Nutrition and Aerobic Exercise - ACS Publications - American

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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.


108

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


Water and

8. GREENLEAF AND HARRS ION

Electrolytes

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

GREENLEAF AND HARRS ION

Electrolytes

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."

Literature Cited 1. Sargent, F., II; Weinman, K. (1963) Physiological variability in young men. In: Physiological Measurements of Metabolic Functions in Man (Consolazio, C.F., Johnson, R.E., and Pecora, L.J., eds.), p. 453, McGraw-Hill, New York, NY. 2. Greenleaf, J.E. (1979) Hyperthermia and exercise. In: Int. Rev. Physiol., Environ. Physiol. III, Vol. 20 (Robertshaw, D., ed.), p. 157, University Park Press, Baltimore, MD. 3. Harrison, M.H., Edwards, R.J. & Fennessy, P.A. (1978) Intravascular volume and tonicity as factors in the regulation of body temperature. J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 44, 69-75. 4. Kozlowski, S., Greenleaf, J.E., Turlejska, E. & Nazar, Κ. (1980) Extracellular hyperosmolality and body temperature during physical exercise in dogs. Am. J. Physiol. 239 (Regulatory Integrative Comp. Physiol. 8), R180-R183. 5. Johnson, R.E. (1964) Human nutritional requirements for water in long space flights. In: Nutrition in Space and Related Waste Problems (Helvey, T.C., ed.), p. 159, National Aeronautics and Space Administration, Washington, DC.




120

6. Oser, B.L. (ed.) (1965) Hawk's Physiological Chemistry. McGraw-Hill, New York, NY. 7. Schloerb, P.R., Friis-Hansen, B.J., Edelman, I.S., Solomon, A.K. & Moore, F.D. (1950) The measurement of total body water in the human subject by deuterium oxide dilution: with a consideration of the dynamics of deuterium distribution. J. Clin. Invest. 29, 1296-1310. 8. Wolf, A.F. (1958) Thirst. Physiology of the Urge to Drink and Problems of Water Lack. C.C. Thomas, Springfield, IL. 9. Greenleaf, J.E., Convertino, V.A. & Mangseth, G.R. (1979) Plasma volume during stress in man: osmolality and red cell volume. J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 47, 1031-1038. 10. Adolph, E.F. & Associates (1947) Physiology of Man in the Desert. Interscience Publishers, Inc., New York, NY. 11. Dill, D.B., Bock, A.V., Edwards, H.T. & Kennedy, P.H. (1936) Industrial fatigue. J. Indust. Hyg. Toxicol. 18, 417-431. 12. Mohsenin, V. & Gonzales, R.R. (1984) Tissue pressure and plasma oncotic pressure during exercise. J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 56, 102-108. 13. Sjogaard, G. & Saltin, B. (1982) Extra- and intracellular water spaces in muscles of man at rest and with dynamic exercise. Am. J. Physiol. 243 (Regulatory Integrative Comp. Physiol. 12), R271-R280. 14. Lundvall, J., Mellander, S., Westling, H. & White, T. (1972) Fluid transfer between blood and tissue during exercise. Acta Physiol. Scand. 85, 258-269. 15. Harrison, M.H., Edwards, R.J. & Leitch, D.R. (1975) Effect of exercise and thermal stress on plasma volume. J. Appl. Physiol. 39, 925-931. 16. Edwards, R.J. & Harrison, M.H. (1984) Intravascular volume and protein responses to running exercise. Med. Sci. Sports Exerc. 16, 247-255. 17. Hagan, R.D., Diaz, F.J. & Horvath, S.M. (1978) Plasma volume changes with movement to supine and standing positions. J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 45, 414-418. 18. Harrison, M.H. (1985) Effects of thermal stress and exercise on blood volume in humans. Physiol. Rev. 65, 149-209. 19. Convertino, V.A., Keil, L.C., Bernauer, E.M. & Greenleaf, J. E. (1981) Plasma volume, osmolality, vasopressin, and renin activity during graded exercise in man. J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 50, 123-128. 20. Convertino, V.A., Keil, L.C. & Greenleaf, J.E. (1983) Plasma volume, renin, and vasopressin responses to graded exercise after training. J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 54, 508-514. 21. Greenleaf, J.E., Convertino, V.A., Stremel, R.W., Bernauer, E.M., Adams, W.C. Vignau, S.R. & Brock, P.J. (1977) Plasma [Na ], [Ca ], and volume shifts and thermoregulation during exercise in man. J. Appl. Physiol: Respirat. Environ. Exercise Physiol. 43, 1026-1032. +

2+



8. GREENLEAF AND HARRISON

Water and Electrolytes

121

22. Greenleaf, J.E., Van Beaumont, W., Brock, P.J., Morse, J.T., & Mangseth, G.R. (1979) Plasma volume and electrolyte shifts with heavy exercise in sitting and supine positions. Am. J. Physiol. 236 (Regulatory Integrative Comp. Physiol. 5), R206-R214. 23. Mellander, S., Johansson, B., Gray, S., Jonsson, 0., Lundvall, J., & Ljung, B. (1967) The effects of hyperosmolarity on intact and isolated vascular smooth muscle: Possible role in exercise hyperemia. Angiologica 4, 310-322. 24. Smith, E.E., Guyton, A.C., Manning, R.D., & White, R.J. (1976) Integrated mechanisms of cardiovascular response and control during exercise in the normal human. Prog. Cardiovasc. Dis. 18, 421-443. 25. Nielsen, M. (1938) Die Regulation der Korpertemperatur bei Muskelarbeit. Scand. Arch. Physiol. 79, 193-230. 26. Maron, M.B. & Horvath, S.M. (1978) The marathon: a history and review of the literature. Med. Sci. Sports 10, 137-150. 27. Mitchell, J.W. (1977) Energy exchanges during exercise. In: Problems with Temperature Regulation during Exercise (Nadel, E.R., ed.), pp. 11-26, Academic Press Inc., NY. 28. Robinson, S. & Robinson, A.H. (1954) Chemical composition of sweat. Physiol. Rev. 34, 202-220. 29. Davies, J.Α., Harrison, M.H., Cochrane, L.A., Edwards, R.J. & Gibson, T.M. (1981) Effect of saline loading during heat acclimatization on adrenocortical hormone levels. J. Appl. Physiol.: Respirât. Environ. Exercise Physiol. 50, 605-612. 30. Gisolfi, C.V. & Copping, J.R. (1974) Thermal effects of prolonged treadmill exercise in the heat. Med. Sci. Sports 6, 108-113. 31. Greenleaf, J.E. & Castle, B.L. (1971) Exercise temperature regulation in man during hypohydration and hyperhydration. J. Appl. Physiol. 30, 847-853. 32. Leithead, C.S. (1964) Disorders of water and electrolyte balance. In: Heat Stress and Heat Disorders (Leithead, C.S. & Lind, A.R., authors), pp. 141-177, F.A. Davis Company, Philadelphia, PA. 33. Ehn, L., Carlmark, B. & Höglund, S. (1980) Iron status in athletes involved in intense physical activity. Med. Sci. Sports Exerc. 12, 61-64. 34. Vellar, O.D. (1968) Studies on sweat losses of nutrients. I. Iron content of whole body sweat and its association with other sweat constituents, serum iron levels, hematological indices, body surface area, and sweat rate. Scand. J. Clin. Lab. Invest. 21, 157-167. 35. Ekblom, Β., Greenleaf, C.J., Greenleaf, J.E. & Hermansen, L. (1970) Temperature regulation during exercise dehydration in man. Acta Physiol. Scand. 79, 475-483. 36. Olsson, K.-E. & Saltin, B. (1971) Diet and fluids in training and competition. Scand. J. Rehab. Med. 3, 31-38. 37. Sawka, M.N., Francesconi, R.P., Pimentai, Ν.A. & Pandolf, K.B. (1984) Hydration and vascular fluid shifts during exercise in the heat. J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 56, 91-96. 38. Greenleaf, J.E. & Greenleaf, C.J. (1970) Human acclimation and acclimatization to heat: a compendium of research. NASA Tech. Memo. X-62,008, 1-188.


122



39. Sciaraffa, D., Fox, S.C., Stockmann, R. & Greenleaf, J.E. (1980) Human acclimation and acclimatization to heat: a compendium of research (1968-1978). NASA Tech. Memo. 81181, 1-102. 40. Sawka, M.N., Toner, M.M., Francesconi, R.P. & Pandolf, K.B. (1983) Hypohydration and exercise: effects of heat acclimation, gender, and environment. J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 55, 1147-1153. 41. Convertino, V.A., Greenleaf, J.E. & Bernauer, E.M. (1980) Role of thermal and exercise factors in the mechanism of hypervolemia. J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 48, 657-664. 42. Shvartz, E., Bhattacharya, Α., Sperinde, S.J., Brock, P.J., Sciaraffa, D., Haines, R.F. & Greenleaf, J.E. (1979) Deconditioning-induced exercise responses as influenced by heat acclimation. Aviat. Space Environ. Med. 50, 893-897. 43. Fox, R.H., Goldsmith, R., Kidd, D.J. & Lewis, H.E. (1963) Acclimatization to heat in man by controlled elevation of body temperature. J. Physiol. (London) 166, 530-547. 44. Greenleaf, J.E. (1964) Lack of artificial acclimatization to heat in physically fit subjects. Nature 203, 1072. 45. Gisolfi, C. & Robinson, S. (1969) Relations between physical training, acclimatization, and heat tolerance. J. Appl. Physiol. 26, 530-534. 46. Convertino, V.A., Shvartz, Ε., Haines R.F., Bhattacharya, Α., Sperinde, S.J., Keil, L.C. & Greenleaf, J.E. (1977) Heat acclimation and water-immersion deconditioning: fluid and electrolyte shifts with tilting. Aerospace Med. Asso. Preprints, 13-14. 47. Senay, L.C., Mitchell, D. & Wyndham, C.H. (1976) Acclimatiza tion in a hot humid environment: body fluid adjustments. J. Appl. Physiol. 40, 786-796. 48. Harrison, M.H., Edwards, R.J., Graveney, M.J., Cochrane, L.A., & Davies, J.A. (1981) Blood volume and plasma protein responses to heat acclimatization in humans. J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 50, 597-604. 49. Pitts, G.C., Johnson, R.E. & Consolazio, F.C. (1944) Work in the heat as affected by intake of water, salt and glucose. Am. J. Physiol. 142, 253-259. 50. Adolph, E.F. (1967) Regulation of water intake in relation to body water content. In: Handbook of Physiology, Section 6: Alimentary Canal, Vol. 1, Control of food and water intake (Code, C.F., ed.), p. 163, American Physiological Society, Washington, DC. 51. Greenleaf, J.E., Brock, P.J., Keil, L.C. & Morse, J.T. (1983) Drinking and water balance during exercise and heat acclima tion. J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 54, 414-419. 52. Sohar, E., Gilat, A.T., Tennenbaum, J. & Nir, M. (1961) Reduction of voluntary dehydration during effort in hot envi ronments. J. Med. Asso. Israel 60, 319-323. 53. Greenleaf, J.E. (1966) Involuntary hypohydration in man and animals: a review. NASA Special Publication 110, 1-34. 54. Greenleaf, J.E. & Sargent, F., II (1965) Voluntary dehydration in man. J. Appl. Physiol. 20, 719-724.



8.

GREENLEAF AND HARRISON

Water and Electrolytes

123

55. Costill, D.L. & Sparks, K.E. (1973) Rapid fluid replacement following thermal dehydration. J. Appl. Physiol. 34, 299-303. 56. Rothstein, Α., Adolph, E.F. & Wills, J.H. (1947) Voluntary dehydration. In: Physiology of Man in the Desert (Adolph, E.F. & Associates, eds.), p. 254, Interscience, New York, NY. 57. Costill, D.L., Bennett, Α., Branam, G. & Eddy, D. (1973) Glucose ingestion at rest and during prolonged exercise. J. Appl. Physiol. 34, 764-769. 58. Krzentowski, G., Jandrain, Β., Pirnay, F., Mosora, F., Lacroix, M., Luyckx, A.S. & Lefebvre, P.J. (1984) Availability of glu cose given orally during exercise. J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 56, 315-320. 59. Greenleaf, J.E. & Brock, P.J. (1980) Na and Ca ingestion: plasma volume-electrolyte distribution at rest and exercise. J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 48, 838-847. 60. Costill, D.L. & Saltin, B. (1974) Factors limiting gastric emptying during rest and exercise. J. Appl. Physiol. 37, 679-683. 61. Hunt, J.N. & Knox, M.T. (1968) Regulation of gastric empty ing. In: Handbook of Physiology, Section 6: Alimentary Canal, Vol. IV, Motility (Code, C.F., ed.), p. 1917, American Phys iological Society, Washington, DC. 62. Erskine, L. & Hunt, J.N. (1981) The gastric emptying of small volumes given in quick succession. J. Physiol. (London) 313, 335-341. 63. Elias, Ε., Gibson, G.J., Greenwood, L.F., Hunt, J.N. & Tripp, J.H. (1968) The slowing of gastric emptying by monosaccharides and disaccharides in test meals. J. Physiol. (London) 194, 317-326. 64. Coyle, E.F., Costill, D.L., Fink, W.J. & Hoopes, D.G. (1978) Gastric emptying rates for selected athletic drinks. Res. Quart. 49, 119-124. 65. Barker, G.R., Cochrane, G.M., Corbett, G.A., Hunt, J.N. & Roberts, S.K. (1974) Actions of glucose and potassium chloride on osmoreceptors slowing gastric emptying. J. Physiol. (London) 237, 183-186. 66. Lee, P.R., Code, C.F. & Scholer, J.F. (1955) The influence of varying concentrations of sodium chloride on the rate of absorption of water from the stomach and small bowel of human beings. Gastroenterology 29, 1008-1015. 67. Ahlborg, B., Bergström, J., Ekelund, L.-G. & Hultman, E. (1967) Muscle glycogen and muscle electrolytes during prolonged physical exercise. Acta Physiol. Scand. 70, 129-142. 68. Karlsson, J. & Saltin, B. (1971) Diet, muscle glycogen, and endurance performance. J. Appl. Physiol. 31, 203-206. 69. Saltin, B. & Hermansen, L. (1967) Glycogen stores and prolonged severe exercise. In: Nutrition and Physical Activity (Blix, G., ed.), p. 32, Almqvist & Wiksells, Uppsala, Sweden. +

2+


124


Ivy, J.L., Miller, W., Dover, V., Goodyear, L.G., Sherman, W.M., Farrell, S. & Williams, H. (1983) Endurance improvement by ingestion of a glucose polymer supplement. Med. Sci. Sports Exerc. 15, 466-471. 71. Greenleaf, J.E. (1982) The body's need for fluids. In: Nutrition and Athletic Performance (Haskell, W., Scala, J. & Whittam, J., eds.), p. 34, Bull Publishing Co., Palo Alto, CA. RECEIVED March 15, 1985 Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on February 13, 2018 | https://pubs.acs.org Publication Date: December 3, 1986 | doi: 10.1021/bk-1986-0294.ch008

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