NUTRITION IN SPORT

intense training can greatly increase daily fluid
requirements.
Electrolytes, primarily sodium chloride and, to
a lesser extent, potassium, calcium and magne-
sium, are contained in sweat. Sweat sodium con-
centration averages approximately 35 mmol · l–1
(range, 10–70 mmol · l–1) and varies depending
upon diet, sweating rate, hydration and heat
acclimation level (Allan & Wilson 1971; Brouns
1991). Sweat glands reabsorb sodium by active
transport, and the ability to reabsorb sodium
does not increase with the sweating rate, so at
high sweating rates the concentration of sweat
sodium increases. Heat acclimation improves the
ability to reabsorb sodium so acclimated persons
have lower sweat sodium concentrations (>50%
reduction) for any sweating rate (Dill et al. 1933;
Basset al. 1955; Allan & Wilson 1971). Sweat
potassium concentration averages 5 mmol · l–1
(range, 3–15 mmol · l–1), calcium averages
1 mmol · l–1 (range, 0.3–2 mmol · l–1) and mag-
nesium averages 0.8 mmol · l–1 (range, 0.2–
1.5 mmol · l–1) (Brouns 1991). Electrolyte supple-
mentation is not necessary, except occasionally
for their first several days of heat exposure where

evidence indicates that this is warranted (Mar-

riott 1994; Convertino et al. 1996), as normal

dietary sodium intake will replenish sweat elec-

trolyte losses (Marriott 1994; Convertino et al.

1996).

During exercise in the heat, a principal

problem is to avoid hypohydration by matching

fluid consumption to sweat loss. This is a difficult

problem because thirst does not provide a good

index of body water requirements (Adolph &

Associates 1947; Hubbard et al. 1984; Engell et al.

1987). Thirst is probably not perceived until an

individual has incurred a water deficit of approx-

imately 2% body weight loss (BWL) (Adolph &

Associates 1947; Hubbard et al. 1984; Armstrong

et al. 1985b). In addition, ad libitumwater intake

during exercise in the heat results in an incom-

plete replacement of body water losses (Adolph

& Associates 1947; Hubbard et al. 1984). Heat-

acclimated persons will usually only replace less

than one half of their fluid deficit when replacing

fluidad libitum(Adolph & Associates 1947). As

a result, it is likely that unless forced hydration

is stressed, some dehydration will occur during

exercise in the heat. Humans will usually fully

effects of dehydration and rehydration 217

3.0

2.5

2.0

1.5

1.0

0.5

0

Sweating rate (l

.h

–1

)

Hot and humid

Cool and dry

160 200 240 280 320

Running speed (m min–1)

16 15 14 13 12 11 10 9 8

Running speed (min km. –1 run)

Fig. 16.1An approximation of.
hourly sweating rates as a
function of climate and running
speed. From Sawka and Pandolf
(1990).