NUTRITION IN SPORT

ments. It has been estimated that the intracellular
compartment can contribute 30–60% of the total;
the interstitial fluid, 30–60% of the total; and the
plasma volume, 8–12% of the total (Mack &
Nadel 1996).
In a study by Costill et al. (1976), eight healthy
men cycled at 70% of V

.
o2max.in an environmental
chamber (Tamb=39°C) until they were progres-
sively dehydrated by 2%, 4% and 6% of their
initial body weight during a single, prolonged
exercise bout. After achieving each level of dehy-
dration, the subjects rested for 30 min in a supine
position while a blood sample and muscle biopsy
were obtained. Plasma and muscle water con-
tents were reduced by 2.4% and 1.2%, respec-
tively, for each percentage decrease in body
weight. Figure 49.2 illustrates the changes in the
plasma, interstitial and intracellular fluid com-
partments at each level of dehydration.
What are the health implications of such major
changes in total body water? Of obvious concern
is the potential for disturbances in thermoregula-
tion. Sawka (1992) concludes from his review of
the literature that hypohydration, consequent
to dehydration, causes greater heat storage (i.e.
increased core temperature) and reduces toler-
ance to heat strain. This is the result of reductions
in the rate of sweating and skin blood flow. Even
with decreased skin blood flow, there is still con-
siderable displacement of blood to the skin for
cooling, making it difficult to maintain central
venous pressure and an adequate cardiac output.
Excessive sweat or urine loss could also result
in large losses of electrolytes, which could pos-
sibly have serious health consequences, such as
cardiac dysrhythmias. However, Costill (1977)
has concluded that even those electrolyte losses
can be large, they are largely derived from the
extracellular compartment, and that losses of
ions in sweat and urine have little effect on the K+
content of plasma or muscle.
Further, concern has been expressed as to the
effects of chronic dehydration on renal function.
Zambraski (1990), in a review of renal function,
fluid homeostasis and exercise, concluded that
exercise, particularly in conjunction with hypo-

hydration, sodium deprivation, and/or heat

stress, presents a major stress to the kidneys.

Renal vasoconstriction and antinatriuretic

responses are increased in magnitude when

dehydration and/or heat stress are combined

with exercise. Exercise proteinuria and haema-

turia have been reported, indicating dramatic

changes in renal function. However, the inci-

dence of acute renal failure is relatively small.

The long-term consequences of repeated

episodes of acute renal stress are unknown.

Repeated bouts of weight cycling have also

weight category sports 641

Total body water loss (l)

–2.2

Levels of dehydration (%)

–4.1 –5.8

0

1.0

2.0

3.0

4.0

30%

60%

10%

52%

10%

38%

50%

39%

11%

3.5

2.5

0.5

1.5

Fig. 49.2Changes in the plasma (
), interstitial ( )

and intracellular water ( ) compartments with

exercise and thermal dehydration of 2%, 4% and 6% of

body weight. Adapted from Costill et al. (1976), with

permission.