drated. Hypohydration was achieved by diuretic
administration (furosemide), which decreased
body weight by 2% and plasma volume by 11%.
Running performance was impaired at all race
distances, but to a greater extent in the longer
races (ª5% for the 5000 and 10 000 m) than the
shorter race (3% for the 1500 m). Burge et al.
(1993) recently examined whether hypohydra-
tion (3% BWL) affected simulated 2000 m rowing
performance. They found that, on average, it
took 22 s longer to complete the task when hypo-
hydrated than when euhydrated. Average power
was reduced by 5% in the hypohydrated state.
Two studies have examined the adverse effects
of hypohydration on moderate to intense cycle
ergometer performance. In both studies, high-
intensity performance tests were conducted
immediately after 55–60 min of cycling during
which volunteers either drank nothing or drank
sufficient fluid to replace sweat losses. Walsh
et al. (1994) reported that time to fatigue when
cycling at 90% V
.
o2max.was 51% longer (6.5 vs.
9.8 min) when subjects drank sufficient fluids to
prevent hypohydration. Below et al. (1995) found
that cyclists completed a performance ride 6.5%
faster if they drank fluids during exercise. The
results of these studies clearly demonstrate the
detrimental effects of hypohydration in submax-
imal exercise performance.
Investigators have documented the effects of
hypohydration on a person’s ability to tolerate
heat strain during submaximal intensity exer-
cise. These studies demonstrate that persons
who drink can continue to exercise in the heat for
many hours, whereas those who under-drink
discontinue because of exhaustion (Adolph &
Associates 1947; Ladell & Shephard 1961; Sawka
et al. 1992). To address whether hypohydration
alters heat tolerance, Sawka and colleagues
(1992) had subjects walk to voluntary exhaustion
when either euhydrated or hypohydrated (by
8% of total body water). The experiments
were designed so that the combined environ-
ment (Ta, 49°C; rh, 20%) and exercise intensity
(47%V
.
o2max.) would not allow thermal equilib-
rium and heat exhaustion would eventually
occur. Hypohydration reduced tolerance time
220 nutrition and exercise
(121–55 min), but more important, hypohydra-
tion reduced the core temperature that a person
could tolerate. Heat exhaustion occurred at a
core temperature approximately 0.4°C lower
when hypohydrated than when euhydrated.
These findings indicate that hypohydration not
only impairs exercise performance, but also
reduces tolerance to heat strain.
Hypohydration increases core temperature
responses during exercise in temperate (Grande
et al. 1959; Cadarette et al. 1984) and hot (Sawka
et al. 1983, 1985) climates. A critical water deficit
of 1% body weight elevates core temperature
during exercise (Ekblom et al. 1970). As the mag-
nitude of water deficit increases, there is a con-
comitant graded elevation of core temperature
during exercise heat stress (Sawka et al. 1985;
Montain & Coyle 1992). Figure 16.4 illustrates
relationships between BWL and core tempera-
ture elevations reported by studies (Adolph &
Associates 1947; Strydom & Holdsworth 1968;
Sawkaet al. 1985; Montain & Coyle 1992) which
examined several hypohydration levels (Sawka
et al. 1996a). The magnitude of core temperature
elevation ranges from 0.1 to 0.23°C for every per-
centage body weight lost. Hypohydration notA
B
C
D01234567
Body water loss (%)1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0Increase in coretemperature ( C)Fig. 16.4Relationship between the elevation in core
temperature (above euhydration) at a given
hypohydration level during exercise with heat stress,
according to different studies: A, 65% V
.
o2max., 33°C db
(Montain & Coyle 1992); B, marching in the desert
(Adolph & Associates 1947); C, 25% V
.
o2max., 49°C db
(Sawkaet al. 1985); D, 45 W, 34°C db (Strydom &
Holdsworth 1968). From Sawka et al. (1996a).