NUTRITION IN SPORT

only elevates core temperature responses, but
it negates the core temperature advantages con-
ferred by high aerobic fitness and heat acclima-
tion (Buskirk et al. 1958; Sawka et al. 1983;
Cadarette et al. 1984).
Hypohydration impairs both dry and evapo-
rative heat loss (or, if the air is warmer than the
skin, dehydration aggravates dry heat gain)
(Sawkaet al. 1985, 1989; Kenney et al. 1990;
Montainet al. 1995). Figure 16.5 presents local
sweating responses (Sawka et al. 1989) and skin
blood flow responses (Kenney et al. 1990) to
hypohydration (5% BWL) during exercise in the
heat. This figure indicates that hypohydration
reduced both effector heat loss responses for a
given core temperature level (Sawka 1992).
Hypohydration is usually associated with either
reduced or unchanged whole-body sweating
rates at a given metabolic rate in the heat (Sawka
et al. 1984). However, even when hypohydration
is associated with no change in sweating rate,
core temperature is usually elevated, so that
sweating rate for a given core temperature is
lower when hypohydrated.

Hyperhydration

Hyperhydration, increased total body water, has
been suggested to improve thermoregulation

during exercise-heat stress above euhydration

levels (Sawka et al. 1996a). The concept that

hyperhydration might be beneficial for exercise

performance arose from the adverse conse-

quences of hypohydration. Studies examining

thermoregulatory effects of hyperhydration

during exercise-heat stress have reported dis-

parate results. Some investigators report lower

core temperatures during exercise after hyperhy-

dration (Moroff & Bass 1965; Nielsen et al. 1971;

Gisolfi & Copping 1974; Nielsen 1974; Grucza

et al. 1987), while other studies do not (Greenleaf

& Castle 1971; Nadel et al. 1980; Candas et al.

1988). Also, several studies (Moroff & Bass 1965;

Nielsen 1974; Lyons et al. 1990) report higher

sweating rates with hyperhydration. In most

studies, heart rate was lower during exercise

with hyperhydration (Sawka et al. 1996a).

We believe that these conflicting results are

due to differences in experimental design and

not hyperhydration per se. For example, studies

(Moroff & Bass 1965; Nielsen et al. 1971; Lyons

et al. 1990) reporting that hyperhydration

reduces thermal strain have not had subjects

fully replace fluid lost during exercise; therefore,

the differences reported may be due to dehydra-

tion causing increased thermal strain during

‘control’ conditions. Maintaining euhydration

during exercise is essential to determine the effi-

effects of dehydration and rehydration 221

1.4

1.0

0.6

0.2

20

16

12

8

4

36 37 38 39 36 37 38 39

(a) Core temperature ( C) (b) Core temperature ( C)

Sweating rate (l

.h

–1

)

Blood flow (ml

.100 ml

–1

.min

–1

)

Fig. 16.5(a) Local sweating rate (Sawka et al. 1989), and (b) forearm skin blood flow (Kenney et al. 1990) responses
for euhydrated (—) and hypohydrated (5% body water loss) (---) persons during exercise with heat stress. From
Sawka (1992).