drated state, acting as a vehicle for the elimina-
tion of metabolic waste products. It is clear
therefore that the total fluid intake after exercise-
induced or thermal sweating must amount to a
volume greater than the volume of sweat that has
been lost if an effective rehydration is to be
achieved. Shirreffs et al. (1996) investigated the
influence of drink volume on rehydration effec-
tiveness following exercise-induced dehydration
equivalent to approximately 2% of body mass.
Drink volumes equivalent to 50%, 100%, 150%
and 200% of the sweat loss were consumed after
exercise. To investigate the possible interaction
between beverage volume and its sodium
content, a relatively low sodium drink (23 mmol ·
l–1) and a moderately high sodium drink (61
mmol · l–1) were compared.
With both beverages, the urine volume pro-
duced was, not surprisingly, related to the bever-
age volume consumed; the smallest volumes
were produced when 50% of the loss was con-
sumed and the greatest when 200% of the loss
was consumed. Subjects did not restore their
hydration status when they consumed a volume
equivalent to, or only half, their sweat loss irre-
spective of the drink composition. When a drink
volume equal to 150% of the sweat loss was con-
sumed, subjects were slightly hypohydrated 6 h
after drinking when the test drink had a low
sodium concentration, and they were in a similar
condition when they drank the same beverage in
a volume of twice their sweat loss. With the high-
sodium drink, enough fluid was retained to keep
the subjects in a state of hyperhydration 6 h after
drink ingestion when they consumed either
150% or 200% of their sweat loss. The excess
would eventually be lost by urine production or
by further sweat loss if the individual resumed
exercise or moved to a warm environment. Cal-
culated plasma volume changes indicated a
decrease of approximately 5.3% with dehydra-
tion. At the end of the study period, the general
pattern was for the increases in plasma volume to
be a direct function of the volume of fluid con-
sumed: additionally, the increase tended to be
greater for those individuals who ingested the
high sodium drink.Food and fluid consumption
In some situations, there may be opportunities to
consume solid food between exercise bouts, and
in most situations it should be encouraged unless
it is likely to result in gastrointestinal distur-
bances. In a study to investigate the role of food
intake in promoting rehydration from a hypo-
hydration of approximately 2% of body mass,
induced by exercising in the heat, a solid meal
plus flavoured water or a commercially available
sports drink were consumed (Maughan et al.rehydration and recovery after exercise 261
800
700
600
500
400
300
200
100
0
01234 56
Time after rehydration (h) MorningCumulative urine volume (ml)Fig. 19.2Cumulative urine
output over time after
rehydration. After exercise-
induced dehydration by
approximately 2% of body mass,
different rehydration drinks in a
volume equivalent to the sweat
loss were consumed, and all the
urine produced was collected. ,
glucose 90 mmol · l-^1 ;, KCl
25 mmol · l-^1 ;, NaCl 60 mmol ·
l-^1 ;, mixture of three drinks. See
text for full explanation. Adapted
from Maughan et al. (1994).