though t h e r e i s no a d a p t a t i o n t o s u c c e s s i v e bouts o f d e h y d r a t i o n , heat a c c l i m a t i z a t i o n seems t o a t t e n u a t e t h e a d v e r s e e f f e c t s o f d e h y d r a t i o n on t h e c a r d i o v a s c u l a r r e s p o n s e s t o heat and e x e r c i s e ( 4 0 ) . N o t h i n g can be g a i n e d , and much can be l o s t , by p e r f o r m i n g p r o l o n g e d e x e r c i s e i n a dehydrated c o n d i t i o n , even f o r acclimatized individuals. I t s h o u l d be emphasized t h a t t h e p h y s i c a l l y c o n d i t i o n e d endurance a t h l e t e needs as much and p r o b a b l y more water t h a n u n t r a i n e d p e o p l e because o f t h e i n c r e a s e d s w e a t i n g o f t h e athlete. Endurance e x e r c i s e t r a i n i n g , when c a r r i e d o u t i n c o o l e n v i r o n ments, i n d u c e s a d a p t i v e responses which a r e q u a l i t a t i v e l y s i m i l a r t o those i n d u c e d d u r i n g heat a c c l i m a t i z a t i o n ; i . e . , i n c r e a s e d sweat r a t e , reduced h e a r t r a t e , and i n c r e a s e d plasma volume ( 4 1 , 4 2 ) . When heat a c c l i m a t i z a t i o n i s induced by r a i s i n g c o r e temperature w i t h s u b j e c t s a t r e s t i n a h o t environment, t h e s e same a d a p t i v e r e s p o n s e s are enhanced t o some degree (41,43). I t has been observed t h a t physi c a l l y f i t s u b j e c t s do not e x h i b i t t h e s e c h a r a c t e r i s t i c r e s p o n s e s when exposed t o a s t a n d a r d a c c l i m a t i z a t i o n regimen i n v o l v i n g i n t e r m i t t e n t w a l k i n g e x e r c i s e i n a hot environment ( 4 4 ) , s u g g e s t i n g t h a t they had a l r e a d y adapted i n t h e same manner as a c c l i m a t i z e d subj e c t s . There appears t o be some a d d i t i v e e f f e c t on t h e magnitude o f the a d a p t i v e responses between t h o s e i n d u c e d by r e s t i n g i n t h e h e a t , and e x e r c i s i n g i n c o o l and h o t environments ( 4 5 ) . The l a r g e s t adapt i v e r e s p o n s e s o c c u r d u r i n g t h e performance o f moderate t o heavy e x e r c i s e i n t h e heat (18,41,42,46-48). T h i r s t and D r i n k i n g D u r i n g E x e r c i s e A l t h o u g h i t has been known f o r many y e a r s t h a t d e h y d r a t i o n and hypoh y d r a t i o n i m p a i r p h y s i c a l performance, many people who engage i n e x e r c i s e and e x e r c i s e t r a i n i n g may not know how s e r i o u s t h a t i m p a i r ment can be o r what t o do about i t , e s p e c i a l l y d u r i n g c o m p e t i t i o n when t h e y a r e c o n f r o n t e d by r u l e s t h a t may p r o h i b i t o r r e s t r i c t l i q u i d consumption. A b e w i l d e r i n g a r r a y o f h y d r a t i o n d r i n k s a r e a v a i l a b l e c o m m e r c i a l l y t h a t bombard t h e i n d i v i d u a l w i t h a v a r i e t y o f c l a i m s , some o f which b o r d e r on t h e l u d i c r o u s . The t r u t h i s s i m p l e ; d u r i n g e x e r c i s e pure water i s b e s t . A l t h o u g h p r e v e n t i o n o f dehydrat i o n by replacement o f a l l f l u i d l o s s e s would be t h e i d e a l p r o c e d u r e f o r m a x i m i z i n g e x e r c i s e performance i n c o o l o r h o t environments ( 4 9 ) , i n p r a c t i c e u n f o r t u n a t e l y , t h i s f u l l replacement i s v i r t u a l l y imposs i b l e t o achieve.


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Stimuli f o r Drinking. T h i r s t s t i m u l a t i o n and t h e a c t o f d r i n k i n g a r e b a s i c p h y s i o l o g i c a l r e s p o n s e s . The t h r e e major c i r c u m s t a n c e s known t o s t i m u l a t e t h i r s t and d r i n k i n g a r e (1) a d e f i c i t o f body water ( h y p o h y d r a t i o n and h y p o v o l e m i a ) , (2) an i n c r e a s e i n t h e o s m o l a l i t y o f the e x t r a c e l l u l a r f l u i d volume ( h y p e r o s m o l a l i t y and hyperosmotemia), and (3) consumption o f d r y food ( p r a n d i a l t h i r s t ) ( 5 0 ) . These t h r e e f a c t o r s can f u n c t i o n i n d e p e n d e n t l y , but they a r e o f t e n i n t e r a c t i v e ; e.g., a hypovolemic s u b j e c t i s o f t e n h y p e r o s m o t i c . I n a d d i t i o n , t h e hormone a n g i o t e n s i n I I a c t s as a s t i m u l a n t f o r d r i n k i n g (dipsogen) i n a n i m a l s , and p o s s i b l y i n man ( 5 1 ) . I n humans e x p e r i e n c i n g t h e r m a l l y s t r e s s f u l c o n d i t i o n s , t h e r a t e of v o l u n t a r y f l u i d i n t a k e under o p t i m a l c o n d i t i o n s f o r d r i n k i n g , i . e . , where c o o l p a l a t a b l e water o r f r u i t j u i c e (52) a r e r e a d i l y a c c e s s i b l e i s , u n f o r t u n a t e l y , o n l y about h a l f t h e r a t e o f water l o s s (51). Unless there i s f o r c e d d r i n k i n g , these s t r e s s e d people a r e almost always i n n e g a t i v e water b a l a n c e . T h i s c o n d i t i o n i s r e f e r r e d t o as i n v o l u n t a r y d e h y d r a t i o n (53)» The maximal s t r a i n on t h e f l u i d e l e c t r o l y t e system o c c u r s when a dehydrated person e x e r c i s e s i n a h o t environment. When t h e f a c t o r s h y d r a t i o n - d e h y d r a t i o n , e x e r c i s e - r e s t , and h o t - c o o l environments a r e s e p a r a t e d , i t i s found t h a t dehyd r a t i o n , e x e r c i s e , and t h e h o t environment a l l have g r e a t e r i n h i b i t o r y e f f e c t s on d r i n k i n g t h a n t h e i r h y d r a t i o n , r e s t , and c o o l e n v i ronment c o n t r o l c o n d i t i o n s . Of t h e s e t h r e e s t r e s s e s , heat exposure has t h e l e a s t i n h i b i t o r y e f f e c t , p r i o r d e h y d r a t i o n has an i n t e r mediate e f f e c t , and moderate e x e r c i s e per se has t h e g r e a t e s t i n h i b i t o r y e f f e c t on v o l u n t a r y r e h y d r a t i o n a f t e r s t r e s s - i n d u c e d f l u i d l o s s ( 5 4 ) . I n s p i t e o f the d i f f e r i n g n a t u r e o f t h e s e v a r i o u s s t i m u l i used t o reduce body w a t e r , t h e r a t e o f r e h y d r a t i o n i s t h e same when f o o d and f l u i d s a r e a v a i l a b l e ad l i b i t u m d u r i n g a c o m f o r t a b l e r e c o v e r y p e r i o d . The more s t r e s s f u l t h e t o t a l c o n d i t i o n , t h e g r e a t e r i s t h e l e v e l o f d e h y d r a t i o n and t h e l o n g e r i t t a k e s t o r e s t o r e t h e l o s t water ( 5 4 ) . I n p r e v i o u s l y d e h y d r a t e d men, f o r c e d f l u i d r e p l a c e m e n t over a 3 h o u r p e r i o d o f t h e f l u i d d e f i c i t f a i l e d t o r e s t o r e plasma volume and plasma o s m o l a l i t y t o p r e d e h y d r a t i o n l e v e l s ( 5 5 ) . The threshold f o r involuntary dehydration i n hydrated subjects occurs w i t h a water (sweat) l o s s o f o n l y 75 g/hr; w i t h heat exposure t h e t h r e s h o l d i s about 275 g/hr (54,56). T h i s means t h a t water l o s s e s below 75 g/hr and 275 g/hr a r e f u l l y r e p l a c e d by d r i n k i n g v o l u n t a r i l y ; above t h e s e t h r e s h o l d s they a r e n o t r e p l a c e d f u l l y . T h i s i s why d r i n k i n g s h o u l d be i n i t i a t e d b e f o r e o r i m m e d i a t e l y upon exposure t o a s t r e s s f u l f l u i d - d e p l e t i n g s i t u a t i o n before f e e l i n g s of t h i r s t a r i s e (30); otherwise, s i g n i f i c a n t l e v e l s of dehydration w i l l occur that cannot be r e s t o r e d e a s i l y by d r i n k i n g . R e s u l t s from a r e c e n t s t u d y (51) i n d i c a t e t h a t heat a c c l i m a t i z a t i o n a c t s t o reduce t h e l e v e l o f i n v o l u n t a r y d e h y d r a t i o n d u r i n g e x e r c i s e i n t h e heat by a p r o g r e s s i v e l y s h o r t e n e d time t o t h e f i r s t d r i n k , a t h r e e f o l d i n c r e a s e i n t h e number o f d r i n k s per exposure, and a s i g n i f i c a n t i n c r e a s e i n t h e mean volume per d r i n k . The r e s u l t i s t h a t v o l u n t a r y d r i n k i n g can be i n c r e a s e d c o m f o r t a b l y from 450 ml/hr t o 1000-1200 ml/hr ( 5 1 ) . Thus, t h e r e appears t o be an a d a p t i v e p h y s i o l o g i c a l r e s p o n s e t h a t a l l o w s one t o i n c r e a s e f l u i d i n t a k e . _

Water, E l e c t r o l y t e , and C a r b o h y d r a t e Replacement D u r i n g E x e r c i s e . To h e l p m i n i m i z e t h e r e q u i r e m e n t f o r water replacement d u r i n g e x e r c i s e , adequate h y d r a t i o n s h o u l d be a t t a i n e d b e f o r e e x e r c i s e commences.



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Some forced drinking may be appropriate, but i t must be planned caref u l l y because the s t a r t of a competitive race i s no time to have a f u l l bladder! Another method of storing water i s by making use of the 2.7 g of water of association with each gram of glycogen. Increasing glycogen stores by consuming additional carbohydrates a few days before exercise has the double advantage of providing addit i o n a l reserves of both energy and water (36,57,58). Enough has been said on the subject of e l e c t r o l y t e intake to strongly suggest that the r u l e i s to avoid s a l t (sodium and potassium). But there w i l l be occasions when s t r i c t adherence to t h i s rule i s inappropriate. Sometimes diet alone may not provide adequate e l e c t r o l y t e s or trace elements, for example when athletes perform where customary foods are unavailable and l o c a l foods are unpalatable. If e l e c t r o l y t e supplements are necessary, they should be taken with meals i n conjunction with f l u i d s . Under no circumstance should s a l t be taken immediately before or during exercise, espec i a l l y i n hot environments when sweating and involuntary dehydration are greatest and the rate of r i s e of hyperosmotemia i s maximal. I f e l e c t r o l y t e consumption i s necessary, f o r example when the exercise must be performed over many hours i n the heat (as i n ultramarathon events), then a very d i l u t e s a l t solution (less than 0.5 grams per 100 m i l l i l i t e r s of H 0) i s much better than s a l t tablets, which can cause gastric i r r i t a t i o n . Calcium should not be used i n rehydration drinks because i t i n h i b i t s the normal hypervolemic response to f l u i d ingestion (59). The ideal time for e l e c t r o l y t e supplementation i s after exercise when a s u f f i c i e n t amount of water can be taken to d i l u t e the s a l t to i s o t o n i c i t y (0.9 grams of NaCl per 100 m i l l i l i t e r s of H 2 O ) , i . e . , the normal concentration of plasma. Normally these e l e c t r o l y t e supplements w i l l be unnecessary as a balanced diet w i l l provide s u f f i c i e n t e l e c t r o l y t e s and trace elements to restore any temporary d e f i c i t . 2

Gastric emptying time (the normal maximal rate being about 600-800 ml/hr (60)) imposes a physiological l i m i t a t i o n upon the rate of f l u i d uptake into the c i r c u l a t o r y system. Acute g a s t r i c discomf o r t usually arises when an attempt i s made to drink large volumes of l i q u i d too quickly during exercise. A modest volume taken at f r e quent intervals (100-125 ml or 4 ounces per 10 min) i s usually a l l that i s comfortably possible, but the b e n e f i c i a l effect can be considerable. Water i s absorbed from the stomach at a rate of about 2.6% of the ingested volume per minute, but at 20% of the ingested amount per minute from the small i n t e s t i n e . Therefore, i t i s cert a i n l y the retention of f l u i d within the stomach that produces the discomfort. The stomach can be envisioned e s s e n t i a l l y as a pump that passes material into the duodenum (61). The rate of g a s t r i c emptying increases i n proportion to the volume ingested (60-62), and the addition of even small amounts of carbohydrates (monosaccharides and disaccharides) has been reported to retard g a s t r i c emptying ( 6 0 , 6 3 - 6 5 ) . The addition of potassium chloride to a test meal slows g a s t r i c emptying s i m i l a r l y to that induced by glucose, but the KC1 i s more nauseating than glucose (65)« Optimal g a s t r i c emptying occurs when s a l i n e , of a concentration that i s nearly isosmotic with the plasma, i s introduced into the stomach (61); hypertonic solutions empty more slowly (61,66). With prolonged exercise i t may be necessary to provide addit i o n a l carbohydrate energy sources since glycogen appears to be the


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p r e f e r r e d s u b s t r a t e f o r energy p r o d u c t i o n (67)» Compared w i t h t h e consumption o f a normal mixed d i e t , consumption o f a c a r b o h y d r a t e e n r i c h e d d i e t p r i o r t o e x e r c i s e can r e s u l t i n s i g n i f i c a n t l y b e t t e r r u n n i n g performance (60). D e p l e t i o n o f muscle g l y c o g e n by e x e r c i s e and subsequent r e p l e n i s h m e n t w i t h enhanced c a r b o h y d r a t e f e e d i n g has a l s o been r e p o r t e d t o i n c r e a s e subsequent p h y s i c a l performance (69). I n g e s t i o n o f a g l u c o s e polymer supplement d u r i n g e x e r c i s e i n c r e a s e s endurance time a t a work r a t e o f 45? o f t h e maximal capa c i t y ( 7 0 ) . Thus, c a r b o h y d r a t e f e e d i n g appears t o enhance e x e r c i s e performance, b u t , as w i t h water and e l e c t r o l y t e s , m o d e r a t i o n i s i m p o r t a n t t o m i n i m i z e the r e t a r d a t i o n e f f e c t on g a s t r i c emptying. Summary D e t e r i o r a t i o n o f p h y s i c a l e x e r c i s e performance due t o d e h y d r a t i o n b e g i n s when body weight d e c r e a s e s by about 1?. U n a c c l i m a t i z e d humans under t h e r m a l and e x e r c i s e s t r e s s w i l l v o l u n t a r i l y d r i n k a t a r a t e o f about h a l f o f the r a t e o f t h e i r f l u i d l o s s e s even a t low r a t e s o f f l u i d l o s s (275 m l / h r ) ; the maximal r a t e o f f l u i d i n t a k e i s about 600-800 m l / h r , t h e normal maximal r a t e o f g a s t r i c emptying. D u r i n g the e x e r c i s e - h e a t a c c l i m a t i z a t i o n p r o c e d u r e , the v o l u n t a r y f l u i d i n t a k e can be i n c r e a s e d from 450 ml/hr t o 1000-1200 ml/hr w i t h no adverse e f f e c t s . E l e c t r o l y t e s u p p l e m e n t a t i o n i n d r i n k i n g f l u i d i s not recommended d u r i n g e x e r c i s e bouts l a s t i n g l e s s t h a n 3 t o 5 hours because the i n c r e a s e d c o n c e n t r a t i o n o f sodium i n t h e plasma a c c e n t u a t e s the h y p e r t h e r m i a . E l e c t r o l y t e and c a r b o h y d r a t e supplementa t i o n i s recommended d u r i n g l o n g e r work o r e x e r c i s e p e r i o d s , espe c i a l l y i n hot e n v i r o n m e n t s , and when r e g u l a r meals a r e n o t a v a i l a b l e . Thus, t h e r e has been no s i g n i f i c a n t e v i d e n c e t h a t would change t h e c o n c l u s i o n o f P i t t s e t a l . i n 1944 ( 4 9 ) : " . . . i n t h e case of w e l l a c c l i m a t i z e d young men whose d a i l y d i e t i s adequate, t h e b e s t performance o f i n t e r m i t t e n t work i n the heat i s t o be a c h i e v e d by r e p l a c i n g water l o s s hour by hour and s a l t l o s s meal by meal."

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