petition is short in weight category sports where
sweating and fluid restriction have been used to
achieve an artificially low body mass.
The primary factors influencing the postexer-
cise rehydration process are the volume and
composition of the fluid consumed. The volume
consumed will be influenced by many factors,
including the palatability of the drink and its
effects on the thirst mechanism, and many differ-
ent formulation options are open. The ingestion
of solid food, and the composition of that food,
will also be an important factor, but there are
many situations where solid food is avoided
between exercise sessions or immediately after
exercise.
Beverage composition
It is well established that plain water consumed
after exercise is not the ideal rehydration bever-
age when rapid and complete restoration of fluid
balance is necessary and where all intake is in
liquid form. Costill and Sparks (1973) demon-
strated that ingestion of plain water after
exercise-induced dehydration caused a large fall
in serum osmolality with a subsequent diuresis:
the result of this stimulation of urinary water loss
was a failure to achieve positive fluid balance by
the end of the 4-h study period. However, when
an electrolyte-containing solution (106 g · l–1car-
bohydrate, 22 mmol · l–1 Na+, 2.6 mmol · l–1 K+,
17.2 mmol · l–1Cl–) was ingested after exercise
which caused a loss of 4% of body mass, the urine
output was less and net water balance was closer
to the pre-exercise level. Nielsen et al. (1986)
showed differences in the rate and extent of
changes in the plasma volume with recovery
from exercise-induced dehydration when differ-
ent carbohydrate-electrolyte solutions were con-
sumed: the plasma volume increase was greater
after drinks with sodium as the only electrolyte
(at concentrations of 43 and 128 mmol · l–1) were
consumed than when drinks containing addi-
tional potassium (at a concentration of 51 mmol ·
l–1) or less electrolytes and more carbohydrate
were consumed. González-Alonso et al. (1992)
have also confirmed that a dilute carbohydrate-
electrolyte solution (60 g · l–1 carbohydrate,
20 mmol · l–1Na+, 3 mmol · l–1K+, 20 mmol · l–1Cl–)
is more effective in promoting postexercise rehy-
dration than either plain water or a low-
electrolyte diet cola: the difference in rehydration
effectiveness between the drinks was a result of
differences in the volume of urine produced. In
none of these studies, however, could the mecha-
nism of the action be identified, as the drinks
used were different from each other in a number
of respects. They did, however, establish that,
because of the high urine flow that ensued, even
drinking large volumes of electrolyte-free drinks
did not allow subjects to remain in positive
fluid balance for more than a very short time.
They also established that the plasma volume
was better maintained when electrolytes were
present in the fluid ingested, and it seemed likely
that this effect was due primarily to the presence
of sodium in the drinks.
The first studies to investigate the mechanisms
that might be involved showed that the ingestion
of large volumes of plain water after exercise-
induced dehydration results in a rapid fall in
plasma osmolality and in the plasma sodium
concentration (Nose et al. 1988a, 1988b, 1988c),
and both of these effects will stimulate urine
output. In these studies, subjects exercised at low
intensity in the heat for 90–110 min, inducing a
mean level of dehydration equivalent to 2.3% of
the pre-exercise body mass, and then rested for
1 h before beginning to drink. Plasma volume
was not restored until after 60 min when plain
water was ingested together with placebo
(sucrose) capsules. In contrast, when sodium
chloride capsules were ingested with water to
give a saline solution with an effective concentra-
tion of 0.45% (77 mmol · l–1), restoration of plasma
volume was complete within 20 min. In the NaCl
trial, voluntary fluid intake was higher and urine
output was less; 29% of the water intake was lost
as urine within 3 h compared with 49% in the
plain water trial. The delayed rehydration in the
water trial was a result of a loss of water as urine
caused by a rapid return to control levels of
plasma renin activity and aldosterone levels.
Therefore, the addition of sodium to rehydra-