cacy of hyperhydration on thermoregulation
during exercise-heat stress. In addition, some
studies (Moroff & Bass 1965; Nielsen et al. 1971)
report that overdrinking before exercise lowered
body core temperature prior to exercise. This was
likely due to the caloric cost of warming the
ingested fluid. Exercise per sedid not exacerbate
the difference that existed prior to exercise.
Hyperhydration in these studies therefore did
not improve heat dissipation during the exercise
period.
While many studies have attempted to induce
hyperhydration by overdrinking water or water-
electrolyte solutions, these approaches have pro-
duced only transient expansions of body water.
One problem often encountered is that much of
the fluid overload is rapidly excreted (Freund
et al. 1995). Some evidence has been accrued that
greater fluid retention can be achieved by drink-
ing an aqueous solution containing glycerol
while resting in temperate conditions (Riedesel
et al. 1987; Freund et al. 1995). Riedesel et al. (1987)
first reported that following hyperhydration
with a glycerol solution, rather than with water
alone, subjects excreted significantly less of the
water load. They found that subjects drinking
glycerol solutions achieved greater hyperhydra-
tion than subjects drinking water while resting
in temperate conditions. Freund et al. (1995)
reported that glycerol increased fluid retention
by reducing free water clearance. Exercise and
heat stress decrease renal blood flow and free
water clearance and therefore both stressors
might reduce glycerol’s effectiveness as a hyper-
hydrating agent.
Lyons et al. (1990) reported that glycerol/water
hyperhydration had dramatic effects on improv-
ing a person’s ability to thermoregulate during
exercise-heat stress. Subjects completed three
trials in which they exercised in a hot (42°C)
climate. For one trial, fluid ingestion was
restricted to 5.4 ml · kg–1 body weight, and in
the other two trials subjects ingested water
(21.4 ml · kg–1) with or without a bolus of glycerol
(1 g · kg–1). Subjects began exercise 90 min after
this hyperhydration period. Glycerol ingestion
increased fluid retention by 30% compared to
222 nutrition and exercise
drinking water alone. During exercise, glycerol
hyperhydration produced a higher sweating rate
(300–400 ml · h–1) and substantially lower core
temperatures (0.7°C) than those produced in
control conditions with water hyperhydration.
These thermoregulatory benefits during exer-
cise-heat stress have not been confirmed. Other
studies report similar core temperatures and
sweating rates between glycerol and water
hyperhydration fluids before exercise (Montner
et al. 1996) in a temperate climate, or as rehydra-
tion solutions during exercise in a warm climate
(Murrayet al. 1991).
Recently, Latzka and colleagues (1997) exam-
ined the effects of hyperhydration on fluid
balance and thermoregulation during exercise-
heat stress. Their approach was to determine if
pre-exercise hyperhydration with and without
glycerol would improve sweating responses
and reduce core temperature. The glycerol and
water dosages were similar to those employed by
Riedeselet al. (1987) and Lyons et al. (1990).
Latzka and colleagues (1997) found that during
exercise (45% V.
o2max.) in the heat (35°C, 45% rh),
there was no difference between hyperhydration
methods for increasing total body water (ª1.5 l).
In addition, unlike euhydration, hyperhydra-
tion did not alter core temperature (rectal or
eosophageal), skin temperature, local sweating
rate, sweating threshold, sweating sensitivity or
heart rate responses. Likewise, no differences
were found between water and glycerol hyper-
hydration methods for these physiological
responses. Latzka and colleagues (1997) con-
cluded that hyperhydration provides no ther-
moregulatory advantage over the maintenance
of euhydration.Conclusion
During exercise, sweat output often exceeds
water intake, producing a body water deficit or
hypohydration. The water deficit lowers both
intracellular and extracellular volume. It also
results in plasma hypertonicity and hypo-
volaemia. Aerobic exercise tasks are likely to be
adversely affected by hypohydration, with the